Clinical and molecular links between COVID-19 and feline infectious peritonitis (FIP)

Arjun N. Sweet, Nicole M. André, Alison E. Stout, Beth N. Licitra and Gary R. Whittaker
Julia A. Beatty, Academic Editor and Séverine Tasker, Academic Editor

Original article: Clinical and Molecular Relationships between COVID-19 and Feline Infectious Peritonitis (FIP)


The emergence of severe acute respiratory syndrome 2 (SARS-CoV-2) has led the medical and scientific community to address questions regarding the pathogenesis and clinical presentation of COVID-19; however, relevant clinical models other than humans are still lacking. In cats, the ubiquitous coronavirus, described as feline coronavirus (FCoV), can manifest as feline infectious peritonitis (FIP), a leading cause of mortality in young cats characterized by severe systemic inflammation. The diverse extrapulmonary symptoms of FIP and the rapidly progressive course of the disease, together with the proximate etiologic agent, represent a degree of overlap with COVID-19. This article reviews the molecular and clinical relationships between FIP and COVID-19. Although there are key differences between the two syndromes, these similarities encourage further investigation of feline coronaviruses as a naturally occurring clinical model for human coronavirus disease.

Keywords: feline infectious peritonitis, SARS-CoV-2, COVID-19, cats

1. Introduction

In the 1960s, feline infectious peritonitis (FIP) was described as a disease in domestic cats and was subsequently found to be of viral etiology, specifically feline coronavirus (FCoV). [1,2]. In most cats, infection with FCoV results in mild to inconspicuous clinical signs, but a small proportion of cats develop severe disease and succumb to the systemic form of the disease known as FIP [3]. In the years since the discovery of FCoV, many features of FCoV have remained misunderstood. Similarly, the COVID-19 pandemic, caused by the emergence of SARS-CoV-2, has raised many equally challenging questions regarding pathogenesis, transmissibility, and treatment. The widespread transmission of FCoV/SARS-CoV-2 and the insidious onset of severe symptoms in both FIP and COVID-19 limit the ability to detect the disease early—what may begin as mild or even mild clinical signs or symptoms can quickly lead to systemic disease [3,4]. We believe that FIP may represent a valuable, naturally occurring extrapulmonary model of COVID-19.

Both FCoV and SARS-CoV-2 belong to the family Coronaviridae [4,5], although to different genera (Figure 1). FCoV, together with similar animal coronaviruses such as canine coronavirus (CCoV) and porcine gastroenteritis virus (TGEV), belong to the genus alpha-coronaviruses. Community respiratory (CAR) human coronaviruses 229E and NL63 are also included in the genus Alphacoronaviruses. [6], the latter being associated with the common cold, hail and possibly Kawasaki disease in children [7]. In contrast, SARS-CoV-2, together with SARS-CoV (the causative agent of the severe acute respiratory syndrome outbreak in 2002-2003) and the Middle East respiratory syndrome coronavirus (MERS-CoV) belong to the genus betacoronaviruses [8], wherein SARS-CoV-2 and SARS-CoV belong to line B (sarbecovirus) and MERS-CoV belong to line C (merbecovirus). Less related beta coronaviruses include human coronavirus CAR OC43 (associated with the common cold), mouse hepatitis virus (MHV), and bovine coronavirus, which is associated with pneumonia and diarrhea in cattle; these viruses are in line A (embekovirus).

Figure 1
Phylogenetic tree of spike proteins of selected coronaviruses. The phylogenetic tree of maximum likelihood was constructed using the MEGAX program (100 bootstraps) from multiple alignment of spike protein sequences. Tip amino acid sequences were obtained from GenBank NCBI. Relevant numbers are: transmissible gastroenteritis virus / TGEV (P07946), severe acute respiratory syndrome coronavirus 2 / SARS-CoV-2 (YP_009724390.1), middle eastern respiratory syndrome / MERS-CoV coronavirus (AFS88 /36) 1. -1 (ACN89742), severe acute respiratory syndrome coronavirus / SARS-CoV (AAT74874.1), feline coronavirus / FCoV-Black (EU186072.1), bovine coronavirus / BCoV (P15777), canine coronavirus / CCo37.14) , human coronavirus / HCoV-OC43 (NC_006213.1), HCoV-229E (NC_002645.1) and HCoV-229E (NC_002645.1).

FCoV can be classified in two ways, the first of which relates to the form of the disease. Feline enteric coronavirus (FECV) is thought to cause a mild gastrointestinal form of the disease, while feline infectious peritonitis virus (FIPV) is associated with a fatal systemic infection known as FIP. [3]. FIPV differs from FECV in its ability to infect and replicate efficiently in monocytes and macrophages [9], causing systemic inflammation. FIPV is associated with a spectrum of clinical sequelae. At one end of the spectrum is effusive or “wet” FIP, which progresses rapidly and involves the accumulation of a highly proteinaceous exudate in the abdominal and/or thoracic cavity. At the other end of the spectrum is non-effusive or “dry” FIP, which can affect many organ systems but is usually characterized by neurological and ocular symptoms. Non-effusive FIP generally has a longer course of disease and is less common than its effusive counterpart. FCoV can also be divided into two serotypes – type I or type II – based on major differences in the spike protein of the virus that affect receptor binding and antibody response [10]. The receptor for FCoV type II is feline aminopeptidase N (fAPN) [11], while the receptor for type I viruses is not identified. Type I FCoV accounts for the vast majority of FIP cases [12].

The classification of SARS-CoV-2 virus into different variants based on genetic mutations is still ongoing as the virus continues to evolve. Viral lines that show the potential for increased transmissibility, treatment resistance, vaccine resistance, or increased morbidity and mortality have been identified as VOCs. The spectrum of diseases associated with COVID-19 is broad, ranging from asymptomatic and mild infections to acute respiratory distress syndrome (ARDS), systemic inflammatory response syndrome (SIRS), and multiorgan failure and death. Systemic inflammation in SARS-CoV-2 is not associated with macrophages and monocytes (as in FIP), but is responsible for a wide range of extrapulmonary symptoms. The SARS-CoV-2 receptor, angiotensin converting enzyme-2 (ACE-2), which plays an important role in the renin-angiotensin system and the development of pro-inflammatory status, appears to be involved. [13]. Multisystem inflammatory syndrome (MIS) in children and adults, as well as the post-acute course of SARS-CoV-2 infection (PASC), also known as "long COVID", are potential consequences of infection with COVID-19.

2. Transfer

As a group, coronaviruses are known for their ability to cause both respiratory and intestinal diseases and are usually transmitted in one or both ways. While FCoV is considered faecal-oral and SARS-CoV-2 is primarily respiratory, patients with COVID-19 may excrete the infectious virus in the faeces. [14], often for a long time, and FCoV can easily become infected by the oronasal route, which is a common method of experimentally vaccinating cats [15].

In most cases, FCoV infection is self-limiting, and although the virus can be detected systemically, replication outside the intestinal epithelium is weak. This form of the virus, referred to as FECV, is easily transmitted by the fecal-oral / oronasal route, with common anchors and swallowing of virus particles during purification being common sources of infection. The current understanding of FIP development involves internal mutation: in a small subset of FECV cases, a complex combination of host and viral factors leads to mutation (s) that allow efficient replication in macrophages and monocytes. [16]. These lethal variants are classified as FIPV and are associated with systemic inflammation, organ failure, and death. FIPV is generally considered non-transmissible because factors that increase its tropism on troprophages appear to limit its faecal-oral spread. [17]. FIP outbreaks were recorded in kennels and shelters. In these situations, congestion stress and high levels of virus in the environment may promote the conversion of FECV to FIPV. There is evidence that some FCoV strains may be more susceptible to this rebirth than others [18,19].

SARS-CoV-2 virus infection is primarily targeted at the respiratory epithelium, but as with FCoV, the virus may appear systemically without appropriate symptoms of infection. [20,21]. Asymptomatic individuals are a well-documented source of SARS-CoV-2 [22,23,24] and delivery includes inhalation of aerosols as well as contact with droplets [25]. The incubation times of SARS-CoV-2 and FECV range from 2 to 14 days [26]. The incubation time of FIP is highly variable, influenced by the time to internal mutation and the individual's immune response. The onset of FIP can occur several weeks to months after the initial infection [27,28,29,30]. Multisystem inflammatory syndrome in children (MIS-C), a severe manifestation of SARS-CoV-2, is also delayed by an initial infection with a median onset of 4 weeks. No viral factors have been associated with the development of MIS-C, but an immune-mediated component is thought.

Vertical transmission of FIP via the placenta or milk is considered rare. In the first experimental study in which a suckling cat was infected, one in four kittens succumbed to FIP [28]. Maternal antibodies appear to be effective in preventing transmission until approximately six weeks of age, when antibody levels decrease and kittens are susceptible to fecal-oral transmission. [31]. However, this maternally acquired immunity can be overcome early in life by high levels of exposure to FCoV – a Swiss study showed that kittens in large herds show infection as early as two weeks of age [32,33]. Vertical transmission poses a risk of SARS-CoV-2 infection. Placental transmission is rare but has been documented in fetuses of SARS-CoV-2 infected mothers. [34,35,36], as evidenced by virus detection in amniotic fluid, neonatal blood, umbilical cord blood and placental tissue. Transmission cases have been documented in both early and late pregnancies, but infection of the neonate with SARS-CoV-2 may not always occur in the uterus. Infection can also occur during childbirth or in close contact with the mother. The neonatal outcomes of COVID-19 infected mothers remain under study, and it is difficult to distinguish between the effects of SARS-CoV-2 infection and maternal comorbidities. Nevertheless, neonatal infection does not appear to be without sequelae, with one analysis showing that approximately 50 % infected neonates exhibited COVID-19-related clinical symptoms, including fever and respiratory and gastrointestinal symptoms. [37].

3. General clinical presentation

Clinical signs associated with both FIP and COVID-19 include fever, diarrhea, depression, weakness, anorexia, and dyspnoea. [1]. Typical manifestations of COVID-19 commonly include non-specific symptoms including fever, dry cough, fatigue, dyspnea, and myalgia [38]. Anosmia (loss of smell) and ageusia (loss of appetite) have also been frequently reported in COVID-19 and are more specific symptomatic indicators of the disease. [39]. Pneumonia, acute respiratory distress syndrome (ARDS) and sepsis may occur. Men appear to be at higher risk of developing more severe manifestations of COVID-19 [40,41], with several small studies confirming the same relationship between males and the development of FIP in cats [42,43].

The classic manifestation of FIP is the formation of effusion in the abdomen and / or thoracic cavity; although this manifestation has also been reported in COVID-19 [44], is very rare. In addition, FIP is manifested in various bodily systems that are similar to the extrapulmonary manifestations of COVID-19 (Figure 2 and Figure 3). The most similar feature of both diseases is endothelial dysfunction. Vasculitis is a hallmark of FIP pathology [45,46] with lesions characterized by perivascular edema and infiltration, vascular wall degeneration and endothelial proliferation [47]. In the case of COVID-19, extrapulmonary symptoms are thought to be caused by virus-mediated endothelitis, which leads to vasculitis, especially in veins with minor arterioles [48,49]. In the following sections, we will describe these extrapulmonary symptoms and point out key similarities and differences.

Figure 2
Summary of systemic clinical signs and pathological conditions associated with FIP. FIP is known to be a systemic infection with a variety of manifestations. Summarized possible systemic clinical signs associated with FIP are summarized, which include organ systems that are also affected by COVID-19. The most common symptoms of FIP are highlighted in red.

Figure 3
Summary of systemic clinical signs, symptoms, and pathologies associated with COVID-19. Respiratory symptoms of COVID-19 are the main manifestation of the disease. However, SARS-CoV-2 infection in humans can also result in a variety of extrapulmonary symptoms. Summarized are the systemic clinical signs and symptoms associated with COVID-19, which include organ systems that are also affected by FIP. The most common symptoms of COVID-19 are highlighted in red. ARDS stands for Acute Respiratory Distress Syndrome.

4. Biomarkers

Inflammatory biomarkers are important as prognostic markers in COVID-19 and as a means of differentiating FIP from other diseases. In FIP, IL-6 expression appears to be increased in the ascitic fluid of FIP-infected cats, presumably through increased expression in the heart and liver. [50,51]. Other acute phase proteins are also elevated in FIP infection. Alpha-1-acid glycoprotein (AGP) has been studied as a diagnostic marker of FIP, but may be elevated in other conditions, thereby limiting its specificity. [52,53]. Serum amyloid A (SAA) is another acute phase protein that appears to distinguish between FIPV and FECV infection, with FIPV-infected cats showing higher levels of SAA compared to FECV-infected cats and control cats without SPF. [54], but has limited utility in distinguishing FIP from other effusive conditions [55].

As with FCoV, subjects with severe COVID-19 have higher SAA levels compared to subjects with milder COVID-19. [56]. Higher SAA levels are also reported in patients who died of COVID-19 compared with those who survived [57]. C-reactive protein (CRP) is another marker that has been shown to be a promising biomarker in both FCoV and SARS-CoV-2 infections. Liver CRP synthesis is induced by IL-6 expression in response to inflammation [58] and is increased in FIP cases [59]. Elevated CRP levels in the early stages of COVID-19 are associated with a more severe course of the disease and higher mortality [60,61,62], which led to the recommendation to use it as a prognostic indicator in risk assessment in patients hospitalized for COVID-19. In contrast, one meta-survey found that IL-6 levels were elevated, but at least one order of magnitude lower in patients with COVID-19 than in patients with ARDS and non-COVID-19-related sepsis, suggesting a different mechanism of immune dysregulation. [63].

The D-dimer, although not specific for COVID-19 or FIP, is another interesting biomarker. D-dimer is released upon fibrin breakdown and is used as a clinical tool to eliminate thromboembolism [64]. Thrombotic events have often been documented in COVID-19 in several organ systems [65,66] and elevated D-dimer levels are associated with higher morbidity and mortality [67,68]. Similarly, thrombotic events can occur in FIP, and high levels of D-dimers, along with other symptoms of disseminated intravascular coagulation (DIC), can be observed in the final stages of FIP in both natural and experimental infections. [69,70].

5. Pathophysiology

5.1. Neurological

FIP is one of the major infectious neurological diseases in cats and the symptoms associated with central nervous system (CNS) infection are well documented. [71]. CNS symptoms are reported in approximately 40 % cases of dry FIP and may manifest as nystagmus, torticollis, ataxia, paralysis, altered behavior, altered mentoring, and seizures [72]. The wide range of symptoms supports the conclusion that the infection is not limited to a specific part of the CNS [73]. CNS infection is restricted to the monocyte and macrophage lines and leads to pyogranulomatous and lymphoplasmacytic inflammation, which usually affects leptomening, choroidal plexus and periventricular parenchyma. [74].

Documentation of neurological symptoms associated with SARS-CoV-2 CNS infection is limited compared to other coronaviruses [75]. The symptoms observed range from headache and confusion to seizures and acute cerebrovascular events. [76]. Virus detection in the brain is rare, suggesting that symptoms may not be directly related to CNS infection. Viral particles have been observed in neural capillary endothelial cells and a subset of cranial nerves, although such detection does not correlate with the severity of neurological symptoms. [77]. There is often no clear evidence of direct infection. Instead, inflammatory mediators, such as activated microglia, have been reported to contribute to microvascular damage and disease. [78,79].

Further comparison of the neuroinflammatory properties of SARS-CoV-2 and FCoV may provide new insights into the neurological manifestations of COVID-19. Further understanding of the neurological symptoms associated with SARS-CoV-2 is necessary to understand the progression of COVID-19 and the extent of CNS infection.

5.2. Ophthalmological

Ocular manifestations of FIP are more common in the dry form of the disease [80]. Mydriasis, iritis, retinal detachment, conjunctivitis, hyphema and keratic precipitates have been observed [81]. The most common ocular manifestation of FIP is uveitis, which can affect both the anterior and posterior uvea [80]. Viral antigen can also be detected in epithelial cells of the nitrating membrane, but viral antigen detection does not distinguish between FECV and FIPV [82].

Ocular manifestations of COVID-19 include conjunctivitis, chemosis, epiphora, conjunctival hyperaemia, and increased tear production [83]. Uveitis—a common ocular presentation of FIP—has also been observed in SARS-CoV-2 infection [84,85]. Tear fluid virus detection has led to concerns about ocular transmission in the first months of the COVID-19 pandemic [83,86]. SARS-CoV-2 RNA was detected in tear secretions and was isolated from ocular secretions, supporting the possibility of ophthalmic transmission [87,88]. Interestingly, in the above case study in China, out of 12 patients with ophthalmic symptoms, only 2 patients returned positive conjunctival tests, suggesting limited sensitivity in detecting virus from conjunctival specimens. [83].

5.3. Cardiovascular

Pericardial effusion is a less common manifestation of FIP, but is well documented in the literature [26,89,90,91]. FCoV was detected in the pericardium of cats with recurrent pericardial effusion, which later developed neurological symptoms [92]. Direct FCoV infection of the heart was documented in a 2019 case study of FIP-associated myocarditis with severe left ventricular hypertrophy and atrial enlargement. [93]. Immunohistochemistry (IHC) revealed the presence of FCoV-infected macrophages and associated pyogranulomatous lesions. [26]. Interestingly, severe SARS-CoV-2 infection with evidence of viral replication in the heart and lungs has recently been documented in cats with pre-existing hypertrophic cardiomyopathy (HCM). [94].

Unlike FIP, heart damage associated with SARS-CoV-2 infection appears to be much more prevalent. A study of 187 patients found that 27.8 % cases of COVID-19 showed evidence of myocardial damage, as evidenced by elevated cardiac troponin (TnT) levels. [95]. High levels of TnT were in turn associated with higher mortality. In a retrospective multicenter study of 68 patients with COVID-19, 27 deaths were attributable to myocardial damage and / or circulatory failure as one of the leading causes of mortality, with elevated C-reactive protein and IL-6 levels associated with higher mortality [96]. The increase in such inflammatory biomarkers in the blood suggests that the rapid inflammatory nature of COVID-19 may have a particularly detrimental effect on heart function. Diffuse edema as well as increased wall thickness and hypokinesis have been reported with COVID-19 infection. [97]. Cardiac tamponade was also observed in patients with COVID-19, with SARS-CoV-2 levels detectable in pericardial fluid. [98]. In contrast to FIP, in which direct invasion of FCoV-infected macrophages into the myocardium was observed in myocarditis, myocardial infection with SARS-CoV-2 virus is not clearly associated with mononuclear cell infiltration or myocarditis. [99]. This leads to considerations of multiple systemic factors in adverse cardiac outcomes – particularly dysregulation of inflammatory cytokines. The impact of SARS-CoV-2 infection on the cardiovascular system is an important element in our increasing understanding of the morbidity and mortality associated with COVID-19.

5.4. Gastroenterological

FCoV is excreted in feces and transmitted by the oronasal route. Initial FCoV infection targets the intestinal tract – infection may be subclinical or cats may develop diarrhea and, less commonly, vomiting. The primary infection lasts for several months and the virus can be shed for months to years [100,101]. Colonic epithelial cells appear to serve as a reservoir for persistent infection and excretion. [21]. The symptoms tend to be mild and spontaneous and only a small proportion of the animals go into the FIP stage. Fibrinous serositis and pyogranulomatous lesions with vasculitis are classic FIP lesions and can be found in the small and large intestines of affected cats. [102]. FIP can cause solitary mass lesions in the intestinal wall, although this is considered a rare presentation (26/156 cats in one study) [103]. These are usually found in the colon or ileocecal junction and have a pyogranulomatous character.

Gastroenterological symptoms are often reported with COVID-19 infection. ACE2, the cellular receptor for SARS-CoV-2, is widely expressed in glandular cells of the gastric, duodenal and rectal epithelium. Viral RNA and nucleocapsid were detected in these tissues [104], which supports their suitability for SARS-CoV-2 replication. Gastrointestinal (GI) symptoms range from general anorexia to diarrhea, nausea, vomiting and abdominal pain [105,106]. Excluding a less specific anorexia symptom, multiple meta-analyzes estimate the prevalence of GI symptoms in patients with COVID-19 at approximately 10 % to 20 %, with diarrhea being the most commonly reported symptom. [106,107,108]. Interestingly, GI symptoms in COVID-19 were observed without accompanying respiratory symptoms [105].

Faecal virus excretion is a major concern for COVID-19, as SARS-CoV-2 RNA may continue to be present in faeces even after reaching undetectable levels in upper airway samples. [109]. Although the detection of viral RNA in faeces alone does not necessarily indicate the presence of infectious virions, viable viral particles have been detected in faeces. [110]. The viral antigen persists in the cells of the gastrointestinal tract as well as in the convalescent phase, up to 6 months after healing [20]. In one case study, persistent colonic infection was associated with persistent gastrointestinal symptoms in a case of "long COVID" [111], which draws a parallel to the role of the colon epithelium as a reservoir for FCoV.

5.5. Dermatological

Dermatological changes have been reported with both SARS-CoV-2 and FIPV infections. Although papular skin lesions are rare, they are the primary dermatological manifestation of FIP, with several available case reports documenting papules. [81,112,113,114]. On histological examination, pyogranulomatous dermatitis, phlebitis, periflebitis, vasculitis and necrosis were reported in several FIP cases. [81,112,113,114,115].

The first report of dermatological manifestations associated with COVID-19 was recorded at Lecco Hospital in Lombardy, Italy [116]. In this study, 18/88 patients (20.4 %) developed a skin disorder, with 8/18 patients observed at the onset of the disease and 10/18 after hospitalization [116]. Clinical signs included erythematous rash (14/18 patients), diffuse urticaria (3/18 patients) and smallpox-like vesicles (1/18 patients) [116]. Lesions were observed mainly on the trunk (torsion) and pruritus was mild or absent [116]. The continuation of the pandemic brought better characteristics of the first observed dermatological symptoms, as well as the identification of rarer presentations. The most common dermatological manifestation of COVID-19 appears to be rash, often characterized by maculopapular lesions. [117,118]. Another predominant dermatological symptom also appears to be urticaria [118,119]. Importantly, neither rash nor urticaria is specific for COVID-19, which limits their positive predictive value. Varicella-like rash has been observed with SARS-CoV-2 infection and may be more specific due to its low prevalence in viral diseases. In particular, with missing lesions in the oral cavity and pruritus observed in COVID-19-associated rash, together with a previous history of varicella infection, the specificity of this presentation is reinforced. [118].

5.6. Teriogenological

Orchiditis and periorchitis with fibrinopurulent or granulomatous infiltrates as well as hypoplastic testes have been observed in several cases of FIP. [1,26,120]. Inflammatory mediators from the tuna surrounding the testicles caused enlargement of the testicles in cats with FIP [26,120]. In effusive FIP, enlargement of the spinal cord due to edema and peritonitis of tuna was observed [16]. Despite the apparent pathology of the male reproductive system in cats, FCoV has not been detected in sperm, which reduces the likelihood of sexual transmission. [121]. The pathology of the female reproductive system in FIP is less documented in the literature, but macroscopic lesions present in the ovaries of FIPV-infected cats have been observed. The surrounding uterine and ovarian vessels of these cats were surrounded by lymphocytes, macrophages, plasma cells and neutrophils. [122].

As with FIP, the pathology of COVID-19 is manifested in the male reproductive system. One study examining the testes of 12 COVID-19 patients found edema as well as lymphocyte and histiocytic infiltration, consistent with viral orchitis. [123]. These samples were also characterized by damage to the seminiferous tubules with a significant effect on Sertoli cells, as well as a reduced number of Leydig cells. In a separate study, germ cell damage was more pronounced despite similar Sertoli cell counts between SARS-CoV-2-infected individuals and uninfected controls, which represents a more direct relationship between infection and fertility. [124]. The extent to which SARS-CoV-2 may persist in the male reproductive tract is still being investigated. Although SARS-CoV-2 has been detected in human semen, it is questionable whether this is a true testicular infection or a consequence of a disrupted blood-epididymal / deferent barrier. [125,126].

Our knowledge of COVID-19 in the female reproductive system is still limited by the amount of literature and sample size of existing studies. Nevertheless, an understanding of the extent of SARS-CoV-2 in the female reproductive tract is essential to recognize any adverse effects on fertility. ACE2 is expressed in the ovaries, oocytes, and uterus, but limited coexpression of proteases such as TMPRSS2 and cathepsins L and B with ACE2 raises questions about the likelihood of ovarian / uterine infection. [127,128]. While in one study of 35 women diagnosed with COVID-19, SARS-CoV-2 was not detected in vaginal fluid or in exfoliated cervical cells, in a case study from Italy, SARS-CoV-2 was detected in vaginal fluid by RT-PCR ( Ct 37.2 on day 7 and Ct 32.9 on day 20 from the onset of symptoms), suggesting that infection of the female reproductive system is possible [129,130].

5.7. Immunological response

FIP is classically characterized as an immune-mediated disease based on early observations of complement and immunoglobulin circulation, even in the form of immune complexes. [131]. Components of type III and IV immune responses have been described [132]. Vasculitis and vasculitis-like lesions are thought to play a role in COVID-19 systemic complications that cannot be explained by direct organ infection, such as microthrombosis in the brain, kidney, spleen and liver. [133]. One type III hypersensitivity report has been identified in the COVID-19 literature [134]; however, immune complexes do not appear to play an important role in COVID-19 pathology. The mechanism of viral clearance and the inflammatory effects of the immune response are important areas of study for both FIP and COVID-19. Previous work investigating SARS-CoV has demonstrated the need for CD4 + T cells for virus clearance [135,136]. T-cell depletion was a recognized consequence of FCoV and was observed to be associated with more severe cases of COVID-19 [137,138,139]. In addition, FIP reduces both regulatory T cells and NK cells in the blood, mesenteric lymph nodes and spleen. [140]. High levels of IL-6 have previously been demonstrated in FIP ascites [50], and similarly, elevated levels of IL-6 appear to be related to disease severity and outcome in patients with COVID-19. [141]. The cytokine storm, characterized by overexpression of inflammatory cytokines, has been implicated in the pathogenesis of both infections. In FIP, this pathology is associated with monocyte and macrophage activation, while in COVID-19, the association with macrophages and monocytes is less clear. [142]. When considering the balance between cell-mediated immunity and humoral immunity, early reports suggested a link to strong humoral immunity leading to FIP [143]. However, humoral immunity may play a more beneficial role in patients with COVID-19 [144], especially given the potential clinical benefit of convalescent plasma / serum [145].

During the development of the SARS-CoV-2 vaccine, the antibody-dependent infection enhancement process (ADE), in which virus-antibody complexes enhance infection, was particularly important. FIPV has been shown to exhibit ADE in the presence of anti-FIPV antibodies [146]. This increase in infection appears to be serotype specific, with passive immunization of cats against FIPV type I or type II leading to ADE only after challenge with the same serotype for which the immunization was performed. [147]. As a result, ADE is a major challenge for the development of FIP vaccines. In diseases caused by human coronaviruses, ADE has yet to be fully understood. SARS-CoV has been found to have higher concentrations of anti-spike antibodies having a higher neutralizing effect, while more dilute concentrations are thought to contribute to ADE in vitro. [148]. In SARS-CoV-2, ADE was observed in monocyte lines but was not related to the regulation of pro-inflammatory cytokines [149]. Spike protein sequence modeling has identified possible ADE mechanisms that involve interaction with Fc receptors on monocytes and adipocytes [150]. If ADE played a role in SARS-CoV-2, the most likely mechanism would be overactivation of the immune cascade through Fc-mediated innate immune cell activation. [151,152]. There is currently insufficient evidence to suggest an ADE with the pathogenesis of SARS-CoV-2 and further research is needed to assess the true extent of the risk.

6. Molecular similarities between FCoV and SARS-CoV-2 spike proteins

Spike protein is a major factor in tissue and cell tropism and binds the cell receptor [153]. It is now well known that SARS-CoV-2 binds angiotensin converting enzyme-2 (ACE-2) as a primary receptor, a common feature of SARS-CoV. There are other binding partners for SARS-CoV-2, including heparan sulfate as a non-specific binding and neuropilin-1 (NRP-1), which may cause viral tropism for the olfactory and central nervous systems. [154,155]. In contrast, most alpha-virus viruses, including FCoV type II, use aminopeptidase (APN) virus entry. [9,153,156]. The receptor for FCoV type I has yet to be elucidated. The spike protein also mediates membrane fusion, which is activated by a complex process controlled by host cell proteases. [153]. While type I FCoV has two protease cleavage activation sites, designated S1 / S2 and S2 ′, type II FCoV has only one cleavage activation site (S2 ′). [10]. In comparison, SARS-CoV-2 is similar to FCoV-1 (and currently unique to SARS-related viruses) in that it has two identified cleavage sites (S1 / S2 and S2 ′), the first of which, the furin cleavage site or FCS is considered a significant factor in pandemic spread [157,158,159]. In both cases, the presence of S1 / S2 cleavage sites distinguishes FCoV-1 and SARS-CoV-2 from their close relatives. The importance of the cleavage activation site appears to be directly related to the proteases required for viral infection, and thus to another component of tissue tropism. In FCoV type I, the transition from FECV to macrophage-tropical FIPV was first demonstrated by amino acid substitutions at the S1 / S2 cleavage site in FIP-confirmed pathological specimens that are thought to reduce proteolytic priming of furin by similar proteases prior to S2-mediated fusion process activation. [72,160,161]. In SARS-CoV-2, TMPRSS-2 or other trypsin-like related proteases are a major activator of fusion and entry at S2 ′ [162] (Table 1), wherein the furin-like proteases prim the tip and S1 / S2 [163] and in particular have been shown to be rapidly regulated after adaptation to Vero E6 cells in culture and possibly also in extrapulmonary human tissues [164]. Thus, there appear to be remarkable similarities in host cell adaptation between the two viruses.

