Feline Infectious Peritonitis

Feline infectious peritonitis (FIP) is a fatal, immune-mediated, and often systemic disease of domestic and wild Felidae caused by a pathogenic biotype of feline coronavirus (FCoV), referred to as feline infectious peritonitis virus (FIPV). The disease arises from a mutation of the ubiquitous, enteric feline enteric coronavirus (FECV), which typically causes subclinical or mild enteritis. The mutated virus acquires the ability to replicate efficiently within macrophages, leading to systemic dissemination and a characteristic pyogranulomatous and vasculitic inflammatory response. FIP manifests in two major clinical forms: the effusive (wet) form, characterized by protein-rich effusions in body cavities, and the non-effusive (dry) form, marked by granulomatous lesions in various organs, including the eyes, kidneys, central nervous system, and liver. A mixed form can occur in some cats. The disease is almost invariably progressive and fatal without treatment, although recent antiviral therapies have shown promise in some cases.

The causative agent is feline coronavirus (FCoV), a positive-sense, single-stranded RNA virus belonging to the family Coronaviridae, genus Alphacoronavirus. Two biotypes exist: feline enteric coronavirus (FECV), which is highly prevalent and usually causes asymptomatic enteric infection, and feline infectious peritonitis virus (FIPV), which arises from spontaneous mutations in the FECV genome, particularly in the spike (S) gene and accessory genes (e.g., 3c, 7b). These mutations enhance the virus's ability to infect and replicate in macrophages, a key step in FIP pathogenesis. The virus is enveloped and relatively fragile in the environment, susceptible to common disinfectants. Transmission occurs primarily via the fecal-oral route, with shedding in feces from infected cats. The virus can also be transmitted via saliva, urine, and fomites. Vertical transmission is rare. The mutation events are believed to occur within the individual cat, and FIPV is not directly transmissible between cats under natural conditions, as the mutated virus is poorly shed. However, some studies suggest that FIPV can be shed in low amounts, but the primary source of infection is FECV.

FIP occurs worldwide, affecting domestic and wild felids, including cheetahs, lions, and tigers. The prevalence of FCoV infection is high in multi-cat households, shelters, and catteries, with up to 90% of cats in such environments seropositive. However, only a small percentage (5-10%) of FCoV-infected cats develop FIP. The disease is most common in cats under 2 years of age, with a peak incidence between 6 months and 2 years, and in older cats (>10 years) due to waning immunity. Certain breeds, such as Abyssinian, Bengal, Birman, Himalayan, Ragdoll, and Devon Rex, may have a genetic predisposition. Intact male cats may be at higher risk. Stress, overcrowding, poor hygiene, and concurrent infections (e.g., feline leukemia virus, feline immunodeficiency virus) increase the risk of FIP development. Seasonality is not well-defined, but outbreaks can occur in shelters. The incidence in single-cat households is low (<1%).

The pathogenesis of FIP begins with the enteric infection by FECV, which replicates in the intestinal epithelium and is shed in feces. In a small proportion of cats, a mutation occurs during viral replication, giving rise to FIPV. The mutated virus gains the ability to infect and replicate efficiently in monocytes and macrophages. This leads to a cell-associated viremia, with the virus disseminating hematogenously to various organs. The infected macrophages trigger a type III hypersensitivity reaction, with the formation of immune complexes (virus-antibody) that deposit in blood vessel walls, leading to vasculitis and increased vascular permeability. This results in the effusive form, with protein-rich fluid accumulation in the peritoneal, pleural, and pericardial cavities. In the non-effusive form, a cell-mediated immune response predominates, leading to the formation of pyogranulomatous lesions in tissues such as the kidneys, liver, eyes, and central nervous system. The disease is characterized by a dysregulated immune response, with an imbalance between pro-inflammatory and anti-inflammatory cytokines, leading to systemic inflammation, fever, and cachexia. The virus also causes direct cytopathic effects in macrophages, leading to tissue necrosis and inflammation.

Predisposing factors for FIP include age (young cats <2 years and older cats >10 years), genetic susceptibility (certain breeds), stress (e.g., rehoming, boarding, surgery), overcrowding, poor sanitation, high FCoV load in the environment, concurrent immunosuppressive infections (FeLV, FIV), and immunosuppressive drug therapy (e.g., corticosteroids). Male cats may be at higher risk. In multi-cat environments, the risk of FCoV infection is high, but the development of FIP depends on the viral mutation rate and the host's immune response. Cats with a weak cell-mediated immune response are more likely to develop the effusive form, while those with a partial cell-mediated response may develop the non-effusive form.

