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 large intestine 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 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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