VirusGroupReceptorConsensus sequence S1 / S2 in circulating virusesConsensus sequence S2 ′ in circulating viruses
SARS-CoV-2Beta-coronavirusACE2SPRRAR | S
(* SHRRAR | S and SRRRAR | S)
FCoV-1Alphacoronavirus (“clade A”)unknownSRRSRR | S (in FECV; mutated in FIPV)KR | S
FCoV-2Alphacoronavirus (“clade B”)APNabsentYRKR | S
Table 1
Summary of SARS-CoV-2 and two FCoV serotypes. The coronavirus spike glycoprotein, mediated by proteolytic cleavage, is a major driver of cell receptor binding and membrane fusion. The Taxonomic classification, host receptor, and amino acid sequences of the proteolytic cleavage site S1 / S2 and S2 ′ are summarized below.
*, Replaced in common variants.

7. Prevention and treatment: From social withdrawal to vaccines

Until now, the role of public health / public health measures has been a major driver in mitigating both the spread of FCoV and SARS-CoV-2. [3,31,165,166]. In this regard, many measures have been introduced for the affected population to reduce social distance, including orders to stay at home, the closure of unnecessary establishments and restrictions on public assemblies. [167]. Although not referred to as social distances, similar methods are often introduced or recommended in cat populations. [3]. Dreschler et al. summarize the methods that have been recommended in cat populations, especially in a multi-cat environment, including reducing the number of cats per room, frequent cage cleaning, and grouping cats according to excretion and / or serological status. [168]. Dreschler argues that quarantining cats exposed to FCoV / FIPV to limit the spread of FCoV in the population is neither effective nor beneficial given the likelihood of widespread FCoV infection in a multi-cat environment as well as the months required for development (and uncertainty in development) FIP. In contrast, quarantine people exposed to SARS-CoV-2 has the potential to reduce the spread of the disease and mortality [169]. Regardless of the extent of grouping or separation, the social difficulties caused by separation must be carefully considered for both cats and humans. In the case of cats, especially in connection with the premature weaning of their mothers, special attention must be paid in the weaning process to ensuring adequate socialization of the kittens. Similarly, in the case of COVID-19, the process of quarantine and / or isolation can be psychologically burdensome for individuals. A thorough cost-benefit analysis must often be carried out to compare the benefits of quarantine and isolation for public health with the negative psychological burden on the persons concerned in order to avoid unnecessary / ineffective quarantine. Where appropriate, justification as well as support to improve well-being should be provided [170].

Although the FIP vaccine is commercially available (Primucell), the benefit of FIP vaccination is still low. Primucell is an intranasal vaccine that uses an attenuated FIPV serotype 2 isolate. Although the FIP vaccine is commercially available (Primucell), the benefit of FIP vaccination is still low. Primucell is an intranasal vaccine that uses an attenuated FIPV serotype 2 isolate (FIPV-DF2), given in two doses 3 to 4 weeks apart to cats at least 16 weeks old. [171]. In a placebo-controlled experimental study in 138 cats, vaccinated cats did not show a significantly lower incidence of FIP compared to controls during the 12-month study period. After adjusting for FCoV titers, cats with lower antibody titers (100 or less) had a significantly lower incidence of FIP at the time of the first vaccination compared to cats with higher titers (400 or more). [172]. However, due to the high prevalence of FCoV, especially in multi-cat environments, attempts to alleviate FIP by vaccinating cats that are FCoV-naive at least 16 weeks of age may not be feasible due to the high potential for FCoV infection during the 16 weeks prior to vaccination. As a result, the American Association of Animal Hospitals and the American Association of General Practitioners for Cats do not recommend FIP vaccination. [173].

ADE remains a major concern of FIP vaccines. Several studies have attempted to reduce the incidence of FIP in experimentally infected cats with recombinant and other experimental vaccines, but ADEs have been reported repeatedly. In one placebo-controlled study in which purebred British Shorthairs and Specific Pathogen Free (SPF) Domestic Shorthairs were vaccinated with one of two recombinant FIPV type 2 (FIPV-DF2) vaccines, both candidate vaccines showed significantly reduced to no protection against challenge FIPV in non-SPF cats - with the majority of non-SPF animals showing ADE [174]. In a separate study, immunization of kittens with a vaccine virus recombined with the FIPV spike glycoprotein gene significantly shortened survival time after FIPV challenge compared to kittens immunized with wild-type vaccine virus. Importantly, low levels of neutralizing antibodies were observed in the group immunized against FIPV [175]. Concerns about ADE after FIPV immunization remain a challenging challenge in FIP prevention.

Unlike the FIP vaccination, the COVID-19 vaccines have played a more significant role in reducing the spread of the infection. Several types of vaccines have been produced that have demonstrated safety and efficacy in preventing symptomatic infection, severe disease, and death from COVID-19—including, but not limited to, mRNA vaccines (Pfizer/BioNTech and Moderna), viral vector vaccines (Janssen, AstraZeneca), and inactivated virus vaccines (Bharat Biotech, Sinovac) [176,177,178,179,180,181]. The first two vaccine platforms use the SARS-CoV-2 spike glycoprotein as the immunogen, while inactivated virus vaccines have the potential to elicit an immune response to viral components other than the spike glycoprotein. Despite the favorable safety profile of COVID-19 vaccines, adverse reactions occurred after vaccination, some of which were mediated by antibodies in analogy to ADE concerns with FIP vaccines. Thrombosis has been a documented problem, especially with AstraZeneca and Janssen vaccines. Although the exact mechanisms are being investigated, the inflammatory response is currently thought to lead to increased levels of platelet-activating antibodies, which bind to platelet factor 4 and lead to a hypercoagulable state. [182,183]. In contrast to the higher incidence of ADE in experimental FIP vaccines, the incidence of thrombotic events after administration of COVID-19 is low. [184].

In addition to the primary endpoints of vaccine studies, which focused on the prevention of symptomatic infection, serious illness, and death from COVID-19, many phase 3 vaccine studies did not address observations to assess the degree of prevention of asymptomatic infection. Beneficial efficacy against asymptomatic infection is important from a public health perspective, especially given that asymptomatic individuals can transmit COVID-19 and that routine monitoring testing is resource intensive and difficult to coordinate on a large scale. [22]. An important contribution towards this field is the real-world studies investigating the efficacy of the vaccine, which show a reduced risk of SARS-CoV-2 infection, as well as a reduced viral load in vaccine "breakthrough" infections. [185,186,187,188]. Such evidence supports the use of SARS-CoV-2 vaccines as a protective measure not only against severe COVID-19, but also as a crucial contribution in the management of the disease.

8. Clinical care and therapeutic options

In 1963, when the first clinical cases of FIP were described (before understanding the viral etiology), it was found that antibiotic treatment was often tried, but it clearly did not bring any benefit. [189]. Since this first report and without an effective vaccine, a number of therapies have been tried in cats with FIP. Ribavirin, a nucleoside analogue, has previously provided promising results against FCoV in an in vitro study [190], however, when administered to cats as an experimental treatment, led to poorer results in some cases [191]. Similarly, at the onset of the COVID-19 pandemic, ribavirin was used in several doses and in combination with other drugs. [192] and a study protocol was designed to examine the benefit in human patients [193]. However, another direct-acting antiviral drug (DAA) (remdesivir), a nucleoside analogue that acts as a chain terminator and raises minor toxicity concerns compared to ribavirin, is rapidly gaining prominence in the treatment of hospitalized patients with COVID-19. . Despite initial enthusiasm, remdesivir has not been shown to be effective in patients with such diseases in robust clinical trials; however, several reports have demonstrated the clinical benefit of the related nucleoside analog GS-441524 in the treatment of cats with FIP, including effusive, non-fusive, and neurological forms of the disease. [194,195,196,197]. At the time of writing, studies on the effectiveness of remdesivir in the treatment of FIP are being conducted in Australia and the United Kingdom. Interestingly, remdesivir is a prodrug form of GS-441524 [195]. Two orally available DAAs have recently entered clinical trials for COVID-19 and are currently awaiting FDA approval; molnupiravir (MK-4482 / EIDD-2801), a modified form of ribavirin, and Paxlovid (a protease inhibitor PF-07321332 in combination with ritonavir, which improves the half-life of PF-07321332) targeting the major protease (Mpro). It is noteworthy that the active substance Paxlovid is related to GC-376 and has previously been shown to be effective in a FIP clinical study. [196]. It will be very interesting to follow the development, FDA approval and use of these DAAs in relation to the relevant diseases caused by SARS-CoV-2 and FCoV.

Due to the inflammatory nature of both FIP and COVID-19, treatment often focuses on controlling the immune response. Although cats with FIP are often given glucocorticoids in an effort to alleviate the inflammatory symptoms of the disease, the clinical benefit is negligible. [198]. The use of corticosteroids in patients with COVID-19 does not appear to be insignificant, with some studies showing negative profiles [199]. However, their administration may be beneficial in severe cases of COVID-19 through the observed reduction in mortality [200,201]. Cyclosporine, an immunosuppressive drug that is often used to prevent organ rejection in transplant patients and to treat some autoimmune diseases, has been studied in both FIP and SARS-CoV-2. An in vitro study with cyclosporin A (CsA) using FCoV type II virus showed a reduction in virus replication [202], treatment of a 14-year-old cat CsA after unsuccessful IFN treatment resulted in clinical improvement, reduction in viral load and survival of more than 260 days [203]. Although there are currently no controlled studies on the use of CsA in the treatment of patients with COVID-19, in addition to safety concerns, potential mechanisms of action have been suggested. [204,205,206]. In addition, the cyclosporin A analogue, alisporivir, has shown in vitro effects on virus replication [207], similar to evidence that cyclophilin A blockade inhibits the replication of other coronaviruses [208].

Many antibiotics have been prescribed for both FIP and COVID-19, but not for their antimicrobial properties, but rather for their anti-inflammatory effects. [198]. For example, doxycycline may have helped prolong survival in cats with FIP [209]. Whether doxycycline would be of benefit to patients with COVID-19 is not currently known, but has been suggested as a possible part of the treatment of the disease. [210].

Interferons have also been studied in the treatment of FIP without a clear link to clinical improvement [211]. In human patients with COVID-19, combination therapy with interferon-β-1b with lopinavir, ritonavir and ribavirin compared with lopinavir and ritonavir alone was associated with a reduced duration of virus excretion and improved clinical outcomes in mild to moderate cases. [212].

Monoclonal antibodies targeting components of the immune response have the potential to reduce inflammatory cytokine levels. A small study in cats experimentally infected with FIPV-1146 demonstrated the benefit of anti-TNF-α in managing the disease [213]. Tocilizumab, an anti-IL-6 monoclonal antibody, was administered to patients with COVID-19 [214]. Due to the different clinical results reported, further research is needed with Tocilizumab [215,216].

The transfer of knowledge between species will undoubtedly affect cats as well as humans and even other species. Although many compounds are effective when studied in vitro, their use in vivo can lead to different results, including toxicity. In addition, the fact that a compound may show promising results in one species does not mean that the same effect will be observed in other species, especially when comparing similar but different viruses and virus-induced diseases.

9. MIS-C and PASC

In April 2020, the UK's National Health Service issued an alert about an increased incidence of multisystem inflammatory syndrome in children - many of whom tested positive for COVID-19 [217]. As the pandemic progressed, studies from other countries examining this inflammatory condition provided more detail towards a clinical understanding of what is now referred to as MIS-C, a rare presentation of COVID-19 in pediatric patients. MIS-C includes several organ systems. Cardiovascular dysregulation in MIS-C is often observed in the form of ventricular dysfunction, pericardial effusion and coronary artery aneurysms [218,219]. Gastrointestinal symptoms mimic appendicitis and include abdominal pain, vomiting, and diarrhea. Terminal ileitis is a common finding on imaging tests [220]. Many patients also experience neurocognitive symptoms, including headache and confusion. More serious neurological complications, including encephalopathy and stroke, are less common [218,221].

One area of significant clinical overlap between FIP and COVID-19 is the rare inflammatory manifestation of SARS-CoV-2 infection – multisystem inflammatory syndrome in children (MIS-C). MIS-C is seen in the pediatric population, just as FIP commonly affects young cats [43]. Like FIP, MIS-C has a systemic presentation involving multiple organ systems—including gastrointestinal, cardiovascular, and hematologic abnormalities [222]. As with the wet form of FIP, pleural effusions and ascites occur in MIS-C. [223]. Both syndromes also show overlap in vascular pathology. FIP shows granulomatous vasculitis, which overlaps with Kawasaki vascular syndrome observed in MIS-C [224]. MIS-C is thought to be a post-infectious disease associated with a previous SARS-CoV-2 infection [223,225]. FIP also has a delayed onset after the first exposure to FCoV and occurs only in a small subset of cases. Although cats with FIP can still shed FCoV in their feces, the mutations associated with the biotype switch from FECV to FIPV are thought not to be transmissible—supporting some degree of similarity in the limited infectious range of both FIP and MIS-C.

Recently, post-acute sequelae of COVID-19 (PASC) has been defined, which includes memory loss, gastrointestinal distress, fatigue, anosmia, dyspnea, etc. and is more often referred to as "long-term COVID". Along with MIS-C, PASC is a very active subject of research, which has been summarized by others [226], and together represent an excellent starting point for the use of feline medicine as a model of coronavirus-induced pathogenesis, possibly in an unexpected way [224].

10. SARS-CoV-2 infection in cats

Cats have now become widespread hosts for SARS-CoV-2 infections, in part due to the relative similarity of human and feline ACE2 receptors. Following cases reported in Hong Kong and Belgium in March 2020, the most notable early natural infection occurred at the Bronx Zoo in New York City, USA. In April, four tigers and three lions showed mild respiratory symptoms from their breeders, and SARS-CoV-2 was detected by PCR and sequencing. [227]. Consequently, infection of both domesticated and non-domesticated cats has become relatively common in cases where owners and caregivers are positive for SARS-CoV-2. From a clinical point of view, SARS-CoV-2 infection in cats is considered to be predominantly asymptomatic, with some animals showing mild respiratory symptoms. [228,229,230]. In general, severe respiratory symptoms do not appear to occur in cats, although severe respiratory distress may in some cases be related to the underlying hypertrophic cardiomyopathy (HCM) of cats. [94]. An increased incidence of myocarditis in dogs and cats has also been reported in the United Kingdom, associated with a sharp increase in variant B.1.1.7 (Alpha). [231]. There is a clear need for further studies in this area, as well as possible links between coronavirus infections in cats and multisystem inflammatory syndrome in children (MIS-C), which, as mentioned above, is a rare manifestation of COVID-19.

Laboratory animal studies have also been key to understanding SARS-CoV-2 infection in cats, which are very susceptible to infection by oronasal challenge. Experimentally infected cats showed mild respiratory symptoms or asymptomatic infection, virus shedding, virus-to-cat virus transmission, and a strong neutralizing antibody response. Recent studies have shown that long-term immunity exists after re-infection of cats, but cats may develop long-term consequences, including persistence of inflammation and other lung lesions. [232]. Overall, as with SARS-CoV in 2003, cats in particular can be an important source of information on the pathogenesis and immune responses elicited by SARS-CoV-2.


Thanks to Annette Choi for helping with Figure 1 and all Whittaker Lab members for helpful discussions during the preparation of this manuscript.

Author shares

All authors have contributed to this article. All authors have read and agreed to the published version of the manuscript.


The work in the author's laboratory is partly funded by research grants from the National Institutes of Health, the EveryCat Foundation and the Cornell Feline Health Center. AES was supported by the NIH Comparative Medicine Training Program T32OD011000. Studies on FIP are also supported by the Michael Zemsky Fund for Cat Diseases.

Conflict of interests

The authors do not indicate any conflict of interest.


Publisher Note: The MDPI remains neutral in terms of jurisdictional claims in published maps and institutional affiliation.


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Read "Clinical and molecular links between COVID-19 and feline infectious peritonitis (FIP)"

History of Feline infectious Peritonitis 1963-2022 – First description to Successful Treatment

Niels C. Pedersen
Center for Companion Animal Health, School of Veterinary Medicine, University of California, 944 Garrod Drive, Davis, CA, 95616, USA
Original article: History of Feline infectious Peritonitis 1963-2022 – First description to Successful Treatment


This article discusses the development of knowledge about feline infectious peritonitis (FIP) from its recognition in 1963 to the present and has been prepared to inform veterinarians, cat rescuers and carers, shelter staff and cat lovers. The causative agent of the feline coronavirus and its relationship to the ubiquitous and minimally pathogenic feline intestinal coronavirus, epizootology, pathogenesis, pathology, clinical signs and diagnostics are briefly mentioned. The main emphasis is placed on the risk factors influencing the incidence of FIP and the role of modern antivirals in successful treatment.


Figure 1. Photo of the author and Dr. Jean Holzworthová (1915-2007) from 1991. Dr. Holzworth was the best feline veterinarian the author knew and was responsible for the first report of FIP as a specific disease. She spent her entire career at Angell Memorial Animal Hospital in Boston.

Feline infectious peritonitis (FIP) was described as a specific disease in 1963 by veterinarians at Angell Memorial Animal Hospital in Boston (Holzworth 1963) (Fig. 1). Pathology records from this institution and Ohio State University failed to identify earlier cases (Wolfe and Griesemer 1966), although identical cases were soon recognized worldwide. The initial pathological descriptions were of diffuse inflammation of the tissues lining the abdominal cavity and abdominal organs with extensive effusion of inflammatory fluid, after which the disease was eventually named (Wolfe and Griesemer 1966, 1971) (Figs. 2, 3). A second and less common clinical form of FIP, which presents with less diffuse and more widespread granulomatous lesions involving organ parenchyma, was first described in 1972 (Montali and Strandberg 1972) (Figs. 3,4). The presence of inflammatory effusions in the body cavity in the common form and the absence of effusions in the less common form led to the names wet (effusion, non-parenchymatous) and dry (non-effusion, parenchymatous) FIP.

The prevalence of FIP appears to have increased during the panzootic disease caused by feline leukemia virus (FeLV) in the 1960s–1980s, when many cases of FIP were found to be associated with FeLV (Cotter et al., 1973; Pedersen 1976a). Subsequent management of FeLV infection in owned cats through rapid testing and vaccination resulted in an increase in FIP cases. However, recent interest in breeding/rescue along with effective treatment has led to increased awareness of the disease and its diagnosis.

Figure 2. Gross necroptic appearance of the abdominal cavity of a cat with acute onset wet FIP. The abdomen is filled with several hundred ml of yellow viscous fluid, the omentum is reddened, edematous and contracted, and fibrin deposits (arrows) are visible on the surface of the spleen and the edges of the liver. A fiber of fibrin can be seen on the spleen
Figure 3. Appearance of an open abdomen at autopsy of a cat that died of a chronic form of effusive FIP. The abdomen is filled with a viscous, yellow-colored exudate, and the omentum is thickened and contracted. The main lesions are in the liver with numerous plaque-like structures (pyogranulomas) on the cover. More circumscribed lesions (granulomas), also oriented on the serous surface, look more fleshy and are elevated above the surface. These lesions also involve the underlying liver parenchyma and are more typical of dry FIP. This is an example of a case of FIP that transitions between the wet and dry forms (arrow).
Figure 4A – Gross section of kidneys of two cats with dry form of FIP. The lesions are superficial and extend into the underlying parenchyma.
Figure 4B - lesions of the dry form of FIP in organs such as the kidneys, cecum, colon and intestinal lymph nodes (Fig. 5) were grossly confused with renal lymphoma.
Figure 5. Gross enlargement of ileo-cecal-colic lymph nodes in a cat with dry FIP.

Etiological factor

The first attempts did not allow identifying the causative agent of FIP, but confirmed its infectious nature (Wolfe and Griesemer 1966). A viral etiology was established in 1968 using ultrafiltrates of infectious material (Zook et al., 1968). The causative virus was subsequently identified as a coronavirus (Ward 1970), which is closely related to enteric coronaviruses of dogs and pigs (Pedersen et al., 1978).

Confusion arose when feline enteric coronavirus (FECV) was isolated from the feces of healthy cats and proved to be indistinguishable from feline infectious peritonitis virus (FIPV) (Pedersen et al., 1981). Unlike FIPV, which readily induced FIP in laboratory cats, experimental infections with FECV were largely asymptomatic. The relationship between the two viruses became clear when FIPVs were found to be FECV mutants that arise in the body of every cat with FIP (Vennema et al., 1995; Poland et al., 1996).

FECV is ubiquitous in feline populations worldwide and is first shed in faeces from approximately 9–10 weeks of age, coinciding with the loss of maternal immunity (Pedersen et al., 2008 ;). The infection takes place via the faecal-oral route and targets the intestinal epithelium, and the primary signs of enteritis are mild or inconspicuous (Pedersen et al., 2008; Vogel et al., 2010). Subsequent faecal excretion occurs from the colon and usually stops after several weeks or months (Herrewegh et al., 1997; Pedersen et al., 2008; Vogel et al., 2010). Immunity is short-lived and repeated infections are common (Pedersen et al., 2008; Pearson et al., 2016). Over time, stronger immunity eventually develops and cats older than 3 years are less likely to shed the infection in their faeces (Addie et al., 2003). FECV is constantly subject to genetic drift into locally and regionally identifiable clades (Herrewegh et al., 1997; Pedersen et al., 2009).

FECV and FIPV are classified as biotypes of the feline coronavirus (FCoV) subspecies. The genomes of FECV and FIPV biotypes are related at >98 %, but with unique host cell tropism and pathogenicity (Chang et al., 2012; Pedersen et al., 2009). FECVs infect the mature intestinal epithelium, whereas FIPVs lose intestinal tropism and acquire the ability to replicate in monocytes/macrophages. The published names FECV or FIPV will be used here when discussing aspects of the disease specific to each biotype, while the term FCoV will be used when discussing features common to both biotypes.

Three types of mutations are involved in the biotype change of FECV to FIPV. The first type, which is unique to each cat with FIP (Poland et al., 1996), consists of an accumulation of missense and nonsense mutations in the c-terminus of the auxiliary 3c gene, often resulting in truncated 3c gene products (Pedersen et al., 2012 ; Vennema et al., 1995). The second type of mutation consists of two specific single nucleotide polymorphisms in the fusion peptide of the S gene, one or the other form being common to >95 % FIPV and absent in FECV (Chang et al., 2012). A third type of mutation, unique to each FIPV isolate and not found in FECV, occurs in and around the furin cleavage motif between the receptor binding domain (S1) and the fusion domain (S2) of the spike gene (S) (Licitra et al., 2013). These mutations have different effects on furin cleavage activity. Together and in an as yet undetermined manner, they are responsible for the shift of the tropism of the host cell from the enterocyte to the macrophage and for the profound change in the form of the disease.

FCoV, and therefore FECV and FIPV, exist in two serotypes identified by antibodies against the viral neutralizing epitope on the S gene (Herrewegh et al., 1998; Terada et al., 2014). Serotype I FCoVs are identified in cat sera and are prevalent in most countries. Serotype II FCoVs result from recombination with the S part of the canine coronavirus gene (Herrewegh et al., 1998; Terada et al., 2014) and are identified by canine coronavirus antibodies. Serotype II FIPVs are easily cultured in tissue culture, whereas serotype I FIPVs are difficult to adapt to growth in vitro. Serotype I and II FECVs were not grown in conventional cell cultures (Tekes et al., 2020).

FIPVs are found exclusively in activated monocytes and macrophages in affected tissues and effusions and are not secreted into the environment. Therefore, cat-to-cat (horizontal) transmission of FIPV is not the main mode of spread. Rather, FIP follows the pattern of an underlying enzootic FECV infection, with sporadic cases and occasional small outbreaks of disease (Foley et al., 1997). These clusters of cases can be mistaken for epizootics. The only report of an epizootic occurrence of FIP was associated with a single serotype II virus that appeared to develop in a shelter housing both dogs and cats (Wang et al., 2013). Horizontal transmission in this case followed an epizootic rather than an enzootic disease model, with infection spreading rapidly to cats of all ages and in close contact with the index case (Wang et al., 2013).

The low incidence of FIP cases in the population suggests that FIPV mutations arise infrequently. However, studies involving FECV infection in immunocompromised cats infected with FIV and FeLV suggest that FIP mutants may be common but only cause disease under certain circumstances. Nineteen cats infected with feline immunodeficiency virus (FIV) for 6 years and a control group of 20 littermates not infected with FIV were orally challenged with FECV (Poland et al., 1996). Cats in both groups remained asymptomatic for two months when two cats in the FIV-infected group developed FIP. In a second study, 26 young cats with enzootic FECV infection from a breeding colony with no history of FIP were contact-exposed to FeLV carriers (Pedersen et al., 1977). Two kittens in the group subsequently developed FIP 2–10 weeks after becoming FeLV viremic. The question remains, how long can FIPV viruses survive in the body before they are eliminated? According to one of the theories, they persist in the body for a certain time and become pathological only if immunity against them is impaired (Healey et al., 2022). This theory is supported by the way immunity to FeLV develops. Most cats resist FeLV by kitten age and develop robust and permanent immunity, but this occurs within a few weeks, during which the virus persists in a subclinical or latent state (Pedersen et al., 1982; Rojko et al., 1982). . Methylprednisolone given during this period, but not after, will abolish developing immunity and lead to a state of persistent viremia.


Epizootiology is the study of the occurrence, spread and possible control of animal diseases and the influence of environmental, host and agent factors. FIP is considered one of the most important infectious causes of death in cats, although there are no precise data on prevalence. It is estimated that 0.3–1.4 % deaths of cats presented to veterinary institutions are related to FIP (Rohrbach et al., 2001; Pesteanu-Somogyi et al., 2006; Riemer et al., 2016) and in some shelters and breeding stations up to 3.6–7.8 % (Cave et al., 2002). FIP is also described as an environmental disease with a higher incidence of multiple cats. Three-quarters of the FIP cases in the currently ongoing treatment study came from the field through foster carers/rescues and cat shelters, 14 % from kennels, and only 11 % from households.1

Studies based on cases observed in academic institutions have demonstrated the influence of age and gender on the incidence of FIP (Rohrbach et al., 2001; Pesteanu-Somogyi et al., 2006; Pedersen 1976a; Worthing et al., 2012; Riemer et al., 2016) . Three-quarters of the cases in these cohorts occurred in cats younger than 3 years of age, and few cases occurred after 7 years of age. This was also confirmed by a current and ongoing field study from the Czech Republic and Slovakia, in which it was found that more than 80 % cases of FIP occurred in cats under 3 years of age and only 5 % in cats older than 7 years (Fig. 6) .1 Earlier institutional studies differed on the effect of sex, but indications were that male cats were slightly more susceptible to FIP than female cats. This was also confirmed by current data from the field, which show a ratio of males to females of 1.3:1.1. It is unclear whether castration affects the incidence of FIP, with some reports suggesting that it may increase susceptibility (Riemer et al., 2016), while others do not report such a clear effect.1

Figure 6. Age of more than 607 cats from the Czech Republic and Slovakia at the time of diagnosis and treatment of FIP.1 Thirty percent of infections were in cats six months of age or younger, 50 % at one year of age, and 85 % at three years of age or younger.

Other environmental and viral risk factors have been implicated in the increased incidence of FIP, but their significance requires knowledge of disease occurrence in their absence. A possible baseline may have been provided by a study of enzootic FECV infection, which had been unrecognized for many years in a well-managed specific pathogen-free breeding colony (Hickman et al., 1995). This colony was kept in strict quarantine free of other infections and the standard of nutrition and husbandry was high. This colony produced hundreds of kittens each year before the first case of FIP was diagnosed. Such observations suggest that FIP may be a rare phenomenon in the absence of risk factors.

The importance of moving to a new home as a risk factor for FIP is only now being appreciated. Breeders, many of whom have not experienced any cases of FIP in their litters, are most concerned about the announcement that one of their kittens has developed FIP shortly after going to a new home. A recent study found that more than half of cats with FIP had experienced a change in environment, shelter or capture in the weeks before the illness.1 Cats are known to hide outward signs of stress, even when suffering from serious internal disease consequences. Even simple procedures such as changing the cage suppress immunity and reactivate latent herpes virus shedding and disease symptoms in cats (Gaskell and Povey, 1977). Stressful situations, even those that seem minor, can cause a decrease in lymphocyte levels and “sickness behavior” (Stella et al., 2013).

Differences in the genetic make-up of enzootic FCoV strains may also contribute to the prevalence of FIP in the population. Serotype II FIPVs are thought to be more virulent than serotype I and more likely to be transmitted from cat to cat (Lin et al., 2009; Wang et al., 2013). It is also possible that certain FECV clades are more susceptible to mutation to FIPV, which should be studied. The author also observed a disproportionately high proportion of cats with neurologic FIP in some regions, suggesting that genetic determinants in certain FCoV strains may be more neurotropic.

Immunodeficiencies associated with retroviruses are associated with susceptibility to FIP. Up to half of FIP cases during the peak of FeLV panzootic disease were persistently infected with FeLV (Cotter et al., 1973; Pedersen 1976a; Hardy 1981). FeLV infection causes suppression of T-cell immunity, which may inhibit the protective immune response to FIP. The importance of FeLV infection for the incidence of FIP has declined significantly since the 1980s, when carrier elimination and vaccination pushed FeLV back into the wild, where exposures are less severe and immunity is the usual outcome. Chronic feline immunodeficiency virus (FIV) infection has also been shown to be a risk factor for FIP in FECV-infected cats under experimental conditions (Poland et al., 1996). In one recent field study, FeLV infection was recognized in 2 % and FIV in 1 % cats treated for FIP.1

The incidence of FIP in purebred cats is reported to be higher than in random breeding cats, with some breeds appearing to be more susceptible than others (Pesteanu-Somogyi et al., 2006; Worthing et al., Genetic predisposition to FIP has been investigated in several Persian cat breeds and is estimated to account for half the risk of the disease (Foley et al., 1997). Some breeds, such as the Birman, are more susceptible to developing dry than wet FIP (Golovko et al., 2013). Attempts to identify specific genes associated with susceptibility for FIP in Burmese cats included several immune-related genes, but none reached the desired significance (Golovko et al., 2013).The largest study of genetic susceptibility to FIP showed that it is extremely polymorphic and reported consanguinity as a major risk factor. breeding (Pedersen et al., 2016).Specific polymorphisms in several genes have also been associated with high levels of FECV shedding among several breeding cat breeds (Bubeniko and et al., 2020).