Clinical signs of FIP are variable and depend on the form and organs involved. The effusive (wet) form typically presents with a history of lethargy, anorexia, weight loss, fever (often unresponsive to antibiotics), and abdominal distension due to peritonitis. Thoracic effusion can cause dyspnea, tachypnea, and muffled heart and lung sounds. Pericardial effusion may lead to cardiac tamponade. The non-effusive (dry) form is more insidious, with chronic fever, weight loss, depression, and organ-specific signs. Ocular signs include uveitis (anterior or posterior), hyphema, chorioretinitis, and retinal detachment. Neurological signs include ataxia, nystagmus, seizures, and paresis. Renal involvement can cause renomegaly, renal failure, and uremia. Hepatic involvement may lead to icterus and elevated liver enzymes. Intestinal granulomas can cause vomiting, diarrhea, and palpable abdominal masses. Some cats may have a mixed form with both effusions and granulomatous lesions. Physical examination may reveal a thin body condition, pale mucous membranes, and a painful abdomen.

Differential diagnoses for FIP include: (1) Bacterial peritonitis (e.g., from gastrointestinal rupture, septic peritonitis) - distinguished by cytology showing degenerate neutrophils and bacteria, positive bacterial culture, and response to antibiotics. (2) Lymphocytic cholangitis - presents with icterus, elevated liver enzymes, and liver biopsy showing lymphocytic infiltration, but no effusions or typical FIP lesions. (3) Toxoplasmosis - can cause uveitis, neurological signs, and fever; diagnosed by serology (IgM/IgG) and PCR, and response to clindamycin. (4) Feline leukemia virus (FeLV) and feline immunodeficiency virus (FIV) infections - can cause immunosuppression and secondary infections; diagnosed by ELISA/PCR. (5) Neoplasia (e.g., lymphoma, adenocarcinoma) - can cause effusions and organomegaly; diagnosed by cytology/histopathology and imaging. (6) Pancreatitis - can cause abdominal pain, vomiting, and fever; diagnosed by serum fPLI and abdominal ultrasound. (7) Heart failure (e.g., hypertrophic cardiomyopathy) - can cause pleural and pericardial effusion; diagnosed by echocardiography and NT-proBNP. (8) Hepatic lipidosis - causes icterus and elevated liver enzymes; diagnosed by liver biopsy. (9) Chronic kidney disease - can cause renal failure and renomegaly; diagnosed by blood work and ultrasound. (10) Systemic fungal infections (e.g., histoplasmosis) - can cause granulomatous lesions and effusions; diagnosed by cytology and fungal culture.

The diagnostic approach to FIP involves a combination of signalment, history, clinical signs, and laboratory tests. A stepwise algorithm is recommended: (1) Initial suspicion based on clinical signs (fever, effusions, uveitis, neurological signs) and risk factors (young age, multi-cat environment). (2) Baseline blood work (CBC, biochemistry, urinalysis) to identify typical abnormalities (lymphopenia, neutrophilia, hyperglobulinemia, hypoalbuminemia, elevated liver enzymes). (3) Serum protein electrophoresis to detect polyclonal gammopathy (elevated beta and gamma globulins). (4) Rivalta test on effusions: a positive result (drop of fluid in acetic acid solution forms a precipitate) is highly suggestive of FIP. (5) Effusion analysis: FIP effusions are typically sterile, with high protein (>3.5 g/dL), low cell count (<5000 cells/µL), and mixed inflammatory cells (lymphocytes, macrophages, neutrophils). (6) Reverse transcriptase polymerase chain reaction (RT-PCR) for FCoV RNA in effusion or blood: a positive result in effusion is highly supportive, but a negative result does not rule out FIP. (7) Immunohistochemistry (IHC) or immunocytochemistry (ICC) for FCoV antigen in macrophages from effusion or tissue biopsy is the gold standard for diagnosis. (8) Serum antibody titers are not diagnostic, as many healthy cats are seropositive. (9) Advanced imaging (ultrasound, MRI) may be used to identify granulomatous lesions or effusions. (10) In cases with neurological signs, cerebrospinal fluid (CSF) analysis may show elevated protein and cell count, and RT-PCR for FCoV can be performed on CSF. (11) If antemortem diagnosis is inconclusive, postmortem examination with histopathology and IHC is definitive.

