Original article: A review on the diagnosis of feline infectious peritonitis
3.3.2022

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 systemic coronavirus (CoV) disease in cats, characterized by fibrinous-granulomatous serositis with protein-rich effusions into body cavities, granulomatous necrotizing phlebitis and periphlebitis, and granulomatous inflammatory lesions in multiple organs (Weiss and Scott 1981; Kipar et al. 2005). Feline CoV (FCoV) is transmitted by the fecal-oral route and primarily infects enterocytes (Pedersen 1995), but subsequently spreads systemically through monocytic viremia (Meli et al 2004; Kipar et al 2005). It has been shown that increased viral replication capacity may be a key feature in the development of FIP, and it is also thought that FIP is caused by mutations in the common feline enteric coronavirus (FECV), which is found in cats worldwide and is not a serious infection (Pedersen et al 2009; Healey et al 2022). Approximately 1 in 10 %-infected cats develop mutations that result in feline infectious peritonitis. In large multi-cat situations, FECV is shed in the feces of most apparently healthy cats and transmission occurs through direct contact with feces or contaminated bedding and other fomites (Pedersen et al 2004). Kittens become infected at approximately 9 weeks of age (Pedersen et al. 2008). The time between 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 noneffusive disease (Pedersen 2014). Even with severe FIP, some cats can live for months. In situations where multiple cats are present, feline enteric coronavirus (FECV) is extremely common and highly contagious. Almost all cats that come into contact with FECV from shedding cats will become ill, but otherwise the infection is usually asymptomatic or causes only mild, transient 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 by the fecal-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 signs such as gait abnormalities or inadequate mentation are nonspecific (Giori et al 2011; Kipar et al 2014). The infection has 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 abdominal distension due to excessive accumulation of fluid in the abdominal cavity. Specificity is always the most important diagnostic value to consider to avoid misdiagnosis of FIP in unaffected cats.

2. Diagnostic tests for feline infectious peritonitis

Diagnosis is based on the cat's age, background, clinical signs, and physical examination. Abdominal distension with ascites, dyspnea with pleural effusion, jaundice, hyperbilirubinuria, palpable masses in the kidneys and/or mesenteric lymph nodes, uveitis, and various neurologic signs associated with brain and/or spinal cord involvement are common in cats with FIP, with retinal changes being the most common ocular involvement. Retinal vascular cuffing may occur, appearing as diffuse grayish lines on either side of the vessels. Granulomatous retinal changes are occasionally seen. FIPV infection has been shown 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). Given the high mortality rate, many veterinarians and pet owners are cautious about making a diagnosis based on “reasonable certainty.” The challenge is to decide whether a test increases the likelihood that the clinical signs are due to FIP (indirect tests) or offers a definitive diagnosis (direct tests). It is essential to recognize that the sensitivity and specificity of any indirect test will vary depending on how likely the cat is to be infected based on other factors. This means that the positive predictive value of a test such as a complete blood count (CBC) or albumin:globulin (A:G) ratio for predicting 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 both confound and aid 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

In cases of suspected FIP with effusion, an effusion sample can be incredibly helpful in establishing the diagnosis and then the hematological findings, so obtaining effusion samples should always be a top priority. In cases of ascites, a sample can be obtained by ultrasound-guided fine needle aspiration or the “flying cat” technique. For identifying small amounts of fluid in the chest and abdomen, ultrasonography provides a useful aid in localizing effusion pockets in the abdomen, while evidence of pericardial effusions can be obtained by muffled cardiac sounds and electrocardiographic changes. Ultrasonography should be used repeatedly to identify any small volume effusions, and ultrasonography can also be used to guide sampling of small pockets of fluid. In cats with pericardial effusions, auscultation of the heart will reveal muffled sounds and the ECG will reveal 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.

Financing

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

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