In females, FIP, usually the wet form, may develop during pregnancy or in the perinatal period. This phenomenon resembles the suppression of immunity in pregnant women and the predisposition to certain infections (Mor and Cardenas 2010). It is not clear whether subclinical FIP is activated by pregnancy or by increased susceptibility to new infection. Maternal infection early in pregnancy results in fetal death and resorption, while later infections often result in abortion (Fig. 7). Kittens can be born healthy, but develop disease in the perinatal period and die. Some babies are born uninfected thanks to the effectiveness of the placental barrier between mother and fetus or thanks to the help of antiviral treatment (Fig. 8).

Figure 7. Aborted kittens from a dam that developed wet FIP late in pregnancy. Miscarriage was the first symptom of FIP, quickly followed by the classic symptoms of abdominal wet FIP. The mother was successfully cured of FIP with the antiviral GS-441524.
Figure 8. This mother developed symptoms of wet abdominal FIP 3 weeks after the onset of pregnancy and was successfully treated with GS-441524. Subsequently, she gave birth to a litter of four kittens by caesarean section, one of which died and three survived and grew up healthy. Treatment was given during the remaining 6 weeks of gestation and continued for 6 weeks during which the kittens were successfully nursed. GS-441524 had no apparent side effects on the mother or the kittens.

A possible increase in the number of cases of FIP was observed in cats older than 10 years in studies conducted 50 years ago (Pedersen 1976a). Slightly more than 3 % cases of FIP in a recent study occurred in cats 10 years of age and older and 1.5 % in cats 12 years of age and older (Fig. 6).1 The occurrence of FIP in the elderly often involves two different scenarios. The first scenario also involves exposure to FECV faecal excretion, but in a unique way. It is common for old cats to mate as kittens and live together in relative isolation unexposed to FECV for many years. One cat in the pair dies, is left alone, and a much younger companion obtained from a rescue organization, shelter, or kennel is brought into the household that has a high probability of excreting FECV. Older cats are also susceptible to the same FIP risk factors as younger cats, as well as other factors associated with aging. The first of these is the impact of aging on the immune system, with the most consequential being the deterioration of cellular immune function (Day 2010). Other risk factors associated with old cats include the debilitating and potentially immunosuppressive effects of diseases such as cancer and chronic diseases of the kidneys, liver, oral cavity and intestines. Some diseases in old cats can be mistaken for FIP or complicate the treatment of FIP if they are present at the same time.

Other risk factors that need further investigation include loss of maternal systemic immunity by separation at birth, early weaning and loss of lactogenic immunity, malnutrition, common kitten infectious diseases, early neutering, vaccination, congenital heart defects, and even a shelter fire (Drechsler et al.), 2011; Healey et al., 2022; Pedersen 2009, Pedersen et al. 2019).1 However, the most important positive risk factor remains the presence of FECV in the population (Addie et al., 1995). The prevalence of FIP in several Persian cat breeds was also related in one study to the proportion of cats that shed FECV at a given time and to the proportion of these cats that are chronic shedders (Foley et al., 1997). The importance of exposure to FECV supports the need to find ways to either prevent infection or reduce its severity. One of the first steps is a better understanding of FECV immunity (Pearson et al., 2019).


The first interface between FECV and the immune system is the lymphatic tissues of the intestine (Malbon et al., 2019, 2020). Although the downstream events leading to FIP are not fully understood, it is possible to speculate based on what is already known about FECV and FIPV infections, other macrophage-tropic infections, and viral immunity in general. During intestinal infection, FECV particles and proteins reach the local lymphatic tissues and are processed by phagocytic cells first into peptides and finally into amino acids. Some of these peptides will be recognized as foreign when arrayed on the cell surface, triggering innate (innate or non-specific) and adaptive (acquired or specific) immune responses (Pearson et al., 2016). FECVs also mutate to FIPV at the same time and in the same cell type. Some of these mutations will allow the virus to replicate in these or closely related cells of a specific monocyte/macrophage lineage.

The host cell for FIPV appears to be a specific class of activated monocytes found around venules on the surface of intestinal and thoracic organs, mesentery, omentum, uveal tract, meninges, choroid and ependyma of the brain and spinal cord, and freely in effusions. These cells belong to the activated (M1) class (Watanabe et al., 2018) and resemble a subpopulation of small peritoneal macrophages described in mice (Cassado et al., 2015). This type of cell arises from circulating bone marrow-derived monocytes that are rapidly mobilized from the blood in response to infectious or inflammatory stimuli. A similar-looking population of activated monocytes has been described around blood vessels in the retina affected by FIP (Ziolkowska et al., 2017). These cells stained for calprotectin, indicating their blood origin. Although FIPV infection occurs initially in smaller activated monocytes, viral replication is most intense in large, vacuolated, terminally differentiated macrophages (Watanabe et al., 2018). The virus released from these cells rapidly infects activated monocytes produced in the bone marrow and drawn to the site from the bloodstream.

The cellular receptor used by FECVs to infect intestinal epithelial cells has not yet been determined. The cellular receptor that FIPVs use to infect activated monocytes is also unknown. RNAs for conventional coronavirus receptors such as aminopeptidase N (APN), angiotensin converting enzyme 2 (ACE2) and CD209L (L-SIGN) were not upregulated in infected peritoneal cells of cats with experimental FIP, and CD209 (DC-SIGN) was significantly underexpressed (Watanabe et al., 2018). An alternative route of infection of activated monocytes may involve immune complexation of the virus and entry into cells by phagocytosis (Dewerchin et al., 2008, 2014; Van Hamme et al., 2008). Activated monocytes in lesions stain strongly positive for FIPV antigen, IgG and complement (Pedersen, 2009) and mRNA for FcγRIIIA (CD16A/ADCC receptor) is markedly increased in infected cells (Watanabe et al., 2018), supporting infection through immune complexation and alternative receptors related to phagocytosis.

Macrophage pathogens are intracellular and elimination of infected cells occurs through lymphocyte-mediated killing. The first line of defense is non-specific lymphocytes, and if they fail, an adaptive immune response to FIPV follows through specific T-lymphocytes. If infected activated monocytes and macrophages fail to be contained and eliminated, they may disseminate locally in the abdominal cavity, possibly from lymph nodes in the lower intestinal region and the site of FECV replication. Spread locally and to distant sites via the bloodstream is by infected monocyte cells (Kipar et al., 2005).

FIP occurs in two basic forms, wet (effusive, nonparenchymatous) (Figures 2 and 3) or dry (noneffusive, parenchymatous) (Figures 4 and 5), with wet FIP accounting for 80 % cases.1 The term "wet" refers to a characteristic fluid discharge in the abdomen or chest (Wolfe and Griesemer 1966, 1971). Wet FIP lesions are dominated by inflammation reminiscent of immediate or Arthus-type hypersensitivity (Pedersen and Boyle, 1980), whereas dry FIP lesions resemble delayed-type hypersensitivity reactions (Montali and Strandberg 1972; Pedersen 2009). The wet and dry forms of FIP therefore reflect competing influences of antibody and cell-mediated immunity and associated cytokine pathways (Malbon et al., 2020, Pedersen 2009). Immunity to FIPV-infected cells, which is the norm, is thought to involve strong cell-mediated responses (Kamal et al. 2019). Dry FIP is thought to occur when cell-mediated immunity is partially effective in suppressing infection, and wet FIP when cellular immunity is ineffective and humoral immune responses predominate.

FIP is considered unique among macrophage infections because it is viral, but the dry form shares many clinical and pathogenic features with feline diseases caused by systemic mycobacterial (Gunn-Moore et al., 2012) and fungal infections (Lloret et al., 2013). . Similarities in pathogenesis also exist between wet FIP and antibody-enhanced viral infections such as dengue fever and dengue hemorrhagic shock syndrome (Pedersen and Boyle 1980; Rothman et al., 1999; Weiss and Scott 1981).

Host responses are thought to solely determine the outcome of FIPV infection and the resulting forms of disease. However, macrophage-tropic pathogens have evolved their own unique defense mechanisms against the host (Leseigneur et al., 2020). One of the mechanisms is the delay of programmed cell death (apoptosis). Delayed apoptosis allows sustained microbial replication and eventual release of more infectious agents, as has also been described in FIPV-infected macrophages (Watanabe et al., 2018). FIPV can also control the recognition and killing of infected activated monocytes by specific or non-specific T-cells. The cell surface targets for T-cells that kill infected cells are likely FIPV proteins (antigens) expressed on major histocompatibility complex class I (MHC-I) receptors. However, surface expression of viral antigens by MHC-I receptors was not detected on FIPV-positive cells collected from FIP tissues or effusions (Cornelissen et al., 2007). DC-Sign has been proposed as a receptor for FIPV (Regan and Whitaker, 2008), but RNA for DC-Sign is markedly underexpressed by infected peritoneal cells, whereas RNA for Fc (MHC-II) receptors is markedly overexpressed and RNA for MHC -I is reduced (Watanabe et al., 2018). This suggests that the normal mode of infection of host cells may be altered by FIPV to favor infection by phagocytosis instead of binding to specific viral receptors on the cell surface, fusion with the cell membrane, and internalization.


Detailed descriptions of the gross and microscopic lesions in the wet form of FIP were first described by Wolfe and Griesemer (1966, 1971). The disease is characterized by vasculitis involving venules in the tissues lining the abdominal or thoracic cavity, organ surfaces, and supporting tissues such as the mesentery, omentum, and mediastinum. The inflammatory process leads to effusions in the abdominal or chest cavity up to a volume of one liter or more (Fig. 2, 3). The underlying lesion is a pyogranuloma, which consists of a focal accumulation of activated monocytic cells in various stages of differentiation, interspersed with non-degenerate neutrophils and sparse numbers of lymphocytes. Pyogranulomas are superficially oriented and appear grossly and microscopically as single and coalescent plaques (Fig. 2).

FIPV antigen is immunohistochemically (IHC) observed only in activated monocytes in lesions and effusions (Litster et al., 2013). Large vacuolated terminally differentiated macrophages are particularly rich in virus (Watanabe et al., 2018), reminiscent of the lepromatous form of leprosy (deSousa et al., 2017). Lymph nodes located near the sites of inflammation are hyperplastic and enlarged.

The relationship between dry and wet FIP was first described in 1972 in a report of cases of unknown etiology with similar pathology (Montali and Strandberg 1972). As the authors state, "this pathological syndrome was characterized by granulomatous inflammation in various organs, but mainly affected the kidneys, visceral lymph nodes, lungs, liver, eyes and leptomeninges". Tissue extracts of these lesions induced wet FIP in laboratory cats, confirming that wet and dry FIP are caused by the same agent.

The gross and microscopic pathology of dry FIP resembles that of other macrophage-tropic infections such as feline systemic blastomycosis, histoplasmosis, coccidioidomycosis (Lloret et al., 2013), tuberculosis and leprosy (Gunn-Moore et al., 2012). Lesions of dry FIP mainly involve the abdominal organs (Figs. 5, 6) and are rare in the thoracic cavity (Montali and Strandberg 1972; Pedersen 2009). Lesions are less widespread and focal than in wet FIP, with a tendency to extend from the serous surfaces into the parenchyma of the underlying organs (Figs. 5, 6). The target of the host immune response are small aggregates of infected monocytic cells associated with venules, similar to pyogranulomas in wet FIP, but surrounded by dense accumulations of lymphocytes and plasma cells and variable fibrosis. The florid hyperemia, edema, and microhemorrhage associated with wet FIP are mostly absent, therefore significant effusions in the body cavities are absent. The host response to foci of infection gives the lesions a gross tumor-like appearance (Figs. 5, 6). Infected activated monocytes in the central focus of infection are less dense and contain lower levels of virus than in the wet form (Pedersen 2009;), a feature of the tuberculoid form of leprosy (de Sousa et al., 2017). Lesions in some places, for example on the wall of the large intestine, can cause a dense surrounding zone of fibrosis, which resembles classic tuberculosis granulomas. Transitional forms also exist between wet and dry forms in a small number of cases and are mostly recognizable at autopsy (Fig. 3).

Ocular and neurological FIP are classified as forms of dry FIP (Montali and Strandberg 1972). However, pathology in the uveal tract and retina and in the ependyma and meninges of the brain and spinal cord is intermediate between wet and dry FIP (Fankhauser and Fatzer 1977; Peiffer and Wilcock 1991). This can be explained by the effect of the blood-ocular and blood-brain barrier in protecting these areas from systemic immune reactions.

Clinical characteristics of FIP

The five most common symptoms in cats with FIP, regardless of clinical form and frequency of occurrence, are lethargy, loss of appetite, enlarged abdominal lymph nodes, weight loss, fever, and deteriorating coat.1 These symptoms can appear quickly, within a week, or they can exist for many weeks or even months before a diagnosis is made. The course of the disease tends to be more rapid in cats with wet FIP than with dry FIP, and growth retardation is common in young cats, especially those with more chronic disease. 20 % cats with fever as the main symptom are eventually diagnosed with FIP (Spencer et al., 2017).

The wet form of FIP occurs in approximately 80 % cases, more often in younger cats, and tends to be more severe and more rapidly progressive than the dry form. Abdominal effusion (ascites) is four times more common than pleural effusion, with abdominal distension (Fig. 9) and dyspnea being common symptoms. Pyrexia and jaundice are more common symptoms in cats with wet than dry FIP (Tasker, 2018).

Figure 9.  Adult longhair cat with chronic abdominal wet FIP. The cat was in acceptable health except for mild weight loss, lethargy, poor coat quality, and occasional low-grade fever. Abdominal distension was not noted for some time, and the abdominal fluid contained relatively low protein and white blood cell counts.
Figure 9. A young cat who presented with rapid onset of high fever, loss of appetite, abdominal distension and abdominal fluid with a high protein and white blood cell content.

Most cats with dry FIP present with disease symptoms limited to the abdomen and/or chest. The most common clinical signs of dry FIP are palpable or ultrasound-identifiable masses in the kidney (Fig. 4), cecum, colon, liver, and associated lymph nodes (Fig. 5). Lesions of dry FIP usually spare the thoracic cavity and rarely occur in the skin, nasal passages, pericardium, and testes as part of a wider systemic disease.

Neurological and ocular disease are the sole or secondary features of 10 % of all FIP cases and are 10 times more often associated with dry than wet FIP (Pedersen 2009). The neurological and ocular forms of FIP have been classified as forms of dry FIP, but it may be more appropriate to classify them as distinct forms of FIP resulting from the modifying effects of the blood-ocular and blood-brain barriers behind which they occur. These barriers have a strong impact on the nature of eye and central nervous system (CNS) disease and response to antiviral therapy.

Clinical signs of neurologic FIP involve both the brain and spinal cord and include posterior weakness and ataxia, generalized incoordination, seizures, mental dullness, anisocoria, and varying degrees of fecal and/or urinary incontinence (Foley et al., 1998; Dickinson et al., 2020) ( Fig. 10). Extreme intracranial pressure can lead to sudden herniation of the cerebellum and brainstem into the spinal canal and spinal shock syndrome. Prodromal symptoms include compulsive wall or floor licking, litter eating, involuntary muscle twitching, and reluctance or inability to jump to high places. Eye involvement may precede or accompany neurological disease. Neurological FIP is a common phenomenon with antiviral therapy, either occurring during treatment of non-CNS forms of FIP or as a manifestation of disease relapse after treatment cessation (Pedersen et al., 2018, 2019; Dickinson et al., 2020).

Figure 10. A young cat with dry FIP and neurological impairment. The cat is lethargic, emaciated and with poor fur. The fur in the perineal area is wet and stained from urinary incontinence.
Figure 11. This cat's right iris staining was the first sign of FIP-associated uveitis. There is a slight haze in the anterior chamber, and there are fibrin deposits rich in red blood cells on the inside of the cornea. The pupils are also unequal (anisocoria).
Figure 12. A young cat with ocular FIP presenting as anterior uveitis in the right eye with secondary glaucoma causing globe enlargement. The iris has changed color due to inflammation, the vessels at the base of the iris are congested, and there is turbidity of aqueous humor and inflammatory products on the back of the cornea. Intraocular pressure is usually low in uncomplicated uveitis but elevated in cats with glaucoma.
Figure 13. This young cat had anterior uveitis, but her FIP therapy with GS-441524 was delayed, allowing glaucoma to develop in both eyes. Treatment cleared the underlying uveitis and greatly improved external health, but secondary glaucoma and blindness persisted.

Eye involvement is usually obvious and is confirmed by ophthalmoscopic examination of the anterior and posterior chambers. Ocular FIP affects the iris, ciliary bodies, retina, and optic disc to varying degrees (Peiffer and Wilcock, 1991; Ziółkowska et al., 2017; Andrew, 2000). The earliest symptom is often a unilateral change in the color of the iris (Fig. 11). The anterior chamber may appear cloudy and may show high protein levels and water turbidity on refraction. Inflammatory products in the form of activated macrophages, red blood cells, fibrin markers and small blood clots are washed into the anterior chamber. This material often adheres to the back of the cornea as keratic precipitates (Fig. 12). The disease can also affect the retina in tapetal and non-tapetal areas and lead to retinal detachment. Intraocular pressure is usually low, except in cases complicated by involvement of the ciliary body and glaucoma (Fig. 12, 13).

FIP Diagnosis

Signaling, environmental history, clinical signs, and physical examination findings often point to FIP (Tasker, 2018). A thorough physical examination should include body weight and temperature, coat and body condition, manual palpation of the abdomen and abdominal organs, gross assessment of cardiac and pulmonary function, and a cursory examination of the eyes and neurological system. Strong suspicion of an effusion in the abdominal or thoracic cavity may warrant confirmatory aspiration and even in-house fluid analysis as part of the initial examination.

Abnormalities in the complete blood count (CBC) and basic serum biochemical panel are important factors in the diagnosis of FIP (Tasker, 2018; Felten and Hartmann, 2019) and monitoring of antiviral therapy (Pedersen et al., 2018, 2019; Jones et al., 2021). ; Krentz et al., 2021) (Fig. 14). Total leukocyte counts are most likely high in cats with wet FIP, but low counts can occur with severe inflammation. A high leukocyte count is often associated with neutrophilia, lymphopenia, and eosinopenia. Mild to moderate non-regenerative anemia is also frequently seen in both wet and dry FIP. Total protein is usually elevated due to elevated globulin levels, while albumin values tend to be low (Fig. 14). This results in an A:G ratio that is often lower than 0.5-0.6 and is considered one of the most consistent indicators of FIP. However, a low A:G ratio can occur in situations where both albumin and globulin are within the reference range or in other diseases. Therefore, the A:G ratio should not be the only FIP indicator and should always be evaluated in the context of other FIP indicators (Tasker, 2018; Felten and Hartmann, 2019). Serum protein values obtained from most serum chemistry panels are usually adequate. Serum protein electrophoresis can provide additional information, especially if protein values from serum chemistry are questionable (Stranieri et al., 2017).

Figure 14. Complete blood count (CBC) (a) of a young cat with acute wet abdominal FIP. Although the leukocyte count was not elevated, relative but not absolute neutrophilia, relative and absolute lymphopenia, relative and absolute eosinopenia, and unresponsive anemia were noted, indicated by low red blood cells, hematocrit, and hemoglobin with a normal reticulocyte count.
Figure 14. Serum biochemical examination (b) of a young cat with acute wet abdominal FIP. Relevant values in the serum chemistry panel were elevated total protein, low albumin, high globulin, low albumin/globulin (A:G) ratio, and elevated total and direct bilirubin. Liver enzymes were normal except for mildly elevated AST and BUN and creatinine were normal, indicating the absence of significant liver or kidney disease. Globulin values are not always given, but a reasonable estimate can be calculated by subtracting the albumin level from the total protein.

Overreliance on CBC and serum biochemistry abnormalities can lead to diagnostic uncertainty when absent, despite the fact that no test value is consistently abnormal in all cases of FIP (Tasker, 2018)1. The biggest differences are between the clinical form of the disease, with leukocytosis and lymphopenia being more common in cats with wet than with dry FIP (Riemer et al., 2016). Hyperbilirubinemia is common in cats with FIP, but especially in cats with wet FIP (Tasker, 2018). The author also found that many cats with primary neurological FIP show minor or no blood abnormalities. Blood test values for FIP also vary from study to study (Tasker, 2018).

A complete analysis of the effusion is important to diagnose wet FIP and to rule out other potential causes of fluid accumulation (Dempsey and Ewing, 2011). It includes color (clear or yellow), viscosity (thin or viscous), presence of precipitates, ability to form a partial clot on standing, protein content, leukocyte count, and differential. The nature of the fluid may vary depending on the duration of the disease and its severity. Effusions in cats with more severe disease usually have protein values close to serum values, are more viscous, contain more leukocytes, are more yellow in color, and have a greater ability to form partial clots on standing. Chronic effusions tend to be less inflammatory in nature, with lower protein and leukocyte counts, less viscous and clearer. These values can be determined on the spot in most clinics. The clotting factor is determined by comparing the fluid collected in the serum and in the anticoagulant tubes after standing. Color and viscosity can be approximated and protein levels can be estimated using a handheld refractometer to determine total solids. Cells are pelleted from the fluid and analyzed on a fast-stained slide using light microscopy, and the leukocyte count and differential are estimated. Cells include nonseptic neutrophils, small and medium-sized mononuclear cells, and large vacuolated macrophages (Fig. 15). It is important to note that effusions can occur in a variety of conditions, such as heart failure, cancer, hypoproteinemia, and bacterial infections. Effusions in these other diseases usually have different identifying features.

Figure 15. Stained smear of peritoneal cells centrifuged from the abdominal fluid of a cat with wet FIP and examined on a fast-stained slide by light microscopy. The predominant cells are large strongly vacuolated macrophages, smaller differentiated activated monocytes and neutrophils. The greatest concentration of viral particles is in the intracytoplasmic vacuoles of macrophages (arrows).
Figure 16. Positive result of the Rivalt test. A small sample of abdominal or thoracic fluid is carefully dropped into a small beaker filled with diluted acetic acid (8 ml of distilled water and 1 drop of concentrated acetic acid). Inflammatory proteins almost immediately precipitate and sink to the bottom (positive). Less inflammatory fluids will form diffuse precipitates (questionable) or diffuse freely in solution (negative).

A positive Rivalt test on abdominal or chest fluid is often used to diagnose FIP as a cause of effusion, and a negative test tends to rule it out (Fischer et al., 2010) (Fig. 16). However, the test may be positive in inflammatory effusions of another cause and negative in some cats with FIP. Therefore, Rivalt's test is most helpful in combination with other clinical findings of FIP and should not replace a thorough fluid analysis (Felten and Hartmann, 2019).

Serum total and direct bilirubin levels are often elevated, especially in cats with wet FIP (Fig. 14), and may be associated with jaundice and bilirubinuria. Hyperbilirubinemia in FIP is not caused by liver disease (Tasker, 2018), but rather by vasculitis, microhemorrhage, hemolysis, and destruction of damaged red blood cells by macrophages locally and in the liver. The released hemoglobin is finally metabolized to bilirubin, which is then conjugated in the hepatocytes and excreted in the urine. Glucuronidation is essential for bilirubin excretion, and genetic disorders affecting glucuronidation in humans prevent its excretion (Kalakonda et al., 2021). Cats as a species are deficient in the enzymes required for glucuronidation, making it difficult to excrete substances such as bilirubin (Court and Greenblatt 2000).

Although FIP can affect the kidneys and liver, it is not severe enough to cause significant loss of kidney or liver function. However, serum tests for blood urea nitrogen (BUN) and creatinine as indicators of kidney disease and alanine aminotransferase (ALT), alkaline phosphatase (ALP), and gamma glutamyltransferase (GGT) as indicators of liver disease are often mildly elevated in cats with FIP, especially with a more acute and serious disease (Fig. 14). Therefore, slightly abnormal test values should not be interpreted excessively if other clinical signs of liver or kidney disease are not present, while their significant increase should point to the possibility of concurrent and possibly predisposing diseases of these organs.

Serum can also be tested for other markers of systemic inflammation, such as increased levels of alpha-1-acid glycoprotein (AGP) (Paltrinieri et al., 2007) and feline serum amyloid A (fSAA) (Yuki et al., 2020). They may also prove useful in monitoring response to antiviral therapy (Krentz et al., 2021).

Radiography can be helpful in identifying chest and abdominal effusions. Abdominal ultrasound can reveal a smaller amount of effusion, identify enlarged mesenteric and ileo-cecal-colic lymph nodes, thickening of the colonic wall and lesions in organs such as the kidneys, liver and spleen (Lewis and O'Brien 2010). It may also be useful in examining the chest for lesions and assisting with needle aspiration or biopsy.

Antibody titers against FCoV have decreased since the first report nearly 50 years ago (Pedersen 1976b). The reference antibody test uses indirect fluorescent antibody staining (IFA) IFA titers ≥ 1:3200 in FIP cats are higher than most FECV-exposed cats (1:25–1:400). Newer tests often use ELISA procedures for rapid in-house or laboratory testing, but are qualitative rather than quantitative. IFA antibody titers decrease during successful antiviral treatment in many cats, but remain high in others (Dickinson et al., 2020; Krentz et al., 2021). Sequential titers can show a gradual increase in titers during the development of FIP (Pedersen et al., 1977), but previous serum samples are rarely available for comparison. Like most tests, FCoV antibody levels should not be used as the sole criterion to diagnose or rule out FIP (Felten and Hartmann, 2019) or to assess treatment success (Krentz et al., 2021).

Reverse transcriptase polymerase chain reaction (RT-PCR) is the primary means of identifying FCoV RNA in inflammatory effusions, fluids, or affected tissues (Felten and Hartmann, 2019). Accessory gene 7b RNA is present at the highest levels in tissues, fluids or exudates infected with FECV or FIPV, making it the most sensitive target for detecting low levels of virus (Gut et al., 1999). RT-PCR for FIPV S gene mutations is often used in samples that are positive for 7b RNA to be specific for FIPV (Felten et al., 2017). Other studies suggest that RT-PCR assays for FIPV-specific S gene mutations have similar specificity for FIP, but at the cost of a significant loss of sensitivity (Barker et al., 2017). A decrease in sensitivity is associated with an increase in the number of false negative results. False-negative RT-PCR tests also occur in samples that do not contain sufficient numbers of infected macrophages or in cats with very low levels of virus. False-negative results are especially common when testing whole blood.

Immunohistochemistry (IHC) detects feline coronavirus nucleocapsid protein in formalin-fixed tissues with high sensitivity and specificity, but is not as popular as RT-PCR (Litster et al., 2013; Ziółkowska et al., 2019). Specimens for IHC must contain intact infected macrophages (Fig. 17), which requires careful separation of cells from effusions and mounting them on slides, or formalin-fixed, paraffin-embedded diseased tissues that show lesions compatible with FIP. The coronavirus antigen in macrophages within a typical FIP lesion or fluid is seen only in FIP, giving IHC a high level of specificity.

Figure 17. Histological section from the thickened colon of a cat with the intestinal form of FIP. The thickened wall contained foci of macrophages (square area), which stained positive (brown-red) for FIPV nucleocapsid protein by immunoperoxidase.

A thorough ophthalmological examination is necessary to diagnose the characteristic changes of FIP (Pfeiffer and Wilcock 1991; Andrew, 2000). A sample of aqueous humor from the anterior chamber of an inflamed eye may also be useful for cytology, PCR and IHC.

Neurological FIP is often diagnosed using contrast-enhanced magnetic resonance imaging (MRI) and is often associated with cerebrospinal fluid (CSF) analysis (Crawford et al., 2017; Tasker, 2018; Dickinson et al., 2020). However, these are expensive procedures that are not always available and carry a certain risk for the cat. MRI lesions include obstructive hydrocephalus, syringomyelia, and herniation of the foramen magnum with contrast enhancement of the meninges of the brain and spinal cord and ependyma of the third ventricle, mesencephalic aqueduct, and brainstem. CSF shows an increased number of proteins and cells (neutrophils, lymphocytes, monocytes/macrophages) and, if present, can be reliable material for PCR or IHC examination.

Neurologic and/or ocular forms of FIP are often confused with systemic feline toxoplasmosis, and many cats with FIP are empirically treated for toxoplasmosis before a diagnosis of FIP is made. Fortunately, the availability of effective treatment for FIP has curtailed this practice. Systemic toxoplasmosis is much less prevalent than FIP, and fewer than 1 % cats with FIP were serologically positive in one field study.1 Therefore, testing or treatment for toxoplasmosis should only be considered once FIP has been adequately diagnosed.

Antiviral treatment as a diagnostic tool

Figure 18. A cat with FIP at the start of treatment with GS-441524 (a) and after 1 week (b). The answer is quick, the fever will disappear within 24-48 hours and the general state of health will improve significantly within 1-2 weeks. This type of response is often used to confirm a diagnosis of FIP.

Situations commonly occur where clinical findings point to FIP but doubts remain. Then there is a choice of performing several diagnostic tests, which may not lead to a more definitive diagnosis. An alternative diagnostic approach is treatment with a suitable antiviral for 1-2 weeks in the correct dose for the suspected form of FIP.2 Treatment often produces clinical improvement in as little as 24-48 hours and this rapidly progresses over the next 2 weeks and the total duration of treatment (Fig. 18). No response to test treatment and/or deterioration in health would indicate the need for further investigation of the cause(s) of ill health.