Hematology: Common findings include normocytic, normochromic, non-regenerative anemia (PCV <25%), neutrophilia with a left shift, lymphopenia (due to lymphocyte depletion), and sometimes thrombocytopenia. Serum biochemistry: Hyperglobulinemia (often >5 g/dL) due to increased gamma globulins, hypoalbuminemia (due to protein loss and inflammation), elevated liver enzymes (ALT, ALP) in hepatic involvement, elevated bilirubin (icterus), and elevated urea and creatinine in renal involvement. Electrolyte imbalances may occur (e.g., hyponatremia, hyperkalemia). Urinalysis: Proteinuria (UPC >0.4) may be present due to glomerulonephritis. Blood gas analysis: May show metabolic acidosis in severe systemic disease. Specific biomarkers: Serum amyloid A (SAA) is often elevated, but not specific. Alpha-1 acid glycoprotein (AGP) is elevated in FIP and can be used as a supportive test. Effusion analysis: Fluid is typically a modified transudate or exudate with high protein (>3.5 g/dL), low cell count (<5000 cells/µL), and a predominance of lymphocytes and macrophages. The Rivalta test is positive in >90% of effusive FIP cases. Serology: FCoV antibody titers are not diagnostic, as they are positive in many healthy cats. PCR: RT-PCR for FCoV RNA can be performed on effusion, blood, CSF, or tissue; a positive result in effusion or CSF is highly supportive, but a negative result does not exclude FIP. Immunocytochemistry on effusion macrophages is highly specific.

Radiography: Thoracic radiographs may reveal pleural effusion (with rounding of lung lobes and fissure lines), alveolar infiltrates, and cardiomegaly due to pericardial effusion. Abdominal radiographs may show loss of serosal detail due to peritoneal effusion, hepatomegaly, renomegaly, and abdominal masses. Ultrasonography: Abdominal ultrasound is useful to detect effusion, mesenteric lymphadenopathy, hepatomegaly, renomegaly, and granulomatous lesions in the kidneys, liver, and intestines. The kidneys may show irregular contours, increased echogenicity, and loss of corticomedullary distinction. The liver may be hypoechoic or hyperechoic with a mottled appearance. Intestinal wall thickening may be seen. Thoracic ultrasound can confirm pleural and pericardial effusion. Echocardiography: May be indicated if pericardial effusion is suspected, to assess cardiac function and rule out primary heart disease. Computed tomography (CT): CT is useful for evaluating the brain in neurological FIP, showing hydrocephalus, periventricular contrast enhancement, and meningeal enhancement. MRI: MRI of the brain can reveal multifocal T2-hyperintense lesions, contrast enhancement, and hydrocephalus. Endoscopy: May be used to obtain biopsies of the gastrointestinal tract if granulomatous lesions are suspected.

Cytology: Fine needle aspirates of effusions or granulomatous masses may show macrophages with intracytoplasmic FCoV antigen (detected by immunocytochemistry). Effusion cytology typically shows a mixed population of lymphocytes, macrophages, and neutrophils, with a low cell count. Histopathology: Definitive diagnosis is made by biopsy of affected tissues (e.g., liver, kidney, omentum, brain). Histopathological findings include pyogranulomatous inflammation with central necrosis, perivascular cuffing, and vasculitis. Immunohistochemistry (IHC) for FCoV antigen in macrophages is the gold standard. Special stains (e.g., Gram, acid-fast) may be used to rule out bacterial or fungal infections. In the non-effusive form, granulomas are often found in the kidneys, liver, lungs, eyes, and central nervous system.

Treatment of FIP has historically been palliative, but recent advances have introduced antiviral drugs with promising results. The current standard of care includes: (1) Supportive care: Fluid therapy to correct dehydration and electrolyte imbalances, nutritional support (e.g., appetite stimulants, feeding tubes), and antiemetics (e.g., maropitant 1 mg/kg IV/SC q24h) if vomiting. (2) Anti-inflammatory/immunosuppressive therapy: Corticosteroids (e.g., prednisolone 1-2 mg/kg PO q12h) may be used to reduce inflammation and immune-mediated damage, but they are not curative and may increase viral replication. (3) Antiviral therapy: The nucleoside analog GS-441524 (remdesivir's active metabolite) has shown efficacy in clinical trials. Dosage: 4-10 mg/kg SC q24h for 12 weeks, with dose adjustments based on response. Another drug, polyprenyl immunostimulant, has been used in non-effusive FIP, but evidence is limited. (4) Other immunomodulators: Feline interferon-omega (1 MU/kg SC q24h for 5 days, then every 48 hours) has been used, but efficacy is variable. (5) Management of effusions: Therapeutic thoracocentesis or abdominocentesis may be needed for respiratory distress, but repeated drainage can lead to protein loss. (6) Treatment of secondary infections: Antibiotics may be indicated if bacterial infection is suspected. (7) In neurological FIP, antiviral therapy may be less effective due to blood-brain barrier penetration, but higher doses of GS-441524 (up to 10-15 mg/kg) may be used. (8) Monitoring: Regular blood work and clinical assessment are essential to adjust therapy. (9) Prognosis: Without treatment, FIP is almost always fatal within weeks to months. With antiviral therapy, some cats can achieve remission, but relapse is possible.