FIP Treatment

Before 2017, there was no cure for FIP, and treatment was mainly aimed at alleviating the symptoms of the disease (Izes et al., 2020). Such supportive treatment was aimed at maintaining good nutrition, controlling inflammation (corticosteroids), changing immune responses (interferons, cyclophosphamide, chlorambucil) and inhibiting key cytokine responses (pentoxifylline and other TNF-alpha inhibitors). Nutritional supplements that were supposed to help specific organ functions were also commonly used, such as one (Polyprenyl Immunostimulant) that was supposed to improve immunity and prolong survival in cats with dry but not wet FIP (Legendre et al., 2017). The effect of good supportive care on survival could not be determined because most cats were euthanized after diagnosis or within days or weeks. The survival rate for even the mildest forms of dry FIP and the most permanent treatment in one study was only 13 % at 200 days and 6 % at 300 days (Legendre et al., 2017).

Many commercially available drugs and compounds inhibit FIPV infection or replication in vitro, some of which are drugs known to inhibit specific HIV or hepatitis C proteins, while others work by inhibiting normal cellular processes that the virus usurps for its own life cycle (Hsieh et al., 2010; Izes et al., 2020; Delaplace et al., 2021). These various drugs and agents include cyclosporine and related immunophilins, several nucleoside and protease inhibitors, vioporin inhibitors, pyridine N-oxide derivatives, chloroquine and related compounds, ivermectin, several plant lectins, ubiquitin inhibitors, itraconazole, and several antibiotics. However, the concentrations required to inhibit viral replication in vitro often approach toxic values for cells. It has also been difficult to transfer favorable in vitro conclusions to animals, and studies in sick cats have rarely followed. Ribavarin inhibits FIPV replication in vitro, but was not effective as a treatment for experimental FIP (Weiss et al., 1993). The efficacy of chloroquine was tested in laboratory cats infected with FIPV, but clinical outcomes in treated cats were only slightly better than untreated ones and hepatotoxicity was demonstrated (Takano et al., 2013). A 3-month-old kitten with chest wet FIP treated with itraconazole and prednisolone developed neurological FIP and was euthanized after 38 days of treatment (Kameshima et al., 2020). Mefloquine also inhibited FIPV replication at low concentrations in cultured feline cells without cytotoxic effects, and preliminary pharmacokinetic studies in cats appeared favorable (Yu et al., 2020), but evidence of its safety and efficacy in clinical trials in cats with FIP has yet to be established. published.

A breakthrough in the treatment of FIP occurred in 2016-2019 when antiviral drugs were reported that target specific FIPV proteins essential for replication. The first of these drugs was GC376, a major protease inhibitor (Mpro ) FIPV (Kim et al., 2016; Pedersen et al., 2018). Protease inhibitors prevent the formation of individual viral proteins by inhibiting their cleavage from polyprotein precursors. GC376 was able to cure all experimentally infected cats and 7 of 21 cats with naturally occurring wet and dry FIP, but was less effective for cats with ocular or neurological signs (Pedersen et al., 2018). The second of these drugs was GS-441514, the active part of the prodrug remdesivir (Gilead Sciences; Murphy et al., 2018; Pedersen et al., 2019). GS-441524 is an adenosine nucleoside analog that blocks FIPV replication by inserting a nonsense adenosine into the developing viral RNA. GS-441524 was also able to cure all experimentally infected cats (Murphy et al., 2018) and 25/31 cats with naturally occurring wet and dry FIP (Pedersen et al., 2019). It has also been shown to be effective at higher doses in several cats with ocular and neurological FIP (Pedersen et al., 2019) and is now the drug of first choice for cats with neurological FIP (Dickinson et al., 2020). GS-441524 has cured thousands of FIP cats from around the world over the past three years, with an overall cure rate of just over 90 % (Jones et al., 2021).1

Although the ability of GC376 and GS-441524 to treat cats has been known for several years, neither is currently legally available in most countries. The rights to GC376 have been purchased by Anivive, but it has not yet been launched.3 Potential conflicts with the development of remdesivir for the treatment of COVID-19 in humans led Gilead Sciences to withhold rights to GS-441524 for animal use, prompting the creation of an unapproved source for GS-441524 from China (Jones et al, 2021).1,2,4 Remdesivir is rapidly metabolized in the body to GS-441524 and has been approved for the treatment of FIP in some countries.2 GS-441524 can also be administered orally in higher doses and is currently commonly used in practice (Krentz et al., 2021).1

The efficacy of drugs such as GC376 and GS-441524 on FIP cats, the use of which preceded the COVID-19 pandemic, has been recognized by researchers investigating related SARS-CoV 2 inhibitors (Yan et al., 2020; Vuong et al., 2021). Remdesivir, an injectable drug called glaucoma (Gilead), has been used worldwide to reduce mortality from COVID-19 (Beigel et al., 2020). GC373, the active form of the prodrug GC376, has undergone simple modifications to increase efficacy and oral bioavailability (Vuong et al., 2021). The GC373-related drug, nirmatrelvir, has been successfully tested against early COVID-19 infections and has been approved for the treatment of early COVID-19 and marketed as paxlovid (Pfizer). Paxlovid consists of two medicines, nirmatrevir and the HIV protease inhibitor ritonavir. Ritonavir is not a significant inhibitor of SARS-CoV 2, but is reported to prolong the half-life of Mprowhen used in combination (Vuong et al., 2020). Nirmatrelvir and paxlovid have not been tested in cats with FIP at present, but based on experience with the closely related drug GC376, oral treatment of some forms of FIP may be important in the future.

Two other nucleoside analogs, EIDD-1931 and EIDD-2801 (Painter et al., 2021), have been investigated for the treatment of multiple RNA virus infections in humans and animals. EIDD-1931 is the experimental designation for beta-D-N4-hydroxycytidine, a compound widely studied since the 1970's. Beta-D-N4-hydroxycytidine is metabolized to a ribonucleoside analog, which is incorporated into RNA instead of cytidine and leads to fatal mutations in the viral RNA strand. The compound is an inhibitor of a wide variety of human and animal RNA viruses, including all known coronaviruses. EIDD-1931 was modified to increase oral absorption and was termed EIDD-2801 (molnupiravir) (Painter et al., 2021). Molnupiravir is deesterified in the body to its active ingredient, beta-D-N4-hydroxycytidine. Therefore, EIDD-1931 and molnupiravir are analogous to GS-441524 and remdesivir. Molnupiravir is marketed for the home treatment of primary COVID-19 under the names Lagevrio (Merck, USA) or Molnulup (Lupine, India).

Both EIDD-1931 and EIDD-2801 have been shown to be effective in inhibiting FIPV in tissue culture (Cook et al., 2021), and EIDD-2801 is currently used to treat some cases of FIP in the field.5,7 The effective concentration of 50 % (EC50) for EIDD-1931 against FIPV is 0.09 µM, EIDD-2801 0.4 µM and GS-441524 0.66 µM (Cook et al., 2021). The percentage cytotoxicity at 100 μM for these compounds is 2.8, 3.8 and 0.0. Thus, EIDD-1931 and -2801 are slightly more inhibitory to viruses, but more cytotoxic than GS-441524. Resistance to GS-441524 has been reported in some cases of FIP (Pedersen et al., 2019) and to remdesivir in patients with COVID-19 (Painter et al., 2021), but these isolates remain sensitive to molnupiravir (Sheahan et al., 2020). This may prove useful in combating resistance to GS-441524 in cats and humans and in developing multidrug therapy to prevent the development of resistance.

What will full approval of medicines like molnupiravir and paxlovid mean for cats? Full human approval should allow veterinarians in most countries to legally procure medicinal products authorized for human consumption for direct use in animals, provided that the guidelines for use in non-food producing animals are followed.6 This requires a reformulation of a medicine made for humans and purchased at a price for humans. Hopefully, antivirals similar or identical to those approved for humans will be licensed exclusively for animals and sold at a much lower price, but this is likely to take years.

Commercial and policy issues that limit the current use of antivirals such as GS-441524 in animal diseases such as FIP are for current cat owners and feline support groups who have already bypassed the current drug approval system and its emphasis on overriding human needs, irrelevant (Jones et al., 2021; Krentz et al., 2021). Advocates of FIP treatment are currently found around the world and often associate under the expanded FIP Warrior brand. Members of these groups often act as intermediaries between owners, veterinarians and antiviral suppliers and often provide advice to those who are unable to obtain veterinary treatment assistance. Some of these groups, such as FIP Warriors Czech Republic / Slovakia7, have placed their experience with FIP treatment on the Internet, where they provide much-needed information about current antiviral treatment.

Current situation of FIP treatment

The current drug of choice for the treatment of FIP is the adenosine nucleoside analog GS-441524, which was first published in the scientific literature under experimental conditions (Murphy et al., 2018) and later against naturally occurring disease (Pedersen et al., 2019). Although initial experimental and field studies of GS-441524 were conducted in collaboration between researchers at Gilead Sciences and the University of California, Davis, the relationship between Remdesivir and GS-441524 and the onset of the COVID-19 pandemic in 2019 led Gilead Sciences to eventually did not grant rights to use GS-441524 to animals on the grounds that it could interfere with the development of Remdesivir for human use.4 Objections to this decision have been raised directly by the company and in several internet forums.4 Subsequent pressure from cat owners, cat rescue groups and cat lovers, along with opportunistic Chinese drug manufacturers, quickly created an alternative unapproved source of GS-441524, its market and treatment network.4  This network has largely bypassed veterinarians, most of whom have decided to wait for the drug to be legalized (Jones et al., 2021). The result of this relationship was an almost seamless transition of FIP treatment with GS-441524 from the laboratory to a rapidly expanding worldwide network of groups, under the umbrella of FIP Warriors (Jones et al., 2021).4,7 

The sale and use of GS-441524 in practice for the treatment of FIP began almost immediately with the first publication of the results of field trials (Pedersen et al., 2019) (Fig. 19).

Figure 19.  Graph of the monthly development of treatment of cats from the Czech Republic and Slovakia since August 2019. This graph comes from the FIP Warrior CZ / SK website.1 These figures reflect the experiences of other FIP Warrior groups around the world. Since 2019, when the first field study GS-441524 was published (Pedersen et al. 2019), thousands of cats have been successfully treated for FIP worldwide. The winter peaks of the disease reflect a late spring and summer increase in the number of kittens born and a high incidence of FIP, which usually begins at 3 to 6 months of age (Fig. 6). This chart is from the FIP Warrior CZ / SK website.1
Figure 20. The main participants in the GS-441524 treatment. This chart is from the FIP Warriors CZ / SK website.1

The fact that GS-441524 is not legally approved for use in animals has prevented many veterinarians from recognizing or participating in this treatment. Only 25 % cats in the CZ / SK treated group received veterinary support during treatment (Fig. 20), although more veterinarians may have been involved in the diagnosis of the disease. Interestingly, this number was higher than the 8.7 % treated cats in the United States that received veterinary care (Jones et al., 2021). However, participants in CZ / SK studies and similar groups around the world are not without medical experience, as many of them are engaged in temporary care / rescue and have had considerable direct and indirect veterinary experience with cat diseases and their treatment and castration programs.

From the first laboratory studies and research of Chinese manufacturers, it was known that GS-441524 can be absorbed orally, although with less efficiency (Kim et al. 2016).9 The first sellers of GS-441524 further investigated this fact and found that effective blood levels could be achieved by increasing the amount administered orally compared to injection.8 Supplements have often been added to GS-441524 oral capsules or tablets, claiming that they increase absorption or have additive therapeutic benefits (Krentz et al., 2011). Most major retailers of GS-441524 now offer oral versions, and oral therapy is becoming increasingly popular either as a single treatment or in combination with GS-441524 (Figure 21). The success of GS-441524 oral therapy did not differ significantly from GS-441524 injection therapy (Figure 22).

Figure 21. Comparison of the use of oral (tablets or capsules) and injectable (subcutaneous) forms of GS-441524 for the treatment of FIP in cats from the Czech Republic and Slovakia. This chart is from the FIP Warriors CZ / SK website.1
Figure 22. There is no significant difference in treatment success with oral administration of GS-441524 compared to subcutaneously administered GS, but the actual amount (mg) of drug administered orally in each dose is up to twice the amount contained in the same dose of GS injection. This chart is from the FIP Warriors CZ / SK.1 website

The recommended dosing schedule for GS-441524 based on published data from field studies (Pedersen et al., 2019) was 4 mg / kg, subcutaneous (SC), daily (q24h), ie 4 mg / kg, SC, q24h. This recommended starting dose for cats with wet or dry FIP without ocular or neurological symptoms tended to increase to 6 mg / kg SC q24h over time (Fig. 23). 8 mg / kg SC q24h is the current recommended dose for cats with ocular symptoms and 10 or 12 mg / kg SC q24h for cats with neurological symptoms.

Figure 23. Daily dose of GS-441524, which was used to treat FIP in cats from the Czech Republic and Slovakia. The usual starting dose was 6 mg / day, with some cats requiring higher doses based on response to treatment, form of the disease and recurrence after treatment appeared to be successful. GS-441524 oral formulations are usually labeled to match the dosage used for the injectable drug, but contain up to twice the labeled amount. This chart is from the FIP Warrior CZ / SK website.1

The optimal duration of treatment, as determined in the initial clinical study, is 84 days (Pedersen et al., 2019). In some cases of acute wet FIP in younger cats, healing has been achieved in 6-8 weeks, but some cats need more than 84 days. As shown in Figure 24.72 % cats were treated for 81-90 days, 19 % longer and only 9 % were treated shorter. Unfortunately, there is no simple and accurate test to determine the moment of cure, and the decision to stop treatment is based on a complete return to health and normal blood test values. Cats treated for much longer than 100 days were usually those requiring a GS dose higher than 12 mg / kg per day by injection or equivalent oral dose, cats that relapsed during the 12-week post-treatment observation period, cats with neurological disease or cats that have become resistant to GS-441524.   

Figure 24. Duration of treatment with GS-141524 in 352 cats successfully treated for all forms of FIP. This chart is from the FIP Warriors CZ / SK website.1
Figure 25. Initial treatment was successful in 88.1 % cats and 6.2 % cats died or were sacrificed either due to insufficient response to treatment, financial reasons or treatment side effects. Another 5.7 % cats relapsed after initial treatment and approximately the same number of cats either recovered or died after further treatment. This chart is from the FIP Warriors CZ / SK.1 website

The treatment success rate for all forms of FIP in cats from the Czech Republic and Slovakia is 88.1 % in the first treatment, but when cats that relapsed after the first treatment and recovered after the second treatment (3.1 %) were included, the overall success rate was more as 91 % (Fig. 25). This cure rate is identical to the cure rate of other groups of FIP fighters (Jones et al., 2021). Treatment success did not differ between cats with wet or dry FIP and without ocular or neurological impairment (Fig. 26). However, the cure rate in cats with ocular and neurological impairments was lower, at 80 % compared to 92 % in all other forms of FIP (Fig. 26).

Figure 26.  Cure rate of cats with wet or dry FIP without ocular or neurological symptoms and cats with ocular or neurological disease as the main feature of their disease. This chart is from the FIP Warriors CZ / SK website.1
Figure 27.  The condition of the cats one year after the successful completion of treatment with GS-441524. This chart is from the FIP Warriors CZ / SK website.1

Cats that have been successfully treated for FIP have been followed for 4 to 5 years, including cases reported in the first field studies. There have been no recurrences or recurrent cases of FIP in this group of first field trials. Data on annual survival are available from a much larger population of the CZ / SK study, which shows that 90.5 % cats are still healthy one year after the end of treatment (Fig. 27). Only 1.3 % of these cats died from causes other than FIP and 8.2 % cohort is currently in an unknown medical condition. The low proportion of cats that died of unknown causes within a year of treatment and their positive response to treatment suggest that FIP has been diagnosed correctly.

EIDD-2801 (molnupiravir) is currently being used in the field for the main treatment and for the treatment of GS-441524-resistant cats.5,7,9 EIDD-1931, the active form of EIDD-2081, needs to be further researched because it is no longer covered by patent protection and is thus easily approved for use in animals if it is found to be truly safe and effective.5 Nirmatrelvir, an oral form of GC373 and a closely related GC376, still needs to be studied for the treatment of FIP.


I am indebted to Ladislav Mihok and his collaborator from "FIP Warriors Czech Republic / Slovakia" for allowing me to share data from their website. This website contains the most important, comprehensive and organized collection of data on FIP antiviral treatment today. The website also contains useful information and advice on starting, conducting and monitoring current treatment. The collection of cats and their data is continuously and regularly updated and at the time of writing this article included more than 600 cats with FIP.


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Read "History of feline infectious peritonitis 1963-2022 - from the first mention to successful treatment"

FIP diagnostics overview

Original article: A review on the diagnosis of feline infectious peritonitis

Jehanzeb Yousufa | Riyaz Ahmed Bhatb | Shahid Hussain Darb | Alisa Shafib | Snober Irshadc | Mohammad Iqbal Yatoob | Jalal Udin Parrahb | Amatul Muheeb | Abdul Qayoom Mirb

Abstract: Feline infectious peritonitis, or simply FIP, is a coronavirus viral disease in cats, usually less than three years old. It is manifested by an extreme inflammatory reaction in the tissues in the abdomen, kidneys and brain. This review article discusses the various diagnostic tests and their benefits in diagnosing cases of suspected FIP for definitive diagnosis. This review can help compare different diagnostic parameters and also raise awareness of their advantages and disadvantages.

Keywords: acute phase proteins, coronavirus, feline infectious peritonitis, Rivalt's test.

1. Introduction

Feline infectious peritonitis (FIP) is a well-known and widespread coronavirus (CoV) -induced systemic disease in cats, characterized by fibrinous-granulomatous serositis with protein-rich exudates in the body cavities, granulomatosis-announcing phlebitis in the gallbladder and periphels Scott 1981; Kipar et al. 2005). Feline CoV (FCoV) spreads via the faecal-oral route and is primarily infected with enterocytes (Pedersen 1995), but subsequently spreads systemically via monocyte viremia (Meli et al 2004; Kipar et al 2005). The increased ability of the virus to replicate has been shown to be a key feature in the development of FIP, and FIP is also thought to be caused by mutations in common feline enteric coronavirus (FECV), which occurs in cats worldwide and is not a serious infection (Pedersen et al 2009; Healey et al 2022). Approximately 10 % infected cats have mutations that result in feline infectious peritonitis. In large multi-cat situations, FECV is excreted in the faeces of most seemingly healthy cats and the transmission occurs through direct contact with faeces or contaminated litter and other fomits (Pedersen et al 2004). Kittens become infected at approximately 9 weeks of age (Pedersen et al. 2008). The time between the onset of clinical signs and death also varies, but younger cats and cats with effusive disease have a shorter course of disease than older cats and cats with non-effusive disease (Pedersen 2014). Even with severe FIP, some cats can live for months. in multi-feline situations, feline enteric coronavirus (FECV) is extremely common and highly contagious. Almost all cats that come into contact with FECV from secreting cats become ill, but on the other hand, the infection is usually asymptomatic or causes only mild temporary diarrhea (Pedersen et al. 2008; Vogel et al. 2010; Ermakov et al. 2021). On the other hand, feline infectious peritonitis virus (FIPV) is not transmitted via the faecal-oral route, but originates from avirulent FECV in a small percentage of infected cats and causes feline infectious peritonitis (FIP) (Pedersen et al 1981; Vennema et al. 1998). Anorexia, lethargy, weight loss, pyrexia, ocular and neurological symptoms such as gait abnormalities or inappropriate mentoring are non-specific (Giori et al 2011; Kipar et al 2014). The infection takes two forms: "wet" and "dry". The dry form causes inflammatory changes around the vessels, seizures, ataxia and excessive thirst, while the wet form leads to enlargement of the abdomen due to excessive accumulation of fluid in the abdominal cavity. Specificity is always the most important diagnostic value to consider in order to avoid misdiagnosis of FIP in unaffected cats.

2. Diagnostic tests for feline infectious peritonitis

The diagnosis takes into account the age, origin, clinical signs and physical examination of the cat. Abdominal distension with ascites, dyspnoea with pleural effusion, jaundice, hyperbilirubinuria, marked masses of the kidneys and / or mesenteric lymph nodes, uveitis and various neurological symptoms are common in cats with effusive (wet) or non-fusive (dry) forms of FIP. and / or spinal cord. Ocular changes often occur in cats with FIP, with the most common ocular disorders being retinal changes. Retinal cuff cuffs may occur, which appear as blurred gray lines on either side of the vessels. Occasionally, granulomatous changes in the retina occur. FIPV infection has been found to be associated with T-cell depletion by apoptosis, although the virus cannot infect CD4 + and CD8 + T-cells (Haagmans et al. 1996; De Groot et al. 2005). Due to the high mortality rate, many veterinarians and pet owners are cautious about a diagnosis based on "reasonable assurance". The challenge is to decide whether the test increases the likelihood that clinical symptoms are caused by FIP (indirect tests) or offers a definitive diagnosis (direct tests). It is important to note that the sensitivity and specificity of any indirect test will vary depending on the likelihood that the cat is infected due to other factors. This means that a positive predictive value of the test, such as complete blood count (CBC) or albumin: globulin (A: G) ratio, to predict FIP will be much higher in cats with FIP-like signaling than in cats with non-FIP signaling. It should be noted that the results of other indirect tests are only estimates and the results of additional indirect tests have the potential to confuse and support the diagnostic process.

3. Diagnostic tests

The problem with FIP diagnostics is that non-invasive tests are not reliable enough. In general, effusion tests have a significantly higher predictive value than blood tests (Stranieri et al. 2018; Hartmann et al. 2003). As a result, the identification of FIP ante mortem in cats without significant effusion is particularly difficult. The most useful ante-mortem indicators are positive anti-Corona (IgG) antibody titers in cerebrospinal fluid (CSF), high total serum protein, and MRI changes such as periventricular contrast enhancement, ventricular dilatation, and hydrocephalus. However, monoclonal antibodies from affected tissues and coronavirus-specific polymerase chain reaction (PCR) are valuable in post mortem evaluations (Foley et al. 1998). As a clear diagnosis cannot be made on the basis of symptoms, medical history and clinical and laboratory indicators alone, these factors should always be considered as a whole, sometimes in combination with other factors such as molecular or even more invasive diagnostic procedures.

3.1. Analysis of effusion samples

If FIP with effusion is suspected, the effusion sample can be incredibly helpful in making a diagnosis and then in hematological findings, so obtaining effusion samples should always be a top priority. In the case of ascites, the sample can be obtained by ultrasound-guided thin-needle aspiration or the "flying cat" technique. To identify small amounts of fluid in the chest and abdomen, ultrasonography provides useful assistance in locating effusion bags in the abdomen, while evidence of pericardial effusions can be obtained through suppressed cardiac echoes and electrocardiographic changes. Ultrasonography should be used repeatedly to identify any small volume effluents, and ultrasonography may also be used to guide the sampling of small bags of fluid. In cats with pericardial effusions, auscultation of the heart detects muffled sounds and the ECG reveals typical changes.
FIP effusions are often clear, viscous / sticky, straw yellow and rich in protein (cytology often describes a dense eosinophilic protein background) with a total protein concentration> 35 g / l (> 50 % globulins). Chlootic effusions are rarely described. FIP effusions are often pyogranulomatous in nature with macrophages, non-degenerate neutrophils and a relatively small number of lymphocytes. As a result, effusions are often referred to as modified transudates based on cell number (<5 × 109 cells / L), but exudates based on protein concentration (greater than 35 g / L). Typical FIP effusions have a low A: G ratio (see above) and an increased AGP content, which is similar to the serum content. A recent study (Hazuchova et al. 2017) found that AGP concentrations in effusions (> 1.55 mg / ml) are more useful (sensitivity and specificity 93 %) in distinguishing FIP cases from cases without FIP than serum AGP levels or other APPs.
The rival test is a simple test that can be used to distinguish transudate from exudate in a effusion sample (Barker and Tasker 2020). Positive results simply suggest that the effluent is exudate and not specific to FIP; positive transudate results have been documented in situations other than FIP (eg, bacterial / septic peritonitis and lymphoma) (Fischer et al. 2012).

3.2. Serum biochemistry

Although the changes in blood biochemistry observed in FIP cases are variable and often non-specific, there are several key anomalies that need to be addressed in order to confirm the diagnosis of FIP.

3.2.1. Acute phase proteins

Many inflammatory and non-inflammatory diseases produce acute phase proteins (APPs) in the liver in response to cytokines released by macrophages and monocytes (particularly interleukins 1 and 6 and tumor necrosis factor α).
AGP is an abbreviation for α1-acid glycoprotein and its examination may help in the diagnosis of FIP. Although the increase in AGP levels (> 0.48 mg / ml) is not specific for FIP, patients with FIP often have significantly high AGP levels (> 1.5 mg / ml). As a result, the magnitude of the increase may be valuable in helping to diagnose FIP, with higher levels more effectively increasing the suspicion index (Giori et al 2011; Hazuchova et al 2017).

3.2.2. Hyperglobulinemia

In 89% cases, hyperglobulinemia is present; often in association with hypoalbuminemia or low-normal serum albumin levels (observed in 64.5 % cases) (Riemer et al. 2016). Hyperproteinemia may not always occur due to the existence of hypoalbuminemia. The albumin: globulin (A: G) ratio is low in hyperglobulinemia and hypoalbuminemia (low-normal albumin concentration) and this parameter can be used to assess the likelihood of FIP in a particular case.

3.2.3. Hyperbilirubinemia

Hyperbilirubinemia occurs in 21-63 % cases of FIP and is more common in effusive FIP, where alanine aminotransferase (ALT), alkaline phosphatase (ALP) and γ-glutamyltransferase enzymes are commonly high (although these may be slightly increased in FIP cases). FIP is rarely associated with hyperbilirubinemia due to immune-mediated hemolytic anemia (IMHA) (Norris et al. 2012) and cats are often not severely anemic. In the absence of high liver enzyme activity or severe anemia, the presence of hyperbilirubinaemia should suggest FIP (note that sepsis and pancreatitis may cause hyperbilirubinaemia without increased liver enzyme activity). Based on a stepwise evaluation of cats with FIP, it was documented that hyperbilirubinemia was more typically recognized in cats just before death or euthanasia than in the first presentation (Harvey et al. 1996). In addition, higher bilirubin levels were observed in cats just before death or euthanasia than in the first presentation.

3.3. Hematology

In FIP, hematological changes are non-specific; however, there are several abnormalities that need to be checked to confirm the diagnosis. Lymphopenia is the most common change (55 - 77%) in cases, with a recent study (Riemer et al. 2016) revealing lymphopenia in only 49.5 % cases of FIP, with neutrophilia (39 - 57 %), left-sided and mild to severe normocytic, normochromic anemia (37-54 %) (Riemer et al. 2016; Norris et al. 2012). Recently, an association between FIP and microcytosis (with or without anemia) has been discovered. FIP can cause severe IMHA with concomitant regenerative anemia; however, this is an unusual phenomenon.

3.4. Serology

ELISAs, indirect immunofluorescence antibody assays, and rapid immunocompromination assays are the most common assays for anti-FCoV antibodies in serum (Addie et al. 2015). In most studies, cells infected with CoV pigs or cats are used as substrate and titers are measured in multiples of serum dilutions. A positive anti-FCoV antibody test means that the cat has been infected with FCoV and has seroconverted (which lasts 2-3 weeks after infection). The tests are therefore of limited clinical importance. Breed-dependent differences in anti-FCoV antibody titers were found, which could indicate differences in the breed's response to FCoV infection (Meli et al. 2013).
Although cats with FIP had higher anti-FCoV antibody titers than cats without FIP, no difference was found between the median anti-FCoV antibody titers in healthy cats and cats with suspected FIP. As a result, the titer in one animal is only marginally useful in identifying cats with FIP (Bell et al. 2006). Many clinically healthy cats (especially those in multi-cat households) have positive and often very high anti-FCoV antibody titers, while 10 % cats with FIP are seronegative, which could be due to virus binding to the antibody and its inaccessibility for serological testing. which also points to problems with interpretation (Meli et al 2013). A negative FCoV antibody test if dry FIP is suspected may be more effective at excluding FIP (Addie et al. 2009). Nevertheless, negative results have been observed in neurological FIP situations (Negrin et al 2007). As a result, doctors disagree on whether to perform a serological test in suspected cases, even though a positive result almost always means FCoV exposure.

3.5. Current trends in diagnostics

The use of anti-coronavirus antibody testing in cerebrospinal fluid (CSF) for diagnosis in cases involving the central nervous system is another breakthrough in which IgG is detected in the CSF. However, in most cases, the antibody was detected only in cats with high serum IgG titers (Boettcher et al. 2007)
An important difference between feline coronavirus and FIP infection is the behavior of the NSP3c gene. The infected tissue isolates from the second case were found to have a disrupted gene 3c, while in the first case the gene was intact. Mutation of the S1 / S2 locus and modulation of the furin recognition site, which is normally present in the S-gene of the enteric coronavirus (Levy and Hutsell 2019), is also a crucial contributing factor.
The diagnostic utility of cerebrospinal fluid immunocytochemistry is also used to diagnose FIP manifested by severe central nervous system involvement. Immunocytochemical staining (ICC) of feline coronavirus antibodies in cerebrospinal fluid macrophages is a highly sensitive test, especially for the diagnosis of ante mortem with a sensitivity of 85 % and a specificity of 83.3 % (Gruendl et al 2017).

4. Conclusions

In cats with suspected FIP, anamnesis, clinical signs and clinicopathological examinations should be correlated. The dry form is more difficult to diagnose than the wet form. In the wet form, laboratory analysis of the fluid can be performed, such as the Rivalt's test. If the test is negative, the probability of FIP is small, but if the test is positive, additional diagnostic tests should follow to confirm FIP. In FIP, the A: G ratio is low because hyperglobulinemia and hypoalbuminemia (low normal albumin concentration) are present, and this parameter can be used to assess the likelihood of FIP in a particular case. Patients with FIP often have significantly high levels of AGP (α1-acid glycoprotein). When distinguishing FIP cases from non-FIP cases, AGP concentrations in effusions (> 1.55 mg / ml) have a sensitivity and specificity of 93 %.

Conflict of interests

The authors declare that they do not have a conflict of interest.


No financial support was provided from any institute or other source.

5. Literature

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Two Vaccine Platforms to Prevent Feline Coronavirus Disease

Original article: Two Vaccine Platforms to Prevent Feline Coronavirus Disease

Principal Investigator: Hector Aguilar-Carreno

Department of Microbiology and Immunology
Sponsor: Cornell Feline Health Center Research Grants Program
Title: Two Vaccine Platforms to Prevent Feline Coronavirus Disease
Project Amount: $69,920
Project Period: July 2021 to June 2022

DESCRIPTION (provided by applicant): 

While most cats infected with Feline Coronavirus (FCoV) develop mild to inapparent diarrheal disease, a subset of them develop the devastating and deadly feline infectious peritonitis (FIP). FCoV can spread via fecal-oral or respiratory routes, particularly in cat shelter environments. Coronaviruses have three envelope glycoproteins, S, E, and M, but the surface protein (S) is in charge of viral entry. S binds the cell surface receptor and then merges the viral membrane to the cell plasma or endosomal membranes, causing viral entry. S also causes cell-cell fusion (syncytia) post-infection. The S protein of most coronaviruses is also highly immunogenic. The devastating FIP disease begs the development of protective vaccines. We identified a small molecule, XM-01, that embeds within viral membranes and inhibits membrane fusion. Importantly, this novel method of inhibition renders virions noninfectious, while maintaining the native conformations of the surface glycoproteins, ideal for eliciting effective immune responses against such viral glycoproteins. Remarkably, vaccination with XM-01-treated influenza virions yielded an increase in neutralizing antibodies and survival rate, and a decrease in morbidity and mortality upon viral challenge in a mouse model, as compared to the traditional formalin-inactivated influenza vaccine. Thus, our Aim 1 will be to determine whether XM-01 can be used to develop a FeCoV inactivated vaccine. Importantly our recent preliminary data includes already determined conditions for complete FCoV inactivation. We will optimize XM-01 inactivation of FCoV in preparation to determine whether this vaccine can yield a robust immune response against this virus. Additionally, our lab has successfully used replication-incompetent vesicular stomatitis virus (VSV)-based pseudotyped virions to vaccinate and protect hamsters against Nipah, Hendra, and Ebola virus diseases with 100% safety and 100% efficacy (manuscript in final revision for Nature Publishing Journals Vaccines). Our Aim 2 will use the replication-incompetent VSV system to develop a vaccine against FCoV. We will optimize incorporation of FCoV-S into VSV virions in preparation to determine whether this vaccine can yield a robust immune response against this virus. As both inactivated and replication-incompetent virions vaccine platforms have been successfully used to prevent other viral diseases, the completion of our Aims will allow our vaccine platforms to readily advance to vaccination clinical trials/licensing. Read "Two vaccination platforms to prevent feline coronavirus disease"

Outbreak of feline infectious peritonitis in a shelter in Taiwan: epidemiological and molecular evidence of horizontal transmission of a new type II feline coronavirus

Ying-Ting Wang,1 Bi-Ling Su,2 Li-En Hsieh,1 and Ling-Ling Chueh1
Original article: An outbreak of feline infectious peritonitis in a Taiwanese shelter: epidemiologic and molecular evidence for horizontal transmission of a novel type II feline coronavirus
Czech translation partially taken from: Results Confirmation of the FIP outbreak in the cat shelter - Sevaron


Infectious feline peritonitis (FIP) is a fatal disease caused by feline coronavirus (FCoV) infection. FCoV can be divided into serotypes I and II. The virus that causes FIP (FIPV) is said to occur sporadically and does not often spread from one cat to another. An outbreak in one animal shelter in Taiwan was recently confirmed. FCoV from all cats in this shelter was analyzed to determine the epidemiology of this outbreak. Thirteen of the 46 (28,2%) cats with typical FIP symptoms were identified. Of these, FIP was confirmed in seven cats by necropsy or histopathological examination. Despite the fact that in this environment with more cats, more than one FCoV was identified, eight cats with symptoms of FIP were reliably found to be infected with FCoV type II. Sequence analysis revealed that FIPV type II, found from feline faeces, body effusions and granulomatous tissue homogenate from cats that underwent FIP, contained identical recombination in all cases. WITH gene. Two cats that succumbed to FIP were found to have an identical nonsense mutation in 3c gene. The excretion of this type II virus in faeces of the effusive form of FIP can be detected up to six days before the animal dies. In general, our data demonstrate that horizontal transmission of FIPV is possible and that FIP cats may pose a potential risk to other cats living in the same environment.


Infectious feline peritonitis (FIP) is a fatal disease of cats caused by feline coronavirus (FCoV) infection. FCoV is an enveloped RNA virus that belongs to the species Alphacoronavirus, family Coronaviridae and in order Nidovirales. The size of the FCoV genome is approximately 28.9 kb, including the nonstructural replication gene; four structural genes that encode spike (S), envelope, membrane, and nucleocapsid proteins; and five helper / nonstructural genes 3abca 7ab[1].

Feline coronaviruses cause mild, invisible, and transient bowel infections and are ubiquitous among cat populations worldwide [2]. They occur in two serotypes, I and II [man]3]. Type I FCoV predominates here, while type II virus represents only 2-30% infections [48]. Following the accumulation of genetic evidence, it is apparent that FCoV type II was formed by two homologous recombinations between FCoV type I and canine coronavirus CoV (CCoV) [9,10]. Both serotypes can mutate in the host, lead to macrophage tropism and a systemic disease called infectious feline peritonitis [cat]2,11,12]. Due to poor virus shedding in FIP studies in cats, mutant FIP viruses (FIP-inducing FCoV, FIPV) appear to be contained only in diseased tissues and are not naturally transmitted in cat-to-cat contact [2,11,13,14].

In this article, we report an epizootic FIP in a shelter in Taiwan that was caused by a new Type II FCoV. Epidemiological and molecular examination of isolates from various healthy and sick cats from this shelter strongly suggests that the virus was introduced by moving kittens from another shelter with subsequent horizontal spread to adult cats with which the new kittens shared the shelter.

Materials and methods

Animals and sampling

A total of 46 cats from a private shelter were included in this study, which ran from September 2011 to August 2012. This shelter houses adult cats and from time to time a few kittens. All the cats were either strayed or rescued, and some of them were obtained from the homes of various private rescue stations where the rescued cats were temporarily housed. Before the onset of the disease, all cats lived together in an indoor environment without cages, sharing food, drink and toilets. Some cats were siblings, others were not related to them (Table 1).

Table 1
Information on all cats from this shelter in which FIP was suspected and in which the disease was confirmed

Hot girlAge 1 Date of admission to the shelter Date of onset of fever Date of death Clinical findings Necropsy findings Effusive / non - fusive
13mJune 16, 2011August 17, 2011September 1, 2011Fever, anorexia, ascites, neurological symptoms  
2a4mAugust 6, 2011ON THE 2September 21, 2011Clinical signs are not available  
3b3mJuly 11, 2011August 18, 2011September 25, 2011Fever, anorexia, weight loss, neurological symptoms  
42.5mJun. 08, 2011August 16, 2011September 28, 2011Fever, ascites, neurological symptoms  
5a4mAugust 6, 2011August 15, 2011October 20, 2011Fever, pleural effusion, diarrhea  
67mApril 24, 2011ON THEOctober 22, 2011Anorexia, weight loss, neurological symptoms  
73y6mResidentON THEOctober 27, 2011 Ascites, jaundice, granulomatous lesions in the kidney, fibrinous peritonitisEffusive
86mJuly 11, 2011ON THEDecember 14, 2011 Granulomatous changes in the kidneys, liver, lungs, brain and eyes Non-fusible
92yResidentON THEDecember 28, 2011 Ascites, pleural effusion and pericardial effusion, granulomatous changes in the kidneys, liver and intestine.Effusive / non - fusive
10 b3mJuly 11, 2011ON THENovember 5, 2011 Granulomatous changes in the kidneys, liver and omentum Non-fusible
11 c1y6mResidentON THEFebruary 14, 2012 Ascites and pleural effusion, jaundice, fibrinous peritonitis, granulomatous changes in the kidneys, liver, lungs and spleen.Effusive / non - fusive
12 c1y6mResidentON THEMarch 19, 2012 Jaundice, fibrinous peritonitis, granulomatous changes in the thoracic and abdominal walls, kidneys, liver, lungs, spleen omenta and eyes.Effusive / non - fusive
131y7mResidentON THEApril 13, 2012 Jaundice, enlargement of the liver and mesenteric lymph nodes, granulomatous changes in the kidneys and lungs.Non-fusible

1 Age of cats at the time of clinical signs of FIP.
2 Not available. 
a, b, c : siblings.

Faeces or rectal samples were taken from all asymptomatic cats at least once to monitor for FCoV. Body swabs, blood samples, swab specimens, including rectal, nasal, oral and conjunctival specimens, were taken as standard from cats that already showed signs of the disease or were suspected of having FIP. In addition to supportive care, cats with suspected FIP were treated with prednisolone (Prelon®, YF Chemical Corp., New Taipei City, Taiwan), benazepril (Cibacen®, Novartis, Barbera del Valles, Spain) and recombinant human interferon alpha (Roferon®-A). , Roche, Basel, Switzerland). Cats that succumbed to the disease were necropsied for pathological confirmation. During necropsy, body exudates were first removed with a needle and syringe, followed by swabs, blood, urine and granulomatous lesions on the internal organs. All samples were frozen at -20 ° C until use. All samples were tested for FCoV nested reverse transcription polymerase chain reaction (RT-nPCR) [man]15]. Samples with positive results were subsequently subjected to further analysis.

Sample preparation and reverse transcription

Swab samples were suspended in 1 ml of water treated with 0.1% diethyl pyrocarbonate (DEPC). Stool samples were suspended with 9x treated water 0.1% DEPC by vortexing. The suspension was centrifuged and the supernatant was transferred to a new tube. About 0.5 g of tissue was frozen and then crushed with a mortar and pestle in the presence of 2 ml of Trizol [16]. Total RNA was extracted from 300 μl of swab suspension, whole blood, faeces suspension, tissue homogenate and body effusion using Trizol. Twenty-one microliters of isolated RNA was reverse transcribed with specific primer N1 (5′-gctacaattgtatcctcaac-3 ′) or P211 [15] with Moloney mouse leukemia reverse transcription (Invitrogen, CA, USA). The reaction was incubated at 37 ° C for 60 min, at 72 ° C for 15 min and finally at 94 ° C for 5 min.

FCoV type determination by nested PCR

Nested PCR was performed for FCoV typing according to the procedures of Addie et al. [5] with a slight modification. After reverse transcription, 5 μl of complementary DNA was added to 25 μl of PCR mix (Invitrogen, CA, USA) according to the manufacturer's instructions for the following primer sets: S1 and Iffs to determine FCoV type I and S1 and Icfs to determine FCoV type II. Nested PCR was performed on 2 μl of the first PCR product using nested primers. The expected size of the second PCR achieved for type I and type II FCoV was 360 and 218 bp. RT-nPCR products were electrophoresed and then the target DNA fragments were purified (Geneaid Biotech, Ltd, Taipei) and sequenced (Mission Biotech, Taipei, Taiwan) - from both orientations.

Gene amplification, sequencing and analysis 3a and 3c  from FCoV type II

For amplification 3a of the FCoV type II gene from FIP cats, a set of specific primers was designed that is able to amplify from WITH type II gene to gen 3a. Complementary DNA, amplified with a primer set, targeted the 3 'end WITH FCoV type II gene (Icfs) and 5 ′ end 3a FCoVe gene (3aR2: 5′-caccaaaacctatacacacaag-3 ′). The temperature cycle was as follows: 5 minutes preheating at 94 ° C; 35 cycles of denaturation at 94 ° C for 20 s, annealing at 50 ° C for 20 s and extension at 72 ° C for 30 s; and final extension at 72 ° C for 5 minutes. This was followed by a second series of amplification using primers nIcfs and 3aR2; the expected product size was about 600 bp. Amplicons were electrophoresed, purified, and sequenced from both orientations to confirm nucleotide sequences.

For amplification 3c of the FCoV type II gene from FIP cats, a set of specific primers was designed that is able to amplify from WITH type II gene to gen 3c. Complementary DNA was amplified with forward primer (Icfs) and reverse primer (E68R: 5′-aatatcaatataattatctgctgga-3 ′ and N21R: 5′-gttcatctccccagttgacg-3 ′, respectively). The temperature cycle was as follows: 5 minutes preheating at 94 ° C; 40 cycles of denaturation at 94 ° C for 30 s, annealing at 46 ° C for 30 s and extension at 72 ° C for 90 s; and final extension at 72 ° C for 7 minutes. Following a second series of amplification using primers nIcfs and E68R, the products were electrophoresed, purified and sequenced from both orientations to confirm nucleotide sequences.

Phylogenetic analysis and recombinant analysis of FCoV type II

Several sequence alignments were performed using ClustalW 2.0 with manual editing in EditSeq (DNASTAR, Madison, USA). Phylogenetic analyzes were performed using MegAlign, version 7.2.1 (DNASTAR, Madison, USA). Bootscan and similar graphs were compiled using SimPlot 3.5.1 software (SCRoftware, Baltimore, USA).

The results

Confirmation of the FIP outbreak in the cat shelter

The shelter has been operating for three and a half years. Prior to August 2011, there were no records of FIP. The kittens (cats 1, 3, 4, 8 and 10) were moved to this shelter between June and July 2011. After arrival, these kittens played together and lived together with adult cats that lived here before. Prior to the outbreak, the kittens were individually taken to a veterinarian for vaccination and adoption visits. Fever was first detected in four kittens (cats 1, 3, 4, 5) within a few days (from 15 to 18 August) (Table 1). Clinical symptoms, e.g. fever, anorexia, neurological symptoms, shortness of breath and enlargement of the abdomen were observed over the next two months and the kittens gradually died between 1 September and 22 October (Table 1). Shelters from the shelter asked for our help on September 27. All cats housed in the shelter for a long time were immediately examined for FCoV using the RT-nPCR method. All FCoV-positive cats were isolated and kept separately. Nevertheless, starting in September, adult cats with FIP (cats 7-13) showed clinical signs similar to kittens, and all of these cats later died.

Six kittens (cats 1-6) with body effusions or neurological symptoms that succumbed in the first two months were not confirmed for necropsy (Table 1). Cat 1 was once brought to our teaching hospital and ascites (free fluid in the abdominal cavity) was taken from her. In cats 7-13, typical symptoms were found, namely ascites or pleural effusions in the body cavity (effusive FIP) and granulomatous lesions in some organs, especially in the kidneys, nuclei, lungs, omentum (forecourt) and eyes (non-effusive FIP). In cats 9, 11 and 12, necropsy showed a mixed form of the disease (Table 2) 1).

A total of 13 of the 46 cats (28.3%) died between September 2011 and April 2012 at FIP. At this time, 33 cats (71.7%) appeared to be clinically healthy and 26 of these asymptomatic cats (78.7%) were positive at least once for FCoV - detected from faeces using the RT-nPCR method. The other seven of these asymptomatic cats were negative for FCoV (Table 2) 2).

Table 2
Detection of the occurrence and type of FCoV from faeces samples in healthy cats from the same shelter

Hot girl
Oct. 2011Feb. 2012Jun. 2012 Jul. 2012
15+ untypable
16++ untypable
19 +untypable
20 +untypable
22++++ untypable
24+ untypable
26 +untypable
33 ++untypable
34++   I
35 ++untypable
38  + untypable
39 ++++I
40 +untypable
41 ++untypable
42  +untypable
46  + untypable

++: FCoV detected in the first round of PCR.
+: FCoV detected only in nested PCR.

FIPV type II was found in all cats that succumbed to FIP

In order to further investigate the relationship between these seven histopathologically confirmed FIP cats, the amplified DNA was typed, sequenced and analyzed. FIPV type II was detected in all eight animals that succumbed to FIP, from swabs, faeces, urine, body effusions, cerebrospinal fluid, and tissue homogenates (Table 3). Type II viruses that cause FIP have been found not only in diseased tissue but also in faeces samples (cats 7, 11, 12 and 13), nasal / oral / conjunctival swab samples (cats 7, 8, 9, 11 and 12). ) and in urine collected by cystocentesis (cat 11) (Table 3). Although no necropsy was performed, ascites from cat 1 - the first cat to die in the shelter at FIP - were available for analysis. This cat was confirmed to be infected with type II virus. In healthy animals, only type I or FCoV was detected from faeces samples without type determination (Table 2) 2). Cats 8, 9 and 13 were infected with both types of FCoV (Table 2) 3). Although it has been found that in this environment with many cats there is more than one type of FCoV, ie. type I, II or non-typed viruses, FCoV type II infection was found in all eight FIP cats, whereas this was not the case in healthy animals (Tables 2 and33).

Table 3
Characteristics 3c FCoV genes obtained from different samples of FIP cats

Hot girlFCoV genotypeWITH instead of gene crossingIntegrity 3c geneb
1   II       4250and intact     
7IIII II IIIIIIIIII 4250intact intactintactintact intact
8III   +IIII+        
9III II +II IIII+4250 G210 *   G210 *G210 *
10     ++IIII II4250    intact  
11IIIIIIII+IIIIII + 4250   E47 *   
12IIII IIII+IIII II+4250G210 *G210 *     
13 I / II  +++IIII+ 4250     Q218 * 

NIGHT, nose / mouth / conjunctival swabs; R / F, rectal swabs or stool samples; A / P, ascites or pleural effusion; CSF, cerebrospinal fluid; Li, liver; Lu, lungs; Ki, kidney; Br, brain; Sp, spleen; Int, gut.
+: FCoV positive, but virus type cannot be determined. -: FCoV negative.
a: FCoV / NTU2 / R / 2003; GenBank: DQ160294.
b: E47 *, G210 * and Q218 *: truncated 3c proteins with premature stop codons at amino acids 47, 210 and 218 were found.

FIPV type II of the same origin was found in cats that succumbed to FIP

To further investigate the relationship of these disease-causing type II viruses, which were isolated from cats that succumbed to FIP, sets of specific primers capable of specifically amplifying from the 3 'end were used to analyze viral sequences. WITH the type II gene has a subsequent gene. The identity of the 620 bp amplicons derived from the seven FIPV type II was approximately 98.7% to 99.8%. Phylogenetic analysis found that the type II FCoVs derived from the outbreak described above were all grouped into a separate cluster, which differs from the other four type II FCoVs currently available at GenBank, i. FIPV 79-1146 (GenBank: {“Type”: ”entrez-nucleotide”, “attrs”: {“text”: ”DQ010921 ″,” term_id ”:” 63098796 ″}} DQ010921), FCoV 79-1683 (GenBank: {“Type”: ”entrez-nucleotide”, “attrs”: {“text”: ”JN634064 ″,” term_id ”:” 384038902 ″}} JN634064), FCoV DF-2 (GenBank: {“Type”: ”entrez-nucleotide”, “attrs”: {“text”: ”DQ286389 ″,” term_id ”:” 87242672 ″}} DQ286389) and FCoV NTU156 (GenBank: {“Type”: ”entrez-nucleotide”, “attrs”: {“text”: ”GQ152141 ″,” term_id ”:” 240015188 ″}} GQ152141) (data not shown).

Recombination at the 3 ′ end WITH of the putative recombination site at nucleotide 4250 was determined in all FCoV type II animals obtained from body effusions and tissue homogenates in cats 1, 7, 9, 10, 11, 12 and 13 (Additional set 1) (Table 3). Sequences above this site show greater similarity to CCoV, whereas sequences beyond this site were more similar to type I FCoV (Fig. 1). 1). Indeed, these findings suggest that FCoV type II, found in all FIP cats, has a common origin.

Figure 1
FIPV recombination from cats 1, 7, 9, 10, 11, 12 and 13 on the S gene. Alignment of the 3 ′ end of the S gene with subsequent FCoV genes isolated from seven FIP cats with FCoV type I and CCoV. The light and dark shaded regions include greater similarity to CCoV and FCoV type I. The predicted recombination event occurred at nucleotide 4250 based on comparison to FCoV NTU2 and is indicated by an arrow. Sequences were obtained from FIPV found in individual samples and tissues and are summarized. NIGHT: swabs from the nose / mouth / conjunctiva; RS: rectal swabs; As: ascites; PE: pleural effusion; Li: liver; Lu: lungs; Ki: kidneys; Br: brain; Sp: spleen; dbd: days before death. GenBank accession number: FCoV C1Je (GenBank: DQ848678), FCoV Black (GenBank: EU186072), FCoV NTU2 (GenBank: DQ160294) and CCoV NTU336 (GenBank: GQ477367).

Identical nonsensical mutation on 3c The gene was found in two cats that succumbed to FIP

In order to further analyze the relationship of these FIPVs, they were 3c genes, a proposed virulence-associated FIP, are amplified from the disease-causing FCoV type II. Mutated 3c genes with identical premature stop codon at nucleotides 628-630 (amino acids 210, G210 *) were found in two FIP cats, cat 9 (ascites, spleen and brain) and 12 (ascites and rectal swabs from the day the cat died and four days previously) (Fig. 2A). It is worth noting that FIPV, obtained from cat 12, showed the same nonsense mutation as the virus in its ascites. Intact 3c the genes were discovered in cats 1, 7 and 10, which had previously succumbed to FIP. Two other clear / different nonsense mutations were found in cats 11 (E47 *) and 13 (Q218 *) (Fig. 1). 2AB, Table 3).

Figure 2:
Alignment of complete FIPV 3c genes from cats 1, 7, 9, 10, 11, 12 and 13. (A) The full length 3c genes analyzed in this study were aligned with FCoV type I, FCoV NTU2. The sequences were obtained from FIPV found in individual samples and tissues and are listed together. The box represents the identified premature stop codons. (B) The diagram shows the location of premature stop codons (PT) of gene 3c from different samples from different FIP cats.

FIPV type II excretion can be detected in the terminal phase in FIP cats

The occurrence of FCoV was continuously analyzed to elucidate a possible route of FIPV secretion and transmission. Disease-associated FCoV type II was found to be excreted by the nasal / oral / conjunctival route and faeces (Table 4). Faecal and nasal / oral / conjunctival type II shedding can be detected from day 6 (cat 11) and from day 4 (cat 12) before death. Viremia can be detected during the terminal stage in cats suffering from FIP up to 18 days before death, and concomitant faecal excretion was detected in one cat (cat 12) (Table 4).

Table 4
Excretion and serotypes of feline coronavirus detected in FIP cats in a cat shelter

Hot girlSampleDays before death
9Feces I            I  II
 NIGHT tampons                 II
 Viremie              II  +
 Efuze              IIII II
11Feces            II II
 NIGHT tampons               II
 Efuze     +           II
12Feces+ +  IIII
 NIGHT tampons      IIII
 Viremie  II++   
 EfuzeII                II

+: FCoV positive; -: FCoV negative.
I, II: FCoV type I or type II.
*: Samples were taken immediately before euthanasia, except for cat 12, which were sampled after death.


The possibility of horizontal transmission is generally questioned in FIP because (i) the occurrence of FIP is sporadic and it is common for only one of them to develop FIP in an environment with a large number of cats [2]; (ii) internal mutation theory, which describes that FIPV is a mutant generated from enteric FCoV in one cat [12,17]; (iii) there is insufficient evidence that the mutant FIPV is eliminated from FIP cats; and (iv) mutations 3c gene is unique for every FIP cat [man]11,13,18]. The current belief is that cats that have succumbed to FIP do not excrete and pass FIPV to other cats [11,13,14,1820]. Our data indicate that this outbreak of FIP was caused by viruses of the same origin. First, all cats that died of FIP had a type II infection, and recombination of these seven type II viruses was located at the same site. Recombination of type II viruses currently available in the genetic bank, i.e. FIPV 79-1146, FCoV 79-1683 and FCoV NTU156, were all unique, specific and occurred independently [9,10]. Second, FIPV, found in three kittens that died within the first two months after the onset of fever, had an intact 3c gene, whereas viruses from cats that survived longer (died four to eight months later) all contained a nonsensical mutation, i. G210 * (cats 9 and 12), E47 * (cat 11) and Q218 * (cat 13). Because the three nonsense mutations found in FIPV in these animals were all located at different sites, the viruses that originally infected these cats should be intact. 3c gene - similar to the virus found in kittens that died earlier. Following infection, local mutations occurred during virus replication in individual cats, resulting in FIPV with 3c a gene that carries meaningless mutations in different places. The finding that viruses, which were identified not only in tissues but also in faecal samples in two cats (cats 9 and 12), had an identical mutation in 3c gene, further confirmed that there was a horizontal transfer (Table 2) 3). Taken together, all of these findings demonstrated that highly virulent FIPV spread horizontally from one animal to another.

This is the first report of an FIPV type II outbreak with evidence of horizontal disease-causing FCoV transmission. The FIP broke out after five kittens (cats 1, 3, 4, 8 and 10) entered this shelter between June and July 2011. Because causative type II viruses with a specific genetic marker in the S gene have been confirmed as feline and canine coronavirus recombination, and some of the kittens that died earlier were found to have lived together or next to dogs between rescue and transport to the shelter. of these kittens may have been the source of this type II virus. Dogs and especially young dogs often shed large amounts of canine coronavirus in their faeces in shelters, and recombination between feline-canine and canine-feline coronavirus is already well documented [man]2123]. In addition, type II causative viruses have been detected in a number of excreta and secretions in cats that have died of FIP (Table 3), demonstrating that it is possible to spread between cats.

Although immediately after the first examination of all animals from this FCoV shelter, FCoV-secreting cats were housed in separate cages and transmission subsequently ceased, mortality at the onset of the disease was high (28%, 13/46). The results of three studies that looked at the outbreak of FIP have been reported earlier. The results of a four-year study conducted at a nearby cat kennel showed an average mortality of 17.3% [24]; the mortality rate from a ten-year study conducted at a nearby kennel was 29.4% (5/17) [25]. Another epidemic study conducted in seven kennels / shelters revealed >10% mortality [20]. The high incidence of FIP in these closed breeding stations could be influenced by genetically predisposed breeding animals. In our study, only a few FIP cats in this shelter were siblings and the other cats were not genetically related. Our study shows that even without the influence of genetic predisposing factors, FIP mortality can be high in a confined environment with a large number of cats if the spread of FCoV, which causes the disease, remains undetected.

In this environment with a large number of cats, three FIP cats were infected not only with FCoV type II, but also co-infected with FCoV type I (Table 3). Type I FCoV was found only in faecal samples, while type II FCoV was found in various samples, including body effusions, granulomatous tissue homogenates, and cerebrospinal fluid. This finding indicates that FCoV type II was a major cause of FIP in these doubly infected animals. This finding is consistent with our previous finding that FCoV type II infection is significantly associated with FIP [4].

The presence of FCoV in whole blood in the terminal phase has been identified previously [26,27]; however, to our knowledge, the presence of FIPV in faeces prior to the final stage of the disease was not published anywhere until our study. The excretion of this type II virus in faeces and by the nasal / oral / conjunctival route can be detected in the effusive form of FIP up to six days before the death of the animal. Another experimental study of the infection showed that inoculated viruses could not be detected until about two weeks after inoculation, before clinical signs of the disease developed [14]. In summary, FIPV transmission could occur at the beginning, before the manifestations of the disease and in the terminal phase. When the disease broke out in our case, all the cats were initially placed together in an open room. After seven cats gradually succumbed to the disease, all FCoV-positive cats were housed separately in cages and kept separately. Isolation probably inhibited disease transmission. This outbreak of disease, which killed 13 cats, allowed us to make it clear that FIPV can be transmitted horizontally and to show that the isolation of sick cats should be taken into account in an environment where more cats are present.

Competitive interests

The authors claim that they have no competitive interests.

Contributions and contributions of authors

YTW performed sampling and preparation, FCoV detection, type determination, amplification 3c gene and other analyzes and compiled a manuscript. The BLS supervised the sampling and treatment of all FIP animals and contributed to the compilation of the manuscript. LEH participated in the amplification 3c gene, genetic analysis and manuscript preparation. The LLC devised the study, participated in the design of the study, coordinated and participated in the preparation of the manuscript. All authors read and approved the final version of the manuscript.

Additional material

Additional file 1:

FIPV recombination site analysis in cats 1, 7, 9, 10, 11, 12 and 13 at WITH gene. Analysis of the plot similarity using the Kimur (two-parameter) distance model, the model of adjacent interconnected trees, and 100 replicates of the bootstrap showed that recombination had occurred and the putative crossing point is indicated by an arrow. 


The authors would like to thank the caregivers in the mentioned cat shelter, without whose help this study would not have been possible.


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Origin of abdominal or thoracic effusions in cats with wet FIP and causes of their persistence during treatment

Niels C. Pedersen, DVM, PhD
Pet Health Center
University of California, Davis

Original article: Origin of abdominal or thoracic effusions in cats with wet FIP and reasons for their persistence during treatment

Origin of FIP exudates. Sweat in wet FIP comes from small vessels (venules) that line the surface of the abdominal and thoracic organs (visceral) and walls (parietal), mesentery / mediastinum, and omentum. The spaces around these vessels contain a specific type of macrophages that come from monocyte progenitors that constantly recirculate between the bloodstream, the interstitial spaces around the venules, the afferent lymph, the regional lymph nodes, and back into the bloodstream. Other sites of this recirculation are located in the meninges, brain ependyma, and uveal eye tract. A small proportion of these monocytes develop into immature macrophages (monocyte / macrophage) and eventually into resident macrophages. Macrophages are constantly looking for infections.