The prognosis for FIP is generally poor to grave without treatment, with a median survival time of days to weeks for the effusive form and weeks to months for the non-effusive form. With the advent of antiviral therapy (GS-441524), the prognosis has improved significantly, with reported survival rates of 50-80% in treated cats, especially if treatment is initiated early. Factors associated with a better prognosis include: non-effusive form, absence of neurological signs, early diagnosis, and good response to therapy. Negative prognostic indicators include: severe neurological involvement, marked hyperbilirubinemia, thrombocytopenia, and lack of response within the first 2 weeks of treatment. Even with successful treatment, long-term remission is possible, but some cats may develop chronic sequelae, such as renal or hepatic dysfunction. Relapse can occur, and re-treatment may be necessary.

Follow-up for cats with FIP depends on the treatment protocol. For cats receiving antiviral therapy (GS-441524), re-evaluation is recommended every 2-4 weeks during the 12-week treatment course. Monitoring should include: physical examination, body weight, CBC, serum biochemistry (especially globulins, albumin, liver enzymes, bilirubin), and assessment of effusions (if present). Imaging (ultrasound) may be repeated to monitor resolution of effusions or granulomas. After completion of therapy, cats should be monitored monthly for the first 3 months, then every 3-6 months for the first year. Long-term monitoring should include blood work and urinalysis to detect chronic kidney disease or other sequelae. For cats receiving palliative care, follow-up is more frequent (weekly) to manage effusions and quality of life. Owners should be educated on the risk of FCoV shedding and the importance of reducing stress and maintaining good hygiene in multi-cat households.

Pearls: (1) FIP should be suspected in any young cat with fever, effusions, or uveitis, especially from a multi-cat environment. (2) The Rivalta test is a simple, inexpensive, and highly sensitive test for FIP effusions. (3) A positive RT-PCR for FCoV on effusion is highly supportive, but a negative result does not rule out FIP. (4) Immunocytochemistry on effusion macrophages is the gold standard antemortem test. (5) Serum globulin >5 g/dL and albumin <2.5 g/dL are classic findings. (6) In neurological FIP, CSF analysis may show elevated protein and cell count, and RT-PCR on CSF can be helpful. (7) Antiviral therapy (GS-441524) is a game-changer, but it is not yet widely available and is expensive. Pitfalls: (1) Do not rely on serum antibody titers for diagnosis, as they are positive in many healthy cats. (2) A negative FCoV PCR on blood does not exclude FIP. (3) Do not perform repeated abdominocentesis for effusions, as it can lead to protein loss and hypovolemia. (4) Corticosteroids may worsen the disease by suppressing cell-mediated immunity. (5) Do not use live vaccines in cats suspected of FIP. (6) In multi-cat households, do not assume that a cat with FIP is contagious to others; FIPV is not directly transmissible, but FECV is.

Key studies and consensus guidelines: (1) Pedersen et al. (2019) demonstrated the efficacy of GS-441524 in treating FIP, with a survival rate of 80% in a field trial. (2) The 2020 ACVIM consensus statement on the diagnosis and treatment of FIP (Tasker et al.) provides evidence-based recommendations, including the use of Rivalta test, PCR, and immunocytochemistry. (3) A study by Addie et al. (2015) highlighted the importance of FCoV mutation in FIP pathogenesis. (4) The European Advisory Board on Cat Diseases (ABCD) guidelines recommend a diagnostic algorithm and supportive care. (5) A meta-analysis by Fischer et al. (2018) evaluated the diagnostic accuracy of various tests, finding that immunocytochemistry and RT-PCR on effusion have high specificity. (6) Recent studies on antiviral therapy have shown that early treatment improves outcomes. (7) The use of feline interferon-omega has been studied, but evidence is limited and inconsistent. (8) Research on immunomodulators, such as polyprenyl immunostimulant, is ongoing, but current evidence is insufficient to recommend routine use.