FIPV is caused by a mutation in feline enteric coronavirus (FECV) present in lymphoid tissues and lymph nodes in the lower intestine. The mutation changes FECV cell tropism from enterocytes to peritoneal-type macrophages. Monocytes / macrophages appear to be the first cell type to be infected. This infection causes more monocytes to leave the bloodstream and begin to turn into macrophages, which continue the cycle of infection. [2]. Monocytes / macrophages do not undergo programmed cell death as usually expected, but continue to mature into large virus-loaded macrophages. These large macrophages eventually undergo programmed cell death (apoptosis) and release large amounts of virus, which then infects new monocytes / macrophages. [1]. Infected monocytes / macrophages and macrophages produce several substances (cytokines) that mediate the intensity of inflammation (disease) and immunity (resistance). [1,2].

Inflammation associated with FIP leads to three types of changes in the venules. The first is loss of vascular wall integrity, micro-bleeding, and leakage of plasma protein rich in activated complement clotting and activation factors and other inflammatory proteins. The second type of damage involves thrombosis and blocking blood flow. The third injury occurs in more chronic cases and involves fibrosis (scarring) around the blood vessels. Variations in these three events determine the amount and composition of exudates according to the four Starling forces that determine the movement of fluids between the bloodstream and interstitial spaces. [3].

The classic effusion in wet FIP is mainly due to acute damage to the vessel walls and leakage of plasma into the interstitial spaces and finally into the body cavities. Protein that escapes into the interstitial spaces attracts additional fluids, which can be exacerbated by blocking venous blood flow and increasing capillary pressure. This type of effusion, known as exudate, also contains high levels of protein, which is involved in inflammation, immune responses and blood clotting.

This fluid also contains a large number of neutrophils, macrophages / monocytes, macrophages, eosinophils and a lower number of lymphocytes and red blood cells. This classic type of fluid has the consistency of egg white and forms weak clots containing a high amount of bilirubin. Bilirubin does not originate from liver disease, but rather from the destruction of red blood cells that escape into interstitial tissue cells and are taken up by monocytes / macrophages and macrophages. Red blood cells break down and hemoglobin is broken down into heme and globin. Globin is further metabolized to biliverdin (greenish color) and finally to bilirubin (yellowish color), which is then excreted by the liver. However, cats lack the enzymes used for conjugation and are therefore ineffective in removing bilirubin from the body. [4]. This leads to the accumulation of bilirubin in the bloodstream and gives the effusion a yellow tinge. The darker the yellow tint, the more bilirubin is in the effusion, the more severe the initiating inflammatory response and the more severe the resulting bilirubinemia, bilirubinuria and jaundice.

The opposite extreme of the classic and more acute effusion in FIP are effusions arising mainly from chronic infections and blockage of venous blood flow and consequent increase in capillary pressure. High capillary pressure results in effusion that is more distant to interstitial fluid than plasma, has a lower protein content, is watery rather than sticky, clear or slightly yellow in color, is not prone to clotting, and has a lower number of acute inflammatory cells such as neutrophils. There are also FIP effusions that are among these extremes, depending on the relative degree of acute inflammation and chronic fibrosis. These transient types of fluids are commonly referred to in the veterinary literature as modified transudate, but this is a misnomer. The modified transudate begins as a transudate and changes as it persists and causes mild inflammation. Low protein and cell effusions in FIP arise as exudates and not as transudates and do not conform to this description. The more correct term is "modified exudate" or "variant exudate outflow".

How long do sweats usually last in cats treated with GS-441524 or GC376? The presence of abdominal effusions often leads to a large dilation of the abdomen and is confirmed by palpation, hollow needle aspiration, X-ray or ultrasound. Cats with thoracic effusions are most often presented with severe shortness of breath and are confirmed by radiological examination and aspiration. Chest effusions are almost always removed to relieve shortness of breath and recur slowly compared to abdominal effusions. Therefore, abdominal effusions are usually not removed unless they are massive and do not interfere with respiration, as they are quickly replaced. Repeated drainage of abdominal effusions can also deplete proteins and cause harmful changes in fluid and electrolyte balance in severely ill cats.

Chest effusions disappear faster with GS-441524 treatment, with improved breathing within 24-72 hours and usually disappearing in less than 7 days. Abdominal effusions usually decrease significantly within 7-14 days and disappear within 21-28 days. The detection of exudates that persist after this time depends on their amount and method of detection. Small amounts of persistent fluid can only be detected by ultrasound.

Persistence of exudates during or after antiviral treatment. There are three basic reasons for the persistence of exudates. The first is the persistence of the infection and the resulting inflammation at a certain level, which can be caused by inappropriate treatment, poor medication or drug resistance. Inadequate treatment may be the result of incorrect dosing of the wrong drug or the acquisition of virus resistance to the drug. The second reason for fluid persistence is chronic venous damage and increased capillary pressure. This may be due to a low-grade infection or residual fibrosis from an infection that has been removed. The third reason for persistence is the existence of other diseases, which can also manifest as exudates. These include congenital heart disease, in particular cardiomyopathy, chronic liver disease (acquired or congenital), hypoproteinemia (acquired or congenital) and cancer. Congenital diseases causing effusions are more common in young cats, while acquired causes and cancer are more commonly diagnosed in older cats.

Diagnosis and treatment of persistent effusions. A thorough examination of the fluid, as described above, is a prerequisite for diagnosis and treatment. If the fluid is inflammatory or semi-inflammatory and the cell pellet is positive by PCR or IHC, the reason for the persistence of the infection must be determined. Was the antiviral treatment performed correctly, was the antiviral drug active and its concentration correct, was there evidence of acquired drug resistance? If the fluid is inflammatory and PCR and IHC are negative, what other diseases are possible? Low protein and non-inflammatory fluids that are negative for PCR and IHC indicate a diagnosis of residual small vessel fibrosis and / or other contributing causes such as heart disease, chronic liver disease, hypoproteinemia (bowel disease or kidneys). Some of the disorders causing this type of effusion may require an exploratory laparotomy with a thorough examination of the abdominal organs and a selective biopsy to determine the origin of the fluid. The treatment of persistent effusions will vary greatly depending on the end cause. Persistent effusions caused by residual small vessel fibrosis in cats cured of the infection often resolve after many weeks or months. Persistent discharges caused in whole or in part by other diseases require treatment for these diseases.

Identification and characteristics of persistent effusions. The presence of fluid after 4 weeks of GS treatment is unpleasant and is usually detected in several ways depending on the amount of fluid and its location. Large amounts of fluid are usually determined by the degree of abdominal dilation, palpation, X-ray and abdominal aspiration, while smaller amounts of fluid are best detected by ultrasound. Persistent pleural effusion is usually detected by X-rays or ultrasound. Overall, ultrasound is the most accurate means of detecting and semiquantitatively determining thoracic and abdominal effusions. Ultrasound can also be used in combination with thin needle aspiration to collect small and localized amounts of fluid.

The second step in examining persistent effusions is to analyze them based on color, protein content, white and red blood cell counts, and the types of white blood cells present. Fluids generated primarily by inflammation will have protein levels close to or equal to plasma and a large number of white blood cells (neutrophils, lymphocytes, monocytes / macrophages and large vacuolated macrophages). Fluids produced by increased capillary pressure are more similar to interstitial fluid with proteins closer to 2.0 g / dl and cell counts <200. The Rivalt test is often used to diagnose FIP-related effusions. However, this is not a specific test for FIP, but rather for inflammatory effusions. It is usually positive for FIP effusions that are high in protein and cells, but is often negative for very low protein and cell effusions. The effluents that are between these two types of effusions will be tested either positively or negatively, depending on where they are in the spectrum.

The third step is the analysis of exudates for the presence of FIP virus. This usually requires 5 to 25 ml or more of fluid. For fluids with a higher protein and cell count, a smaller amount may suffice, while for fluids with a low protein and cell count, a larger amount is required. The freshly collected sample should be centrifuged and the cell pellet analyzed for the presence of viral RNA by PCR or cytocentrifuged for immunohistochemistry (IHC). The PCR test should be for FIPV 7b RNA and not for specific FIPV mutations, as the mutation test does not have sufficient sensitivity and does not provide any diagnostic benefits [5]. Samples that are positive by PCR or IHC provide definitive evidence of FIP. However, up to 30 % samples from known cases of FIP may have a false negative test either due to an inappropriate sample and its preparation, or because the RNA level of the FIP virus is below the level of detection. It is also true that the less inflammatory the fluid, the lower the virus levels. Therefore, effusions with lower protein and white blood cell levels are more likely to be tested negative because viral RNA is below the detection limit of the test.


[1] Watanabe R, Eckstrand C, Liu H, Pedersen NC. Characterization of peritoneal cells from cats with experimentally-induced feline infectious peritonitis (FIP) using RNA-seq. Vet Res. 2018 49 (1): 81. doi: 10.1186 / s13567-018-0578-y.

[2]. Kipar A, Meli ML, Failing K, Euler T, Gomes-Keller MA, Schwartz D, Lutz H, Reinacher M. Natural feline coronavirus infection: differences in cytokine patterns in association with the outcome of infection. Vet Immunol Immunopathol. 2006 Aug 15; 112 (3-4): 141-55. doi: 10.1016 / j.vetimm.2006.02.004. Epub

[3] Brandis K. Starling's Hypothesis, LibreTexts. _(Brandis)/04%3A_Capillary_Fluid_Dynamics/4.02%3A_Starling%27s_Hypothesis

[4]. Court MH. Feline drug metabolism and disposition: pharmacokinetic evidence for species differences and molecular mechanisms. Vet Clin North Am Small Anim Pract. 2013; 43 (5): 10391054. doi: 10.1016 / j.cvsm.2013.05.002

[5]. Barker, EN, Stranieri, A, Helps, CR. Limitations of using feline coronavirus spike protein gene mutations to diagnose feline infectious peritonitis. Vet Res 2017; 48: 60. Read "Origin of abdominal or thoracic effusions in cats with wet FIP and causes of their persistence during treatment"

Acute phase proteins in cats

April 2019
Rita Mourão Rosa, Lisa Alexandra Pereira Mestrinho
Original article: Acute phase proteins in cats

ABSTRACT: Acute phase proteins (APPs) are proteins synthesized and released mainly by hepatocytes during cell damage or invasion of microorganisms. This article provides an overview of the use of APP in cat diseases, identifies their usefulness in the clinical setting, and analyzes 55 published papers. Serum amyloid A, alpha-1 acid glycoprotein and haptoglobin are indicators that the authors consider useful in monitoring the acute inflammatory response in cats. Although APP measurement is still not routinely used in veterinary medicine, along with clinical signs and other blood parameters, they are clinically of interest and useful in diseases such as feline infectious peritonitis, pancreatitis, renal failure, retroviral and calicivirus infections. Although there are commercially available kits for measuring feline APPs, standardization of tests for technical simplicity, greater species specificity, and less associated costs will allow for routine use in feline practice, as is the case in the human field.
keywords: inflammation, acute phase proteins, cat.


Acute phase response (APR) is an early non-specific systemic innate immune response to a local or systemic stimulus that helps treat and restore homeostasis and minimize tissue damage when an organism is affected by trauma, infection, stress, surgery, neoplasia, or inflammation (GRUYS et al. , 2005; CRAY et al., 2009; ECKERSALL AND BELL, 2010). In this reaction, we observe several different systemic effects: fever, leukocytosis, hormonal changes - mainly cortisol and thyroxine concentrations, with secondary catabolic status and serum muscle, iron and zinc depletion (CERÓN et al. 2005, JAVARD et al. 2017).
Cytokines IL-1β, TNF-α, and especially IL-6, and approximately 90 minutes after injury, increase protein synthesis in hepatocytes, lymph nodes, tonsils, and spleen, as well as blood leukocytes. These newly formed proteins are called acute phase proteins (APPs) (TIZARD, 2013b).

Acute-phase proteins

APP concentrations may increase (APP positive) or decrease (APP negative) in response to inflammation (PALTRINIERI et al., 2008) (JOHNSTON & TOBIAS, 2018). They can activate leukocytosis and complement, cause protease inhibition, lead to blood clotting and opsonization - a defense mechanism that leads to the elimination of infectious agents, tissue regeneration and restoration of health (CRAY et al., 2009). APP can have two functions, pro- and / or anti-inflammatory, which must be fine-tuned to promote homeostasis (HOCHEPIED et al., 2003).

According to the size and duration of the reaction following the stimulus, three main groups of APP are distinguished (MURATA et al., 2004; PETERSEN et al., 2004; CERÓN et al.). Positive APP can be divided into two groups: the first group includes APP with an increase of 10 up to 1000-fold in humans or 10- to 100-fold in domestic animals in the presence of inflammation - e.g. c-reactive protein (CRP) and serum amyloid A (SAA). The second group are APPs, which increase 2 to 10-fold in an inflammatory response - e.g. haptoglobin and alpha-globulins. The last group included negative APP, in which the concentration decreases in response to inflammation - e.g. albumin (KANN et al., 2012).

Acute phase positive proteins

Positive APPs are glycoproteins whose serum concentrations, when stimulated by pro-inflammatory cytokines, increase by 25 % during the disease process and are released into the bloodstream. These concentrations can be measured and used in diagnosis, prognosis, monitoring of response to treatment, as well as general health screening. They can also be considered as quantitative biomarkers of the disease, highly sensitive to inflammation but not very specific, as an increase in APP can also occur in non-inflammatory diseases (CERÓN et al., 2005; ECKERSALL and BELL, 2010).

Positive APPs respond to cytokines differently, and these groups fall into two main classes. Type 1 APP, which includes AGP, complement component 3, SAA, CRP, haptoglobin and hemopexin, is regulated by IL-1, IL-6 and TNF-α as well as glucocorticoids. Type 2, which includes three fibrinogen chains (α-, β- and γ-fibrinogen) and various inhibitory proteases, is regulated by cytokines IL-6 and glucocorticoids (BAUMANN et al., 1990; BAUMANN & GAULDIE, 1994).

In cats, APP SAA or alpha-1-acid glycoprotein (AGP) is the most important. Blood SAA levels may indicate inflammatory conditions such as feline infectious peritonitis (FIP) and other infectious diseases such as calicivirus infection, chlamydia, leukemia and infectious immunodeficiency, as they increase 10- to 50-fold (TIZARD, 2013b). SAA can also be increased in other diseases, such as diabetes mellitus and cancer. Haptoglobin usually increases 2- to 10-fold and is particularly high in FIP (TIZARD, 2013b). Table 1 summarizes the individual positive APPs in the context of feline disease.

Acute phase negative proteins

The most significant negative APP is albumin, whose blood concentration decreases during APR due to amino acid aberrations towards the synthesis of positive APPs (CRAY et al., 2009; PALTRINIERI, 2007a). Other negative APPs are transferrin, transthyretin, retinol ligand, and cortisol binding protein, proteins involved in vitamin and hormone transport (JAIN et al., 2011).

Acute phase proteins in cat disease

Unlike cytokines, which are small in size and rapidly filtered by the kidney, acute phase proteins have a higher molecular weight (greater than 45 kDa) and consequently remain in plasma for longer (SALGADO et al., 2011).

APP levels can only indicate inflammation, and consequently their concentrations can help diagnose and monitor the disease. APP can help detect subclinical inflammation, distinguish acute from chronic disease, and predict its course (VILHENA et al, 2018; JAVARD et al., 2017). Because APRs begin before specific immunological changes occur, they can be used as an early marker of disease before leukogram changes occur, with their magnitude related to disease severity (PETERSEN et al., 2004; CÉRON et al., 2005; VILHENA et al., 2005). , 2018). For this reason, disease monitoring can be considered one of the most interesting and promising applications of APP.

APP levels along with clinical signs and blood tests have been evaluated in a variety of animal diseases (ie, FIP, canine inflammatory disease, leishmaniasis, ehrlichiosis, and canine pyometra) and have been shown to be useful in diagnosis, response to treatment, and prognosis (ECKERSALL et al. ), 2001; MARTINEZ-SUBIELA et al., 2005; SHIMADA et al., 2002; JERGENS et al., 2003; GIORDANO et al., 2004; PETERSEN et al., 2004; DABROWSKI et al., 2009; VILHENA et al., 2018).

To obtain complete information on APR, one major and one moderate positive as well as one negative APP should be evaluated simultaneously (CERÓN et al., 2008). High concentrations of major APP are usually associated with infectious diseases, usually systemic bacterial infection or immune-mediated disease (CERÓN et al., 2008; TROÌA et al., 2017). Although APPs should be analyzed along with white blood cell and neutrophil counts, they are most sensitive in the early detection of inflammation and infection (CERÓN et al., 2008; ALVES et al., 2010). However, the specificity of these proteins is low in determining the cause of the process, and also increases in physiological conditions such as pregnancy (PALTRINIERI et al., 2008).

APPThe disease
Induced inflammation and surgery
Various diseases (pancreatitis, renal failure, FLUTD, tumors, diabetes mellitus; kidney disease, injury, etc.)
FeLV; hemotropic mycoplasma infections
Hepatozoonfelis and Babesia vogeli infection
FIV cats treated with recombinant feline interferon
AGPChlamydophila psittaci infection;
Pancreatitis and pancreatic tumors
Lymphoma and other tumors
Induced inflammation and surgery
FIV cats treated with recombinant feline interferon
Abscesses, pyothorax, adipose tissue necrosis
Various diseases (FLUTD, tumors, diabetes mellitus, kidney diseases, injuries, etc.)
Induced inflammation and surgery
Abscesses, pyothorax, adipose tissue necrosis
Various diseases (FLUTD, tumors, diabetes mellitus, kidney diseases, injuries, etc.)
Hepatozoonfelis and Babesia vogeli infection
FeLV, hemotropic mycoplasmas
CRPFIV cats treated with recombinant feline interferon
Induced inflammation and surgery
Table 1 - Acute phase proteins studied for feline diseases.
Legend: Serum amyloid A (SAA), α1-acid glycoprotein (AGP), systemic inflammatory response syndrome (SIRS), feline lower urinary tract disease (FLUTD), feline infectious peritonitis (FIP), feline leukemia virus (FeLV), immunodeficiency virus cats (FIV); feline calicivirus (FCV).

Figure 1 shows the expected behavior of acute phase positive proteins based on revised studies. AGP, SAA and haptoglobin have been identified as useful indicators for monitoring the acute inflammatory response in cats (WINKEL et al., 2015; PALTRINIERI et al., 2007a, b; KAJIKAWA et al., 1999). APPs in cats were first identified after comparative measurements in the serum of clinically normal and diseased animals, in experimentally induced inflammation studies, and in postoperative studies (KAJIKAWA et al., 1999). The concentration of SAA reportedly increased first, followed by an increase in AGP and haptoglobin, in contrast to a less pronounced increase in CRP (KAJIKAWA et al., 1999). One study showed that CRP behaves similarly to SAA and AGP in cat inflammation (LEAL et al., 2014).

Serum Amyloid A

SAA is one of the major APPs in several species, important in both humans and cats (KAJIKAWA et al., 1999). It modulates the immune response by attracting inflammatory cells to tissues and leading to the production of multiple inflammatory cytokines (GRUYS et al., 2005; TIZARD, 2013a). Its concentration can increase more than 1,000 times in an inflammatory condition, which we then understand as inflammation (TAMAMOTO et al., 2013). However, such an increase can be observed in both non-inflammatory and inflammatory diseases and neoplasms (TAMAMOTO et al., 2013). According to a study in cats that underwent surgery, SAA levels begin to increase approximately 3 to 6 hours, peaking 21 to 24 hours after surgery (SASAKI et al., 2003).

Figure 1 - Idealized behavior of acute phase proteins in cats after inflammatory stimuli. The values representing the changes cannot be considered absolute. Increase in serum amyloid A (SAA) 3 to 6 h after challenge, peak at 21 to 24 h, peak size 10 to 50 times its basal plasma concentration. Alpha 1 acid glycoprotein (AGP) increases 8 h after challenge, peak at 36 h, size at peak time 2 to 10 times its baseline plasma concentration. Haptoglobulin (Hp) increase 24 h after challenge, peak 36 to 48 h, peak size 2 to 10 times its basal plasma concentration. C-reactive protein (CRP) increased 8 h after challenge, peak at 36 h, peak size 1.5 times its basal values.

Alpha 1-acid glycoprotein

Alpha 1-acid glycoprotein (AGP) is an acute phase-reactive protein found in the serum mucoid portion of serum (SELTING et al., 2000; WINKEL et al., 2015). Like most positive APPs, AGP is a glycoprotein synthesized predominantly by hepatocytes in APR and released into the bloodstream (CÉRON et al., 2005).

AGP can be used to monitor early interferon treatment in cats infected with feline immunodeficiency virus (FIV) (GIL et al., 2014). AGP as well as haptoglobin (Hp) are increased in anemic cats suffering from pyothorax, abscesses or fat necrosis (OTTENJANN et al., 2006).

Changes in AGP in feline neoplasia do not appear to be consistent across studies. Some of them do not describe any changes in cats with lymphoma (CORREA et al., 2001). Others point to an increase in both AGP and SAA in cats with sarcomas, carcinomas, or other round cell tumors (SELTING et al., 2000; TAMAMOTO et al., 2013; MEACHEN et al., 2015; HAZUCHOVA et al., 2017).

AGP is important as an indicator test for FIP, which is used specifically in Europe (CECILIANI et al., 2004). GIORI et al. examined the specificity and sensitivity of several tests in 12 cats, with 33.33 % cats being FIP negative based on histopathology and immunohistochemistry and 66.66 % cats being FIP positive confirmed by histopathology and immunohistochemistry. This author concludes that immunohistochemistry must always be performed to confirm FIP, but high concentrations of AGP can help support the diagnosis of FIP if immunohistochemistry cannot be performed and histopathology is not convincing.


Haptoglobin (Hp) is one of the most important acute phase proteins in cattle, sheep, goats, horses and cats (TIZARD, 2013a), synthesized mainly by hepatocytes but also by other tissues such as skin, lungs and kidneys (JAIN et al, 2011 ). Hp binds to iron molecules and makes them inaccessible to invasive bacteria, thereby inhibiting bacterial proliferation and invasion. Subsequently, it also binds to free hemoglobin, thus preventing its oxidation with lipids and proteins (TIZARD, 2013a), which justifies a reduction in Hp in case of hemolysis.

In cats, Hp usually increases 2- to 10-fold in inflammatory conditions, and is particularly high in FIP (TIZARD, 2013a). However, both Hp and SAA did not provide sufficient support to distinguish FIP from other causes of effusion compared to AGP (HAZUCHOVÁ et al., 2017).

Measurement APP

The serum is composed of a large number of individual proteins in which the detection of changes in its fractions can provide important diagnostic information (ECKERSALL, 2008).

Ideally, measurement of all serum proteins should be available so that they can be used as a diagnostic tool in relation to inflammatory diseases.
Currently, APPs (Table 2) can be determined by enzyme-linked immunosorbent assay (ELISA), radioimmunoassay, nephelometry, immunoturbidimetry (IT), Western blot, and messenger ribonucleic acid (mRNA) analysis (CÉRON et al., 2005; PALTRINIERI et al., 2008; SCHREIBER et al., 1989). Although some human APP tests have been automated for veterinary medicine, species-specific tests are still limited. Cross-species differences in APP and the limited availability of cross-reactive agents have so far contributed to the low routine level of APP determination in veterinary laboratories, especially in cats. Regardless, the technology is evolving and routine monitoring of clinically relevant APPs in cats can be expected in the near future.


Acute phase proteins in cats are biomarkers suitable for monitoring inflammation, along with other clinical and laboratory findings that are useful in diagnosing subclinical changes, monitoring the development and effect of the disease in the body, as well as in evaluating the response to treatment.

In cats, SAA APP, which is most pronounced in response to inflammation, is followed by AGP and haptoglobin, in contrast to CRP, which is used in other species.

Although there are commercially available kits for determining feline APPs, standardization of tests for technical simplicity, higher species specificity with lower associated costs will allow routine use in feline practice, as is done in human medicine.

Radioimmunoassay24 to 48 hours to obtain results, specific operator skills required
ELISACommercially available species-specific kitsLack of automation, expensive, some "between-run" inaccuracy
Immunoturbidimetry30 minutes to obtain results, customizable with a biochemical analyzer
Western BlotLong time for immunoblot processing
Nephelometric immunoassaysThey depend on the cross-reactivity of the increased antiserum
Table 2 - Advantages and disadvantages of possible APP measurement techniques.

Appendix: APP and their position in the electrophoretogram

Although there are tests directly for a specific APP, it is useful to know in which region the electrophoretograms are located.

Electrophoretogram demonstration (Serum protein electrophoresis output)
Serum proteinElectrophoretic region
α1-acid glycoproteinα1 (alpha-1)
Serum Amyloid Aα (alpha)
Haptoglobinα2 (alpha-2)
Ceruloplasmin α2 (alpha-2)
Transferrinβ1 (beta-1)
C-reactive proteinγ (gamma)
Position of serum proteins in electrophoretogram


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KURIBAYASHI, T. et al. Alpha 1-acid glycoprotein (AAG) levels
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MARTÍNEZ-SUBIELA, S. et al. Analytical validation of commercial techniques for haptoglobin determination, C reactive protein and amiloid A series in canines [Analytical validation of commercial techniques for haptoglobin, C reactive protein and serum amyloid A determinations in dogs]. Archivos de Medicina Veterinaria, v.37, n.1, 2005. Available from:. Accessed: Jan. 13, 2019. doi: 10.4067 / S0301-732X2005000100009.

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Alternative treatments for cats with FIP and natural or acquired resistance to GS-441524

Niels C. Pedersen, Nicole Jacque, 3.11. 2021
Original article: Alternative treatments for cats with FIP and natural or acquired resistance to GS-441524

SC - subcutaneous
IV - intravenous
IM - to the muscle
PO - per os - orally
SID - once a day
BID - 2x this
q24h - once every 24 hours
q12h - once in 12 hours


Antiviral resistance is well documented in diseases such as HIV / AIDS and hepatitis C. In some cases, this resistance is present in the infecting virus, but is more often due to long-term drug exposure. Resistance to GC376 [1] and GS-441524 [2] has also been documented in cats with naturally acquired FIP. Resistance develops based on mutations in regions of the viral genome that contain targets for the antiviral drug. For example, several amino acid changes (N25S, A252S or K260N) were detected in the GIP376-resistant FIPV isolate (3CLpro). [3]. A change in N25S in 3CLpro was found to cause a 1.68-fold increase in 50 % GC376 inhibitory concentration in tissue cultures [3]. Resistance to GC376, although recognized in initial field trials, has not yet been described. GC376 is less popular in the treatment of FIP and is not recommended for cats with ocular or neurological FIP. [1].

Natural resistance to GS-441524 was observed in one of 31 cats treated for naturally occurring FIP [2]. One of the 31 cats in the original GS-441524 field study also appeared to be resistant, as viral RNA levels did not decrease throughout the treatment period and the symptoms of the disease did not abate. Although this virus has not been studied, resistance to GS-5734 (Remdesivir), a prodrug of GS-441524, has been established in tissue culture by amino acid mutations in RNA polymerase and corrective exonuclease. [4].

Resistance to GS-441524 has been confirmed in a number of cats that have been treated for FIP with GS-441524 in the last 3 years, especially among cats with neurological FIP [5]. Resistance to GS441524 is usually partial and higher doses often cure the infection or significantly reduce the symptoms of the disease during treatment. Interestingly, resistance to GS-441524 has also been found in patients with Covid19 treated with Remdesivir [12]. An immunocompromised patient developed a prolonged course of SARS-CoV-2 infection. Remdesivirus treatment initially alleviated symptoms and significantly reduced virus levels, but the disease returned with a large increase in virus replication. Whole genome sequencing identified an E802D mutation in nsp12 RNA-dependent RNA polymerase that was not present in pre-treatment samples and caused a 6-fold increase in resistance.

Although the history of molnupiravir and its recent use in the treatment of FIP has been described [6], there are currently no studies documenting natural or acquired resistance to molnupiravir. Molnupiravir has been shown to function as an RNA mutagen causing several defects in the viral genome [7]while remdesivir / GS-441524 is a non-binding RNA chain terminator [8], which suggests that its resistance profile will be different.

Overcoming resistance to GS-441524

Drug resistance can only be overcome in two ways: 1) by gradually increasing the dose of the antiviral to achieve drug levels in body fluids that exceed the resistance level, or 2) by using another antiviral that has a different mechanism of resistance, either alone or in combination. So far, the first option has been chosen, which has proved effective in many cases. However, resistance to GS-441524 may be complete or so high that increasing the dose is no longer effective. In such cases, the second option is increasingly used. Currently available alternatives to GS-441524, although still from an unapproved market, are GC376 and molnupiravir.

Antiviral drug treatment regimens for resistance to GS-441524

GC376 / GS-441524

The combined GS / GC regimen has been shown to be effective in cats treated with GS-441524 at doses up to 40 mg / kg without cure due to resistance to GS-441524. It is better to intervene as soon as resistance to GS-441524 is detected, which will allow the cat to be cured sooner and at lesser cost to the owner.

Rainman is the current supplier of GC376, which comes in 4 ml vials at a concentration of 53 mg / ml.

GS / GC dosage: The dose of GS (SC or PO equivalent) in combination antiviral therapy is the same as the dose needed to adequately control the symptoms of the disease. This is usually the last dose used before the end of treatment and relapse. To this dose of GS-441524, GC376 is added at a dose of 20 mg / kg SC q24h regardless of the form of FIP. This is sufficient for most cats, including many cats with neuro FIP, but some will need higher doses. If remission of clinical signs is not achieved or blood tests are of concern, the dose of GC376 is increased by 10 mg / kg up to 50 mg / kg SC q24h.

Duration of treatment: An eight-week combination GC / GS treatment is recommended, which is added to previous GS monotherapy. Some cats were cured at 6 weeks of combination therapy, but relapse is more likely than at 8 weeks.

Side effects: Most cats have no serious side effects. However, about one in five cats may experience nausea or discomfort at the beginning of treatment and sometimes longer. These side effects do not appear to be dose dependent and can be treated with anti-nausea drugs such as Cerenia, Ondansetron or Famotidine. Ondansetron appears to have performed better in some cats.


Molnupiravir has been reported to be effective in monotherapy in cats with FIP by at least one Chinese retailer GS-441524 [9], but there are no reports of its use in cats with resistance to GS-441524. However, resistance to GS-441524 is unlikely to spread to molnupiravir. The fact that it has been found to be effective as an oral medicine also makes it attractive for treatment alone, as many cats resistant to GS-441524 have suffered from injections for a very long time.

A field study of molnupiravir reportedly consisted of 286 cats with various forms of naturally occurring FIP, which were examined in pet clinics in the United States, the United Kingdom, Italy, Germany, France, Japan, Romania, Turkey and China. Among the 286 cats that participated in the trial, no deaths occurred, including seven cats with ocular (n = 2) and neurological (n = 5) FIP. Twenty-eight of these cats were cured after 4-6 weeks of treatment and 258 after 8 weeks. All treated cats remained healthy 3-5 months later, a period during which cats that were not successfully cured would be expected to relapse. These data provide convincing evidence of the safety and efficacy of molnupiravir in cats with various forms of FIP. However, we hope that this field study will be written in the form of a manuscript, submitted for review and published. Nevertheless, it is now sold to cat owners with FIP. At least one other major retailer of GS-441524 is also interested in using molnupiravir for FIP, indicating a demand for further treatment of cats with FIP antivirals.

Molnupiravir dosage: The safe and effective dosing of molnupiravir in cats with FIP has not been established based on closely controlled and monitored field studies such as those performed for GC376 [1] and GS-441524. [2]. However, at least one seller from China in his flyer for a product called Hero-2801 [9] provided some pharmacokinetic and field trials of Molnuparivir in cats with naturally occurring FIP. This information does not clearly state the amount of molnupiravir in one of their “50 mg tablets” and the actual dosing interval (q12h or q24h?). The dose used in this study also appeared to be too high. Fortunately, the estimated starting dose of molnupiravir in cats with FIP can be obtained from published studies on EIDD-1931 and EIDD-2801. [15] in vitro on cell cultures and laboratory and field studies GS-441524 [14,18]. Molnupiravir (EIDD-2801) has an EC50 of 0.4 μM / μl against FIPV in cell culture, while the EC50 of GS-441524 is approximately 1.0 μM / μl. [18]. Both have a similar oral absorption of approximately 40-50 %, so an effective subcutaneous (SC) dose of molnupiravir would be approximately half the recommended starting dose of 4 mg / kg SC q24h for GS441524. [14] or 2 mg / kg SC q24h. The oral (PO) dose would be doubled to account for less effective oral absorption per 4 mg / kg PO q24h dose. The estimated initial effective oral dose of molnupiravir in cats with FIP can also be calculated from the available Covid-19 treatment data. Patients treated with Covid-19 are given 200 mg of molnupiravir PO q12h for 5 days. This dose was, of course, calculated from a pharmacokinetic study performed in humans, and if the average person weighs 60-80 kg (70 kg), the effective inhibitory dose is 3,03.0 mg / kg PO q12h. The cat has a basal metabolic rate 1.5-fold higher than humans, and assuming the same oral absorption in both humans and cats, the minimum dose for cats according to this calculation would be 4.5 mg / kg PO q12h in neocular and non-neurological forms of FIP. If molnupiravir crosses the blood-brain and blood-brain barriers with the same efficacy as GS-441524 [3,18], the dose should be increased to 1,51.5 and 2,02.0-fold to ensure adequate penetration into aqueous humor and cerebrospinal fluid for ocular cats (88 mg / kg PO, q12 h), respectively. neurological FIP (~ 10 mg / kg PO, q12 h). These doses are comparable to those used in ferrets, where 7 mg / kg q12h maintains sterilizing blood levels of the influenza virus drug (1.86 μM) for 24 hours. [10]. Doses in ferrets of 128 mg / kg PO q12h caused almost toxic blood levels, while a dose of 20 mg / kg PO q12h caused only slightly higher blood levels. [10].

Molnupiravir / GC376 or Molnupiravir / GS-441524

Combinations of molnupiravir with GC376 or GS-441524 will be used more and more frequently, not only to synergy or complement their individual antiviral effects, but also as a way to prevent drug resistance. Medicinal cocktails have been very effective in preventing drug resistance in HIV / AIDS patients [11]. However, there is currently insufficient evidence on the safety and efficacy of the combination of molnupiravir with GC376 or GS-441524 as initial treatment for FIP.

Case studies

Rocky - DSH MN Neuro FIP

A 9-month-old neutered domestic shorthair cat obtained as a rescue kitten had several weeks of seizures with increasing frequency, ataxia and progressive paresis. The blood tests were unremarkable. FIP treatment was started at a dose of 15 mg / kg BID GS-441524, which decreased to SID for about a week. The cat showed improvement, seizures stopped, and mobility increased within 24 hours of starting treatment. Within 5 days of treatment, the cat was able to move again. However, approximately 2 weeks after the start of treatment, the cat experienced loss of vision, decreased mobility, recovery of seizures and difficulty swallowing. Dose adjustments of levetiracetam and prednisolone were made, as well as a change in the composition of GS-441524, followed by a temporary improvement in motility and swallowing and a reduction in seizures, but overall the cat's condition worsened. The dose of GS-441524 was gradually increased to 25 mg / kg, with little or no improvement. At this point, GS was taken orally at a dose of 25 mg / kg (estimated to be approximately 12.5 mg / kg) and within 3 days, the cat began to move, improved vision, and stopped seizures with increased energy and appetite. Improvement in cats continued for approximately 4 weeks with oral administration of GS-441524, then stopped for approximately 3 weeks before rapidly progressing paresis. Oral doses up to 30 mg / kg SC equivalent have been tested but have no effect. GS-441524 was then injected at a dose of 20 mg / kg and the cat was able to move again within 4 days with good appetite and energy. After 2 weeks, a dose of GC376 20 mg / kg BID was added to the dosing regimen. The cat terminated 6 weeks of the GS441524 and GC376 combination therapy and then discontinued the treatment. Although the cat has certain permanent neurological deficits, its condition is stable, it has good mobility, appetite and activity for 9 months after the end of antiviral treatment.

Rocky's video:

Bucky - DSH MN Neuro / Eyepiece FIP

A four-month-old neutered domestic shorthair cat obtained as a rescue kitten was presented with a monthly history of lethargy and a progressive history of ataxia, hind limb paresis, spades, uveitis, anisocoria, and urinary and stool incontinence. Blood tests were mostly uncommon, with the exception of mild hyperglobulinemia. The A / G ratio was 0.6. The cat was treated with 10 mg / kg GS-441524 SC SID for 3 weeks. Activity, mentation and uveitis improved within 72 hours of starting treatment. During the first 2 weeks, a slow improvement in mobility and eye symptoms was observed, but then a plateau was reached. After 3 weeks, the dose of GS-441524 was increased to 15 mg / kg GS-441524 SC SID due to persistent neurological and ocular deficits. In addition, enlargement of the left eye due to glaucoma was noted at this time and the eye continued to swell until it was removed at week 8 of treatment.
Due to persistent weakness / lack of pelvic coordination and increasing lethargy, dose GS-441524 was increased to 20 mg / kg SC SID [or equivalent oral dose] at week 9 and 20 mg / kg SC BID was added to the regimen a few days later. GC376. Significantly increased activity and willingness to jump on elevated surfaces occurred within 48 hours of starting GS376 treatment. The combination treatment of GS-441524 and GC376 was maintained for 8 weeks. The cat has residual incontinence problems after treatment, but is otherwise clinically normal 6 months after treatment.

Boris - Maine Coon MI wet eye FIP

The five-month-old intact (uncastrated) Maine Coon cat, obtained from the breeder, had lethargy, anorexia, abdominal ascites, cough, anemia and neutrophilia. No biochemical analysis was performed to establish the diagnosis. The cat was treated with 6 mg / kg GS-441524 SC SID for 8 weeks. After six weeks of treatment, X-rays revealed nodules in the lungs, and after 8 weeks, hyperglobulinemia persisted. The GS-441524 dose was then increased to 8 mg / kg SC SID for 4 weeks. There was little improvement in blood tests and X-rays and the dose of GS-441524 was increased to 12 mg / kg SC SID over 4 weeks, followed by an increase to 17 mg / kg over 11 weeks, 25 mg / kg over 4 weeks and 30 mg / kg for 4 weeks. After 25 weeks of treatment, ultrasound revealed pleural abnormalities on the left side and X-rays showed no improvement in the pulmonary nodules. In addition, uveitis and retinal detachment have been reported in the right eye. Pulmonary aspirates that showed FIP-compliant inflammation were collected. After 33 weeks of treatment, 20 mg / kg SC BID GC376 was added to the regimen and the combined treatment of GS-441524 and GC376 was continued for 12 weeks. Increased activity was noted over several days. Over the course of 5 weeks, the weight gain accelerated, the cough subsided and the energy level increased. Blood tests showed an improvement in the A / G ratio, and chest X-rays showed a reduction in the lungs. After 84 days of combination antiviral therapy, the A / G ratio was 0.85 and the cat appeared clinically normal. The cat is currently 3 months after treatment.


  1. Pedersen NC, Kim Y, Liu H, Galasiti Kankanamalage AC, Eckstrand C, Groutas WC, Bannasch M, Meadows JM, Chang KO. Efficacy of a 3C-like protease inhibitor in treating various forms of acquired feline infectious peritonitis. J Feline Med Surg. 2018; 20 (4): 378-392.
  2. Pedersen NC, Perron M, Bannasch M, Montgomery E, Murakami E, Liepnieks M, Liu H. efficacy and safety of the nucleoside analog GS-441524 for the treatment of cats with naturally occurring feline infectious peritonitis. J Feline Med Surg. 2019; 21 (4): 271-281.
  3. Perera KD, Rathnayake AD, Liu H, et al. Characterization of amino acid substitutions in feline coronavirus 3C-like protease from a cat with feline infectious peritonitis treated with a protease inhibitor. J. Vet Microbiol. 2019; 237: 108398. doi: 10.1016 / j.vetmic.2019.108398
  4. Agostini ML, Andres EL, Sims AC, et al. Coronavirus susceptibility to the antiviral remdesivir (GS5734) is mediated by the viral polymerase and the proofreading exoribonuclease. MBio 2018; 9. DOI: 10.1128 / mBio.00221-18.
  5. Pedersen NC. 2021. The neurological form of FIP and GS-441524 treatment.
  6. Pedersen NC. The long history of beta-d-n4-hyroxycytidine and its modern application to treatment of covid019 in people and FIP in cats. cats /.
  7. Agostini, ML et al. Small-molecule antiviral beta-dN (4) -hydroxycytidine inhibits a proofreading-intact coronavirus with a high genetic barrier to resistance. J. Virol. 2019; 93, e01348.
  8. Warren, TK et al. Therapeutic efficacy of the small molecule GS-5734 against Ebola virus in rhesus monkeys. Nature 2016; 531, 381–385.
  9. FIP Warriors CZ / SK - EIDD-2801 (Molnupiravir)
  10. Toots M, Yoon JJ, Cox RM, Hart M, Sticher ZM, Makhsous N, Plesker R, Barrena AH, Reddy PG, Mitchell DG, Shean RC, Bluemling GR, Kolykhalov AA, Greninger AL, Natchus MG, Painter GR, Plemper RK . Characterization of orally efficacious influenza drug with high resistance barrier in ferrets and human airway epithelia. Sci Transl Med. 2019; 11 (515): eaax5866.
  11. Zdanowicz MM. The pharmacology of HIV drug resistance. Am J Pharm Educ. 2006; 70 (5): 100.doi: 10.5688 / aj7005100
  12. Gandhi, S, Klein J, Robertson A, et al. De novo emergence of a remdesivir resistance mutation during treatment of persistent SARS-CoV-2 infection in an immunocompromised patient: A case report. medRxiv, 2021.11.08.21266069AID
Read "Alternative treatment of cats with FIP and natural or acquired resistance to GS-441524"

FIP treatment with subcutaneous remdesivir followed by GS-441524 oral tablets

Richard Malik DVSc PhD FACVS FASM Center for Veterinary Education, University of Sydney
Original article: Treatment of FIP in cats with subcutaneous remdesivir followed by oral GS-441524 tablets

Translator's note: The article contains information about the real content of GS-441524 in tablets. However, this content may not correspond to the "equivalent" amount of GS-441524 in tablets from other manufacturers, where the actual content of GS-441524 is always slightly higher due to the known reduced oral bioavailability of the drug. Therefore, it is not possible to easily and unambiguously compare the recommended dosage of GS-441524 from BOVA in Australia and in our country.


Infectious feline peritonitis (FIP) is an infectious disease, especially of young cats. It occurs when a feline enteric coronavirus that multiplies in the gut undergoes a critical mutation that changes its tissue tropism from enterocytes to macrophages. The FIP virus then circulates in the body in macrophages - this is the ultimate mechanism of the Trojan horse. This leads to disseminated infection and the development of fibrinoid necrotizing vasculitis and serositis due to the deposition of immune complexes consisting of feline antibodies and FIP viral antigens.

In general, there are two forms of FIP - effusive ("wet") FIP and non-effusive ("dry") FIP. The disease process itself can occur in the abdomen, thoracic cavity, pericardium, eyes or central nervous system. Combinations of dry and wet FIP with various tissues are not uncommon.

Until recently, the diagnosis of feline infectious peritonitis (FIP) was a death judgment for a feline patient. In recent years, however, this vision has been turned upside down as a result of the pioneering work of Professor Niels C. Pedersen and colleagues at UC Davis.

Over the last 12 months, many veterinarians in Australia have also successfully managed many cases of FIP using remdesivir and GS-441524.

Omega-interferon (Virbagen) and polyprenyl immunostimulant (PPI) were the first drugs used to treat FIP, and both had some effects in some patients. Omega interferon has been useful in cases of effusive ("wet") FIP, often combined with low-dose prednisolone according to the Ishid protocol, while PPI, pioneered by Al Legendre, has been more useful in cases of non-fusible FIP. In some cases, both drugs were used at the same time. The problem was that both forms of therapy were often expensive, especially when both drugs were used, so that although patients improved and could have transient clinical remissions during treatment, permanent clinical cures were rare. As a result, most veterinarians still considered the diagnosis of FIP a prelude to euthanasia.

That all changed a few years ago thanks to the culmination of FIP's lifelong research. Niels Pedersen. Niels is an amazing North American veterinarian of Danish descent. He grew up on a chicken farm and originally wanted to be a clinician for large animals, but with great foresight he decided on a scientific career. Shortly after graduating, he traveled to Canberry to the John Curtin School of Medical Research at ANU, where in the late 1960s he received a PhD in kidney transplant rejection immunology from Professor Bede Morris, using sheep as an experimental model to study lymphocyte kinetics.

When Niels returned to UC Davis, he focused on studying infections and immunity. Although he has contributed to a large number of topics in internal medicine and the genomics of dogs and cats, FIP has become his favorite disease due to its commonness and current complexity. His studies date from the 1980s, when he specialized in diagnostics, virology and pathogenesis, to the present, with an increasing focus on therapy.

Niels, in collaboration with colleagues from Kansas State University, has shown that a purposefully designed protease inhibitor GC-376 could prevent and cure experimentally induced FIP in laboratory cats.1,2 Field clinical trials with cats with naturally occurring disease have been disappointing, especially when cats have had an ocular form of FIP or CNS disease. He did not give up, so he switched to another drug - GS-4415243,4 - a nucleoside analogue developed by the North American pharmaceutical company Gilead. This molecule has been shown to be much more effective than GC-376 in the treatment of FIP, both in experimental infections and in spontaneous cases of FIP. Starting with pharmacokinetics and dose escalation studies using a wide range of clinical cases, Niels and colleagues found that the required dose depended on whether the patient had dry or wet FIP and whether the eye or central nervous system (CNS) was affected.5

Surprisingly, Gilead, the manufacturer who developed GS441524, has not yet shown interest in developing this molecule for the treatment of cats. To fill the gap for effective FIP therapy around the world, various laboratories in China and Eastern Europe have begun producing GS-441524 and selling it on the black market.

The wide availability of the GS-441524, often of high quality and initially very high price, provided dedicated owners with a way to save their cats with FIP. Studies by clinical pathologist Samantha Evans of Ohio State University have indicated a cure rate of approximately 80 % in the field. Until recently, the procurement of the drug was complicated and full of problems, which at some level were circumvented by various "FIP Warriors" groups on Facebook. Unfortunately for Australian cat lovers, APVMA and Vet Boards finally understood what was going on and the Border Force made it much more difficult to obtain GS-441524 and its safe import for veterinary use. Regulatory and Veterinary Committees' warnings against prosecutors were directed against veterinarians who allowed cats with FIP to be treated with black market drugs.

Ironically, the COVID 19 pandemic provided a new solution to this problem. Gilead developed remdesivir (GS-5734) as a drug for the treatment of hepatitis C, Ebola and human coronavirus disease. Remdesivir is a prodrug of GS-441524, which contains an additional chemical side chain (including a phosphate group) to improve intracellular penetration (Figure 1B). Remdesivir (as a product of Veklura) obtained a temporary marketing authorization (for two years) from TGA in July 2020 for the treatment of SARS-CoV-2 infections in human patients with COVID-19. This registration process would normally take several years, but the severity of the pandemic has accelerated this process, taking into account preliminary data from clinical trials. As remdesivir became a licensed human drug and Gilead licensed production worldwide, it meant more access to quality raw material. This circumvented the problems with the use of the drug purchased on the black market, as well as the problems of unknown purity and consistency of the product over time.

In 2020, the veterinary compounding company BOVA Australia provided reliable supplies of remdesivir in a suitable format for IV and subcutaneous application. Studies in Australia have determined that the shelf life after reconstitution exceeds 12 days and have confirmed in vitro efficacy against coronaviruses in tissue cultures. The analytical purity of the drug is regularly checked by HPLC. Over the past year, veterinarians in every Australian state have used remdesivir to treat cats with FIP. There have been a number of effusive and non-fusive cases, including some cats with ocular disorders (uveitis) and others with multifocal CNS disease. Based on treatment of approximately 500 cats treated between October 2020 and November 2021, remdesivir has been shown to be highly effective in managing FIP infections. It allows for a slightly simpler subcutaneous administration and the injection appears to be slightly less painful compared to GS-441524 and does not cause the local injection site reactions observed with GS-441524 injection. Remdesivir was originally used exclusively in Australia, although it has also been available in the UK from BOVA UK for the last 2 months.

The molecular weight of remdesivir is 603 g / mol, while the molecular weight of GS-441524 is 291 g / mol. This could suggest that treatment of cats with remdesivir requires approximately twice the dose of GS-441524, although this does not take into account the possible improvement in intracellular penetration of remdesivir into certain tissues compared to GS-441524. The proposed dose of remdesivir in human patients with COVID19 is 200 mg intravenously (IV), followed by 100 mg IV daily. For a 70 kg human patient, this represents a daily dose of 1.3 mg / kg, so using allometric scaling, a dose of 5-10 mg / kg per day was considered correct for a cat. However, our experience with the first 500 cases was that many cats eventually needed a higher dose of remdesivir for permanent cure, so we adjusted our recommended dosage upwards (see below). Remdesivir provides BOVA as a sterile 10 mg / ml solution ready for use in a 10 ml vial.

Figure 1. (A) BOVA Remdesivir reconstituted and ready for treatment. After reconstitution, the contents of the vial are stable for at least 120 days at 5 ° C - and the vial is usually consumed in 3-7 days. It is best to store the vial in the refrigerator. (B) The pathway that remdesivir travels intracellularly to activate as GS-441524.

At present, Australia and the United Kingdom are the only countries where remdesivir is readily available by prescription for veterinary use. However, veterinarians in India, New Zealand, South Africa and parts of Europe have also started using human medicine suppliers to access the medicine.


Figure 2: Amazingly comprehensive and practical overview of FIP diagnostics by Severine Tasker.

A complete differential diagnosis of FIP is beyond the scope of this article, but readers are strongly encouraged to read the excellent article by Séverine Tasker in the Journal of Feline Medicine & Surgery. 6

Although FIP can occur in cats of any age, most cases occur in kittens and cats less than 3 years of age. Persistent and often high fever that does not respond to antibiotic therapy (and often NSAIDs) is a common finding, as is increased plasma total protein levels due to elevated globulin concentrations (diffuse gammopathy in serum electrophoresis). In effusive or "wet" FIP, the albumin to globulin ratio may drop to <0.45. Acute phase reactants such as serum amyloid A and α1-acid glycoprotein tend to be markedly elevated. Many cats with FIP also exhibit secondary immune-mediated hemolytic anemia, increased AST and ALT activities, and jaundice.

Diagnostic imaging is crucial for early diagnosis, which has been greatly facilitated by the introduction of digital radiology and the widespread availability of diagnostic ultrasound in small animal practice. Pleural effusion is readily recognizable from chest X-rays, while abdominal effusion is best detected by ultrasound (Figure 3), especially if high frequency probes are available. It is worth noting that in some cases, the fluid pockets may be focal and localized. Often there is some fluid around the kidney under the kidney sheath, kittens may have scrotal edema, while in rare cases the discharge is limited to the pericardial sac. But the key is - to look for (i) effusion in any body cavity, (ii) granulomas in the kidneys, liver or lungs, (iii) enlarged intra-abdominal and mesenteric lymph nodes (Figure 5) or marked thickening of the iliac-ecological area (f focal FIP ’) ( Figure 5). Chest X-rays after drainage of pleural effusion may show changes corresponding to viral pneumonia.

Figure 2: (A) Ultrasound of the abdomen showing abundant highly echogenic fluid (fibrin fibers) in cats with high protein ascites due to effusive FIP. (B) The fusion contains a viscous yellow to straw-colored liquid. (C) An X-ray of the abdomen with the appearance of cut glass indicating fluid in the abdomen.

If you see an effusion - puncture - because fluid is the best diagnostic sample.

Figure 3: Marked mesenteric lymphadenomegaly in a cat with dry FIP.

A fluid with a high protein content, often yellow to straw in color, is characteristic (Figure 3B). If you see granuloma in the organ or if the lymph nodes are clearly enlarged - do FNA (thin needle aspiration biopsy), apply a smear, use RapidDiff staining and look for neutrophils and macrophages (pyogranulomatous inflammation) without visible infectious agents (Figure 4). The two diseases most commonly confused with FIP in adult cats are lymphoma and some types of lymphocytic cholangitis (associated with high protein ascites).

Figure 4: RapidDiff stained aspirate with a thin needle from the mesenteric lymph nodes of a 4-year-old oriental cat with dry FIP. Distinctive macrophages are the key to cytological diagnosis. Photo courtesy of Trish Martin.

Of course, effusive disease is much easier to diagnose because ascitic, pericardial or pleural fluid provides a suitable sample that can be examined cytologically, by fluid analysis and immunofluorescence (IFA) for FIP antigen, or reverse transcriptase PCR to detect FIP nucleic acid. IFA is performed at VPDS, B14, University of Sydney (via Vetnostics, QML, ASAP, VetPath, Gribbles or IDEXX). However, it is usually the cheapest way to send the sample directly to the university laboratory.

Dry FIP is more problematic because it usually requires a thin-needle aspiration biopsy of pyogranulomatous lesions in the liver, kidneys, or abdominal lymph nodes. Occasionally, cases of wet FIP may show fluid samples that are negative for IFA and / or PCR testing, but the patient is still likely to have FIP, which is reflected in a favorable response to remdesivir or GS-441524 treatment.


Since October 2020, we have been treating cats with FIP with remdesivir (IV and SCI) and more recently with GS-441524 (oral), so our protocols are constantly evolving with experience. About 500 cats have been treated so far. We try to avoid being too prescriptive in our recommendations, as we suspect that there is no one-size-fits-all protocol and that each case presents unique circumstances, including patient size, whether the cat is still "happy" and reasonably , or is depressed and dehydrated. An important factor is the emotional and financial commitment of the owner. A key feature that needs to be mentioned is that both drugs are very safe, even in sick cats and kittens.

Note that the following recommended doses are higher than those originally recommended a year ago. Although lower doses worked in many patients, we found that this was often the wrong economic consideration, as disease recurrence at the end of treatment and the development of viral resistance during treatment appear to be related to insufficient initial dosing. So we have learned to be more aggressive from the beginning, which is cheaper in the long run (ie 2nd therapy is not required)

Our greatest experience is with remdesivir. This drug is expensive and the owner has to commit to a costly treatment process that takes 3 months. For most clients, this represents an emotional and financial burden. My view is that in many cases it is better to spend money on antiviral therapy as such than on extensive diagnostics and monitoring.

Figure 5: Significant thickening of the ilico-ecological area of the Devon Rex cat with the so-called "focal FIP", a common form of non-fusive FIP. Photo courtesy of Penny Tisdall.

One of the approaches in newly diagnosed cats with severe disease is hospitalization of cats during the first 3-4 days of treatment. Patients begin treatment with remdesivir when receiving IV fluid therapy (typically 2-4 ml / kg / hr; first day Hartmann's solution or Plasmalyte followed by 0.45 % NaCl and 2.5 % dextrose containing 20 mmol KCl / l). On the 1st day of hospitalization, remdesivir is administered in a high dose intravenously (10-15 mg / kg diluted in 10 ml with saline and is given SLOWLY for 20-30 minutes or longer, manually or by infusion pump; in human patients, administration lasts 2 hours. ) to achieve an increased starting dose of drug distribution volume. This achieves fast antiviral efficacy. In cases with CNS disease, we recommend a daily IV dose of 20 mg / kg. Many cats may appear slightly depressed several hours after IV remdesivir infusion. In human patients, remdesivir may cause infusion-related reactions, including low blood pressure, nausea, vomiting, sweating or chills, but we have not observed these events in our feline patients.

The advantage of starting treatment intravenously is that dehydration, if present, is corrected and you have IV access if you need to take other medicines (eg anticonvulsants, corticosteroids). Importantly, once an IV catheter is inserted, daily injections of remdesivir do not cause any pain or discomfort. However, if the cat eats and is diagnosed in the early stages of the disease, then IV therapy is not required and the same doses can be given subcutaneously, saving a lot of money.

FIP cats treated with remdesivir typically improve significantly during the first 2-3 days. However, we found that cases of effusion, and especially those that resulted in pleural effusion prior to treatment, should be closely monitored, as the combination of the antiviral effect of remdesivir and a higher than maintenance dose of crystalloids may lead to transient worsening of pleural effusion. This requires drainage twice a day using a 19G butterfly needle (1.1 mm - cream color) and a 3-way stopcock (ideally using an ultrasonic guide to find the best place to insert the needle). These "secondary" pleural effusions can be fatal if not detected in time and appear to occur in approximately 1 in 10 cases of effusive FIPs treated with remdesivir.

Another problem that occasionally occurs at this time is the development of neurological symptoms, including seizures. Our view is that this is not the effect of the drug as such, but rather the unmasking of the subclinical CNS FIP. Such cats require careful monitoring, while the development of seizures requires the use of anticonvulsant drugs such as midazolam (0.3 mg / kg IV), alfaxane or propofol (administered IV to be effective), followed by levetiracetam (Keppra) (10- 20 mg / kg, PO every 8 hours). Phenobarbitone is a reliable anticonvulsant, but it tends to increase the metabolism of many drugs, and levetiracetam is probably safer until we better understand the pharmacokinetics and metabolism of remdesivir and GS-441524. Some doctors also administer dexamethasone or prednisolone as a single treatment to relieve CNS inflammation.

Although advocating initial IV therapy for the most severe cases of FIP, cats and kittens that are still "happy" and eating do not require IV therapy at first and may instead begin subcutaneous injections at 10-12 mg / kg / day (20 mg / kg in CNS diseases). This is, of course, much cheaper because cats or kittens do not have to be placed in an infusion pump and hospitalized in a stressful environment. For clients who have financial limits, this may be a more appropriate way to start therapy. Some skilled colleagues, such as Jim Euclid, have developed a hybrid approach where kittens receive subcutaneous fluids daily as a bolus with injected remdesivir.

The cats were then given continuous subcutaneous injections of remdesivir. It originally took 84 days, and such cases accounted for most of the cases we have dealt with so far. Recently, we have been using aggressive IV / SCI remdesivir for initial therapy, and then cats are switching to oral GS-441524 for 10 weeks of consolidation therapy.

After the initial use of lower doses, which were not successful in every patient, we now use the following treatment protocols:

  • for cats with wet FIP: 10-12 mg / kg once daily (SID) for 2 weeks
  • for cats with severe eye impairment: 15 mg / kg SID by subcutaneous injection (SCI) for 2 weeks; Cats with severe uveitis should also be given topical corticosteroids (Before Forte or Maxidex) for 2-3 days (no longer!) and atropine eye ointment.
  • for cats with neurological FIP with CNS symptoms: administer 20 mg / kg SID SCI for 2-4 weeks. 5

It is important that owners are properly instructed on how to optimally administer daily injections. Cats will perceive the injection as less painful if the remdesivir solution in the syringe is allowed to warm to room temperature instead of being refrigerated. In addition, if you teach them simple tasks such as using a new needle when injecting (ie use a needle other than the one used to draw the medicine from the vial) and using 21G (0.8mm - green) or 23G diameter needles (0.6mm - blue), injections will be more tolerable. Although 21G needles are larger, some cats may have the advantage of injecting faster. Alternatively, for simplicity, veterinarians can prepare injections for the whole week, which they will keep in the refrigerator, and will give a new injection every day.

For cats that continue to perceive SC injections as painful, we used gabapentin orally (50 to 100 mg per cat) and / or transmucosally or SC administered buprenorphine 30-60 minutes before sedation / analgesia injection. The area to be injected can also be trimmed so that a topical EMLA cream can be applied 30 minutes before the injection. BOVA produces a faster-acting local anesthetic gel that may be useful in some patients. In exceptional cases, we inserted a cephalic catheter every 4-5 days so that owners could administer IV therapy instead of SC injections. Injection site reactions reported with GS-441524 injected abroad do not appear to occur with remdesivir.

After 2-4 weeks of taking remdesivir and after the abdominal fluid has disappeared and the ocular and CNS symptoms have improved or disappeared, we are now proposing a switch to GS-441524 tablets. This is done for 3 reasons: (i) it reduces costs (ii) eliminates the pain problem of SC injections (iii) in some patients it is more effective. Remdesivir injections are probably more reliable than oral GS-441524, and in the worst cases, you might choose to give them for 4 weeks, but for most cats, 2 weeks and comfort and lower oral formulation costs outperform everything else.

The use of GS-441524 tablets is relatively new in Australia, but is widely used overseas. The recommended oral dose of GS441524 is usually the same as the SCI / IV remdesivir dose: wet cases of FIP receive 10-12 mg / kg PO SID, ocular cases 15 mg / kg PO SID and CNS cases 20 mg / kg (or higher). GS-441524 is more economical and even safer than remdesivir. In CNS cases where high doses are administered, it is probably best to administer 10 mg / kg PO every 12 hours (BID) to circumvent the "ceiling" effect referred to in the limited absorption of high doses.

Figure 6. Focal dry FIP with pyogranulomatous inflammation of intra-abdominal lymph nodes. Instead of exploratory laparotomy, lymph node biopsy, histology, and immunohistology, 3 days of remdesivir IV treatment may be more cost-effective if FIP is highly suspected. Enlarged lymph node FNA is probably an ideal diagnostic option for physicians with this set of skills.

Why are the dosages about the same? At mg / kg, GS441524 has twice as many active molecules as remdesivir (due to the difference in their molecular weight), but the bioavailability of GS-441524 is only 50 % (only half of what is given is absorbed, and this is affected by feeding and also the effect of the ceiling dose) - so these two factors cancel each other out.

We recommend that GS-441524 tablets be given with a small treat to mask the tablet, with the main meal being served 1 hour later. The tablets provided by BOVA are 50 mg tuna-flavored tablets, with four score lines, so they can be divided into halves or even quarters.

In situations where owners cannot afford full treatment, we use mefloquine (Lariam; 5 mg / kg orally once daily in capsules or 62.5 mg twice a week) after initial treatment with remdesivir / GS-441524.

Phillip McDonagh, Jacqui Norris, Merran Govendir and colleagues at the Sydney School of Veterinary Science have shown that mefloquine has an antiviral effect. 7 This is probably due to the fact that mefloquine usurps the biochemical intracellular pathways used by the FIP virus, a mechanism that has recently been demonstrated with clofazimine. 8 (anti-leprosy medicine), and several other medicines. In several cats, where owners could not afford a complete treatment with remdesivir, mefloquine proved to be effective in reaching the limit of clinical cure.

The main advantage of buying remdesivir and GS-441524 from BOVA for the treatment of FIP cases is that the products we use are subject to quality control. It's just a prescription with the client's name and address, the patient's name and the dose to be given, and the compounder can usually deliver the vials or tablets to any veterinarian in Australia within 24-48 hours.

At present, the price is 100 mg of vials of remdesivir 250$ plus GST and postage (the total price is usually about 280$). GS-441524 is sold in packs of 10 tablets for 600$ plus shipping and handling. By purchasing more vials and tablets at the same time, of course, postage and handling fees will be reduced. We believe that most owners will feel much more comfortable getting a product from a well-known Australian company than sending money overseas and hoping that drugs of unknown quality on the black market will reach Australia safely without being detained by customs.

There is no reason why a well-motivated veterinarian would not be able to handle these cases in his own practice. This is often more convenient for the owner, especially if they struggle with daily injections and need a practice near them.

Figure 7: Gs-441524 tablets from BOVA Australia. They are tuna flavored. They can be divided into halves or even quarters. MUCH EASIER than injections for most cats. Less stress and less cost.

Veterinarians who wish to explore this option or have general questions about FIP case management may email Sally Coggins (, Richard Malik (, David Hughes (, Grette Howard ( or Professor Jacqui Norris (, for advice on diagnosis or treatment. Many Australian veterinarians interested in FIP have gained considerable expertise in the management of these cases. For example, Andrew Spanner in Adelaide treated more than 20 cases with excellent results. Thus, there are already many feline medicine physicians and internal medicine specialists with experience in the treatment of FIP, and so veterinarians who are hesitant to treat their own cases have the opportunity to recommend these specialists to their clients.

Physicians who accept FIP cases from GPs include: QLD Rhett Marshall, Marcus Gunew, Alison Jukes, Rachel Korman; NSW Katherine Briscoe, Michael Linton, Randolph Baral, Melissa Catt; VIC - Carolyn O'Brien, Keshuan Chow, Amy Lingard; WA-Martine Van Boeijen and Murdoch University Veterinary Hospital; TAS Moira van Dorsselaer.

All of these doctors (and probably even more we don't know about) are happy to accept cases for diagnosis and therapy. Everyone is probably happy to discuss case management with you.

Figure 8: Bengal kitten with CNS and ocular FIP (A: before) and (B: after) after Remdesivira. This cat also had pulmonary granulomas.

Sally Coggins, working with Lara Boland, Emily Pritchard, Associate Professor Mary Thompson and Professor Jacqui Norris at the Sydney School of Veterinary Science, is interested in treating cases with comprehensive diagnosis and free monitoring. It will be part of Sally's doctoral program, so you will help her advance in her studies by sending her cases. We hope that through these studies, we will get a better idea of how quickly cats respond and when exactly treatment can be safely stopped. Owners will only have to pay for remdesivir and GS-441524 for therapy. This group is also interested in treating cases with interferon-omega and mefloquine.

In most cases, FIP is doing very well with GS-441524 or remdesivir. Niels Pedersen has gathered an amazing resource for veterinarians interested in FIP case management - - pedersen - research / . The site also provides some recommendations on how to monitor cats during treatment. I'm not very protocol-oriented, so the key things for me to keep track of are appetite, attitude, activity levels, and changes in body weight and fitness over time. Most physicians like to monitor serum hematology and biochemistry every month to ensure that all measurable abnormalities improve, although this can be stressful for the patient and increase treatment costs. The trade-off is taking a few drops of blood to monitor PCV, total plasma protein (TPP) using refractometry, and plasma color to determine if anemia is improving, jaundice is subsiding, and gamma globulin levels are lowering, resulting in lower TPP.

Do not worry about transient increases in globulin levels at the start of treatment; when high protein effusions are absorbed, a lot of immunoglobulins enter the patient's plasma. This phenomenon may be common until the 8th week of treatment, but disappears by the 12th week.

Figure 9: Transverse plane MRI image in contrast to T1 weighting. Note: dilatation of the lateral ventricles with very slight emphasis on the ependymal lining (orange arrows). Image courtesy of Christine Thomas.

And what about a kitten with multifocal CNS disease, where FIP CNS is the most likely cause of clinical symptoms? The traditional approach is serology (to rule out cryptococcosis and toxoplasmosis), a good history and thiamine test to rule out vitamin B1 deficiency, followed by MRI scans (Figure 9) and CSF collection for fluid analysis and multiplex neuro-qPCR analysis). This approach is very expensive and there is also a certain risk of anesthesia and especially CSF collection. We found that a 3-5 day intravenous or sc. Remdesivir therapy can be used as a therapeutic test in cats with probable CNS FIP and is a cost-effective alternative to complete diagnostic processing, which can cost 3-5000$ or more.

Similarly, if exploratory laparotomy, abnormal tissue biopsy, histology, and immunohistochemistry for FIP antigen are used to diagnose dry intra-abdominal FIP versus 3-5 days of treatment with remdesivir or GS-441524, a drug test may be considered, which is a better choice from in terms of patient well-being and reduced costs. Most cats with non-fusive FIP experience rapid improvement with antiviral therapy, with normalized fever, improved appetite, and better overall attitude within 2 to 3 days. If the patient does not respond to antiviral therapy, then exploratory laparotomy and representative organ biopsy are reasonable, as the main differential diagnoses are lymphoma and lymphocytic cholangitis.

This is a matter of personal approach for each doctor. FNA for cytological and sometimes immunohistochemical examination or PCR is a convincing non-invasive option where this expertise is available, but sometimes it does not give a definitive answer. Some veterinarians insist on tissue diagnosis and positive immunohistology or PCR in each patient, while others would like to "treat treatable" with a 3-5-day remdesivir / GS-441524 application and proceed to exploratory laparotomy only when there is no clear response to therapy.

It is incredibly satisfying to see the transformation of cats and kittens, which are not well, into normal and happy cats. It's really something that will lift your spirits as a doctor. It's good science and good veterinary medicine!


In the past, the diagnosis of FIP was an intellectual exercise so that we could end the suffering of a cat or kitten with the certainty of an accurate diagnosis. Now, thanks to FIP's lifelong study, Dr. Niels Pedersen, we are able to successfully treat perhaps 80 % or more cats with FIP if the client has sufficient funds. It is too early to predict whether or how many will be repeated later.

There is a need for intensive study in diagnosis and case management, but with the necessary effort, a good veterinarian should be able to work with a determined owner to achieve a clinical cure. The most important thing is not to put too many obstacles in the way of the dedicated owner and support him during the 12-week marathon treatment course by helping him find the best way to treat his patient. This may include sedative / analgesic treatment to help the cat improve controllability and prevent discomfort when the client brings their cat to the clinic daily for remdesivir injections or switching to GS-441524 tablets when the stress from the injections is too great for the owner. It is important to go a long way and a payment plan can be provided that will allow determined clients to improve the affordability of treatment.

Finally, the impact of COVID-19 on coronavirus research has been indeed profound, and several very promising drugs are under development, such as molnupiravir from Merck and another oral drug from Pfizer.


2-step approach to therapy


IV / SC injections of Remdesivir

  • For cats with wet FIP: 10-12 mg / kg remdesivir by subcutaneous injection (SCI) once daily (SID) for 2 weeks
  • For cats with eye: 15 mg / kg SID remdesivir SCI for 2 weeks
  • For cats with neurological symptoms of FIP and CNS: remdesivir 20 mg / kg SID for 2 weeks


Switch to GS-441524 tablets after 2 weeks of remdesivir injection

  • For cats with wet FIP: 10-12 mg / kg GS-441524 oral SID for 10 weeks
  • For cats with eye impairment: 15 mg / kg SID GS-441524 oral SID for 10 weeks
  • For cats with neurological symptoms of FIP and CNS: GS-441524 10 mg / kg oral BID (20 mg / kg / day) for 10 weeks


  1. Kim, Y .; Liu, H .; Galasiti Kankanamalage, AC; Weerasekara, S .; Hua, DH; Groutas, WC; Chang, KO; Pedersen, NC Reversal of the progression of fatal coronavirus infection in cats by a broad-spectrum coronavirus protease inhibitor. PLoS Pathog. 2016, 12, e1005531.
  2. Pedersen, NC; Kim, Y .; Liu, H .; Galasiti Kankanamalage, AC; Eckstrand, C .; Groutas, WC; Bannasch, M .; Meadows, JM; Chang, KO Efficacy of a 3C-like protease inhibitor in treating various forms of acquired feline infectious peritonitis. J. Feline Med. Surg. 2018, 20, 378–392.
  3. Murphy, BG; Perron, M .; Murakami, E .; Bauer, K .; Park, Y .; Eckstrand, C .; Liepnieks, M .; Pedersen, NC The nucleoside analog GS-441524 strongly inhibits feline infectious peritonitis (FIP) virus in tissue culture and experimental cat infection studies. Vet. Microbiol. 2018, 219, 226–233.
  4. Pedersen, NC; Perron, M .; Bannasch, M .; Montgomery, E .; Murakami,
    E .; Liepnieks, M .; Liu, H. Efficacy, and safety of the nucleoside analog GS441524 for treatment of cats with naturally occurring feline infectious peritonitis. J. Feline Med. Surg. 2019, 21, 271–281.
  5. Dickinson PJ, Bannasch M, Thomasy SM, et al. Antiviral treatment using the adenosine nucleoside analogue GS-441524 in cats with clinically diagnosed neurological feline infectious peritonitis. Journal of Veterinary Internal Medicine. 2020. doi: 10.1111 / jvim.15780.
  6. Tasker S. Diagnosis of feline infectious peritonitis: Update on evidence supporting available tests. Journal of Feline Medicine and Surgery.
    2018; 20 (3): 228-243. doi: 10.1177 / 1098612X18758592
  7. McDonagh, P .; Sheehy, PA; Norris, JM Identification, and characterization of small molecule inhibitors of feline coronavirus replication. Vet. Microbiol. 2014, 174, 438–447.
  8. Yuan, S., Yin, X., Meng, X. et al. Clofazimine broadly inhibits coronaviruses including SARS-CoV-2. Nature (2021).
  9. - THE BEST resource on the internet or anywhere for FIP.
Figure 10: Two cats with dry FIP after successful therapy. As an avid young veterinarian wrote to me not so long ago - "that's why I did science!"


2 kg kitten with wet FIP
4 × 100 mg remdesivir vials - 1000$
35 × 50 mg tablets GS-441524 - 2100$
Manipulation and GST - 30$ plus 310$ = 340$
A total of 3440$, approximately 290$ per week for 12 weeks

4 kg cat with dry FIP
7 × 100 mg remdesivir vials - 1750$
70 × 50 mg tablets GS-441524 - 4200$
Handling and GST 30$ plus 600$
A total of 6550$, about 545$ per week for 12 weeks

Figure 11: Two siblings who developed FIP and were successfully treated with remdesivir and GS441524.
Read "FIP treatment with subcutaneous remdesivir followed by oral tablets GS-441524"

The long history of Beta-d-N4-hydroxycytidine and its modern application to treatment of Covid-19 in people and FIP in cats.

Niels C. Pedersen DVM, PhD
Original article: The long history of Beta-d-N4-hydroxycytidine and its modern application to treatment of Covid-19 in people and FIP in cats.

Beta-d-N4-hydroxycytidine is a small molecule (nucleoside) that was studied in the late 1970s in the former Soviet Union as part of biological weapons research [2]. The weaponization of diseases such as smallpox was a worldwide threat, but the danger of using the smallpox virus for this purpose was too great. Smallpox was eradicated from the world, virtually all stocks were destroyed and further research was banned. This led the US and the Soviet Union to research other RNA viruses as biological weapons and antivirals to defend against them. The Venezuelan equine encephalomyelitis virus (VEEV) was one of the first viruses to be seriously considered for use as a biological weapon. [3]. VEEV is transmitted to humans by mosquito bites and causes high fever, headaches and encephalitis with swelling that can be fatal. Beta-d-N4-hydroxycytidine has been found to inhibit not only VEEV replication but also a wide range of alphaviruses, including Ebola, chikungunya, influenza virus, norovirus, bovine diarrhea virus, hepatitis C virus and respiratory syncytial virus. [3-8]. The first reports of an inhibitory effect of beta-d-N4-hydroxycytidine on human coronavirus NL63 date back to 2006 [9]. Recent studies have confirmed its inhibitory effect on a wide range of human and animal coronaviruses [8].

An important part of the recent history of beta-d-N4-hydroxycytidine is associated with the Emory Institute for Drug Development (EIDD) [1], where he received the experimental designation EIDD-1931. The US government has provided significant financial support for the study of antivirals against alphaviruses in institutions such as Emory since 2004. [10]. In 2014, the Defense Threat Reduction Agency provided institutional support to find an antiviral compound against VEEV and other alpha-coronaviruses. "N4-Hydroxycytidine and its derivatives and antiviral uses" were included in U.S. Patent Application 2016/106050 A1 of 2016 [11]. Additional funding in 2019 was provided by the National Institute of Allergies and Infections for fellowship of the esterified beta-d-N4-hydroxycytidine precursor (EIDD-2801) for the treatment of influenza. [10]. The stated purpose of the chemical changes of EIDD-2801 was to increase its oral bioavailability, which would ultimately allow beta-d-N4-hydroxycytidine to be administered as pills and not as injections. In 2019/2020, the focus of research changed from influenza to SARS-CoV-2 [2]. The commercialization of EIDD-2801 was entrusted to Emory's Ridgeway Biotherapeutics subsidiary, which subsequently worked with Merck on a lengthy and costly FDA approval process. The current version of EIDD-2081 for field testing was named Molnupiravir.

Beta-d-N4-hydroxycytidine, the active substance in Molnupiravir, exists in two forms as tautomers. In one form, it acts as a cytidine with a single bond between the carbon and the N-OH group. In its other form, which mimics uridine, it has an oxime with a double bond between the carbon and the N-OH group. In the presence of beta-d-N4-hydroxycytidine, viral RNA-dependent RNA polymerase reads it as uridine instead of cytidine and inserts adenosine instead of guanosine. Switching between forms causes inconsistencies during transcription, which results in numerous mutations in the viral genome and a cessation of viral replication. [8].

Merck's commitment to conditional and full FDA approval of Molnuparivir continues. In its statement, Merck stated: [12] "In anticipation of the results of the MOVe-OUT program, Merck manufactures Molnupiravir at its own risk. Merck expects to produce 10 million therapeutic doses by the end of 2021, with more expected to be produced in 2022. Merck is committed to providing timely access to Molnupiravir worldwide, if authorized or approved, and plans to introduce access to tiered prices based on World Bank admission criteria that reflect countries' relative ability to fund their pandemic health response. As part of its commitment to extend the global approach, Merck has previously announced that it has entered into non-exclusive voluntary licensing agreements for Molnupiravir with established generic manufacturers to accelerate the availability of Molnupiravir in more than 100 low and middle income countries (LMICs) following approval or emergency approval by local regulatory agencies. . " This "generosity" is unlikely to apply to use in animals.

Drugs to inhibit the current Covid-19 pandemic have been the subject of accelerated field trials in the last two years, and one of them, Remdesivir, has been approved for use in hospitalized patients in record time. Last year, Molnupiravir was submitted for conditional approval as an oral medicinal product for home treatment of the infection at an early stage. [12]. However, anti-coronavirus compounds have been developed previously for another common and serious feline disease, feline infectious peritonitis (FIP). These drugs include a protease inhibitor (GC376) [13] and an RNA-dependent RNA polymerase inhibitor (GS-441524), which is an active ingredient of Remdesivir [14]. The success of antiviral drugs in the treatment of FIP prompted a recent study by EIDD-1931 and EIDD-2801 for their ability to inhibit FIPV in tissue cultures. [15]. The effective EC50 concentrations for EIDD-1931 against FIPV are 0.09 μM, EIDD-2801 0.4 μM and GS441524 0.66 μM [15]. The percentage of cytotoxicity at 100 μM is 2.8, 3.8 and 0, respectively. Therefore, EIDD-1931 and EIDD-2801 are slightly more effective at inhibiting viruses, but also more cytotoxic than GS-441524. These laboratory studies suggest that EIDD-1931 and EIDD-2801 are excellent candidates for the treatment of FIP.

Although EIDD-1931 and EIDD-2801 are a great promise for the treatment of FIP, there are several obstacles that will make the legal use of these compounds unlikely in the near future. GS-441524, the active form of Remdesivir and patented by Gilead Sciences, was investigated for use in cats with FIP shortly before the Covid-19 pandemic. FIP research [14] therefore stimulated the potential use of Remdesivir against Ebola and not SARS-like coronavirus [14]. Although these studies were conducted in collaboration with scientists from Gilead Sciences, the company refused to grant GS-441524 rights to treatment in animals as soon as it became clear that there was a much larger market for Covid-19 in humans. [16]. Similarly, my attempts over the past 2-3 years at Emory, Ridgeback Biotherapeutics, and Merck Veterinary Division to investigate EIDD-1931 and EIDD2801 for the treatment of FIP in cats have either remained unanswered or rejected, no doubt for similar reasons why Gilead refused to grant rights for GS-441524. However, the great worldwide need for FIP treatment quickly supported the unapproved market for GS-441524 from China. The same need to treat FIP has recently aroused interest in Molnupiravir, also from China.

Situation with EIDD-1931 vs. EIDD-2801 / Molnupiravir and GS-441524 vs. Remdesivir raises the question of why some medicines are being converted to prodrugs for marketing purposes [17]. Remdesivir was reportedly esterified to increase antiviral activity, although studies in cats showed that GS-441524 and Remdesivir had similar viral inhibitory activity in tissue culture. [18]. However, Remdesivir was found to be poorly absorbed by the oral route and was therefore conditionally approved for injectable use only. EIDD-2801 was designed to increase the oral absorption of EIDD-1931, although previous research has shown that EIDD-1931 is well absorbed orally without esterification. [6]. The motives for the commercialization of Remdesivir instead of GS-441524 for human use have been scientifically questioned, as it appears to be better in several ways without further modification. [17]. Why EIDD-2801 was chosen for commercialization, when EIDD-1931 would be cheaper, 4 times more effective against viruses and one third less toxic than EIDD-2801 [15]? The strength of patent rights and the longevity of patents may be more important factors in these decisions. [16,17,19].

One of the problems in the treatment of FIP in cats is the blood-eye and blood-brain barriers, which become very important when the disease affects the eyes and / or the brain. [13, 14, 20]. This problem has been largely overcome in the treatment of ocular and neurological forms of FIP with GS-441524 by gradually increasing the dose to increase blood levels and thus drug concentrations in the ventricular fluid and / or brain. [20]. GC376, one of the most effective antivirals against FIP virus in culture [17], is not effective against ocular and neurological FIP due to the inability to get enough drug to these sites, even if the dose is increased several times[14]. Fortunately, it appears that EIDD-1931 can reach effective levels in the brain, as indicated by studies in horses with VEEV infection. [3]. Drug resistance is another problem that now occurs in some cats treated with GS-441524, especially in individuals with the neurological form of FIP. Long treatment procedures and difficulties in transporting enough drug to the brain support the development of drug resistance.

The short-term and long-term toxic effects of the drug candidate on the test person or animal are crucial. GS-441524 showed lower toxicity in cell cultures than GC376, EIDD-1931 and EIDD-2801 [15]. Most important, however, is the toxicity that manifests itself in vivo. GC376 is one of the drugs with the highest coronavirus inhibitory effect [15], but slows the development of adult teeth when given to young kittens [13]. No serious toxicity was observed during nearly three years of field use of GS-441524, reflecting the complete absence of cytotoxic effects in vitro at concentrations up to 400 µM. [18]. However, EIDD-1931 and EIDD-2801 show significant cytotoxicity at 100 μM [15]. Therefore, the ability of EIDD-1931 to make fatal mutations in RNA has been raising a number of questions for some time. [8, 21, 22]. This was the main reason why the application for the treatment of diseases was still delayed. However, the current recommended duration of treatment with Covid-19 Molnupiravir is only 5 days at the initial stage of treatment. [10]. However, the recommended duration of FIP treatment with GS-441524 is 12 weeks [14], which represents a much longer time for the manifestation of toxicity. Therefore, close observation of cats during treatment with EIDD-1931 or EIDD-2801, whether short-term or long-term, will be important.

All existing antiviral drugs have led to the development of drug resistance through mutations in the viral genome. Although Remdesivir appears to be less susceptible to such mutations compared to drugs used in viral diseases such as HIV / AIDS, resistance is well documented. [23-25]. Resistance to GS-441524 in cats treated for FIP was observed at a higher frequency, especially in cats with neurological FIP, where it is more difficult to deliver sufficient drug to the brain [13, 14, 20]. Resistance to GS-441524 in cats is also likely to be a major problem, as cats with FIP are often treated for 12 weeks or longer, while Remdesivir (and Molnupiravir) are recommended for only five days during the initial viremic stage of Covid-19. [16]. The problem of drug resistance in HIV / AIDS treatment is effectively addressed by using a cocktail of different drugs simultaneously with different resistance profiles. Mutants resistant to one drug will immediately inhibit other drugs, thus preventing their positive selection during treatment. Inhibition of resistance is particularly strong when the two drugs attack different proteins involved in virus replication. For example, GC376 is a protease inhibitor [13], while GS-441524 acts on an RNA-dependent RNA polymerase [18]. However, GC376 is not as well absorbed across the blood-brain barrier. Although the necessary research has not yet been performed, there appears to be no cross-resistance between GS-441524 and Molnupiravir and is as effective as GS-441524 in crossing the blood-brain barrier. [3]. This makes Molnupiravir (or 5-hydroxycytidine) an important contribution to the future treatment of FIP.

As expected, Molnupiravir has recently been tested on cats with FIP by at least one Chinese retailer, GS-441524, and preliminary results are available on the FIP Warriors website. [26]. Field trials included 286 cats with various forms of naturally occurring FIP observed at pet clinics in the United States, the United Kingdom, Italy, Germany, France, Japan, Romania, Turkey, and China. The 286 cats that participated in the study, including seven cats with ocular (n = 2) and neurological (n = 5) FIP, did not die. Twenty-eight of these cats were cured after 4-6 weeks of treatment and 258 after 8 weeks. All treated cats were healthy after 3-5 months, a period during which relapses would be expected to relapse unsuccessfully. These data provide convincing evidence of the safety and efficacy of Molnupiravir in cats with various forms of FIP. However, we hope that this field study will be written in manuscript form, submitted for review and published. Either way, Molnupiravir is already marketed to owners of cats with FIP. At least one other major retailer of GS-441524 is also interested in using Molnupiravir for FIP, indicating a demand for additional antiviral drugs for cats with FIP.

Safe and effective dosing for Molnupiravir in cats with FIP has not been published. However, at least one vendor from China provided certain pharmacokinetic and field test data for Molnuparivir in cats with naturally occurring FIP in a leaflet for the product Hero-2081. [26]. However, this information does not clearly indicate the amount of Molnupiravir in one of their "50 mg tablets" and the actual dosing interval (q12h or q24h?). Fortunately, the estimated starting dose of molnupiravir for cats with FIP can be obtained from published in vitro cell culture studies of EIDD-1931 and EIDD-2801. [15] and laboratory and field studies GS-441524 [14,18]. Molnupiravir (EIDD-2801) has an EC50 of 0.4 μM / μl against FIPV in cell culture, while the EC50 of GS-441524 is about 1.0 μM / μl. [18]. Both have a similar oral absorption of about 40-50 %, so the effective subcutaneous (SC) dose for Molnupiravir would be approximately half the recommended 4 mg / kg SC every 24 hours of the initial dose for GS441524. [14] or 2 mg / kg SC q24h. The per-os (PO) dose would be doubled to account for less effective oral absorption at a dose of 4 mg / kg PO every 24 hours. The estimated initial oral dose of molnupiravir for cats with FIP can also be calculated from the available Covid-19 treatment data. Patients treated for Covid-19 are given 200 mg of molnupiravir PO q12h for 5 days. This dose was evidently calculated from a pharmacokinetic study performed in humans, and if the average person weighs 60-80 kg (70 kg), the effective inhibitory dose is 3,03.0 mg / kg PO q12h. The cat has a basal metabolic ratio 1.5 times higher than humans, and assuming the same oral absorption in both humans and cats, the minimum dose for cats according to this calculation would be 4.5 mg / kg PO every 12 hours. Assuming that molnupiravir crosses the blood-brain barrier and the blood-brain barrier as efficiently as GS-441524 [3,18], the dose would be increased ~ 1.5 and ~ 2.0-fold to allow adequate penetration into the aqueous humor and cerebrospinal fluid for cats with ocular (~ 8 mg / kg PO, q12 h) or neurological FIP (~ 10 mg / kg PO, q12h). The treatment will last 10-12 weeks and the monitoring of the response to treatment will be identical to GS-441524 [14, 20]. These recommendations are based on published data assumptions and further experience with Molnupiravir will be required in this area. Molnupiravir is unlikely to be safer and more effective than GS-441524 in the treatment of FIP, but a third antiviral drug may be particularly useful in preventing resistance to GS-441524 (as a cocktail of antivirals with different resistance profiles) or in treating cats that no longer respond. good on GS-441524. It is largely unknown whether Molnupiravir will be without long-term toxic effects, as the active substance N4-hydroxycytidine is an extremely potent mutagen. [21] and the duration of FIP treatment is much longer than with Covid-19 and there is a likelihood of major side effects.

It is a pity that EIDD-1931 (N4-hydroxycytidine), the active substance in Molnupiravir, has not received much attention in the treatment of FIP cats than Molnupiravir. EIDD-1931 has a 4-fold greater inhibitory effect against the virus than Molnupiravir (EC50 0.09 vs. 0.4 μM) and the percentage of cytotoxicity is slightly lower (2.3% vs. 3.8% at 100 μM) [15]. N4-hydroxycytidine is also efficiently absorbed orally [3], which was downplayed in the development of EIDD-2801 (Molnupiravir). This scenario is identical to the GS-441524 vs. Remdesivir, the second of which, Remdesivir, was chosen for commercialization, although current research suggests that GS-441524 would be the best candidate.[17].


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Read "A long history of Beta-d-N4-hydroxycytidine and its modern application for the treatment of Covid-19 in humans and FIP in cats."