Feline infectious peritonitis

Early signs are non-specific: lethargy, poor appetite, weight loss or stunted growth in kittens, and a fever that does not respond to treatment. Many cats develop effusions, most often abdominal, also pleural or pericardial, and disease with effusion usually progresses within days to weeks. Without effusion the course is slower, over weeks to months, with signs from the organs involved: enlarged abdominal lymph nodes, liver, kidney or intestinal lesions, uveitis and neurological signs such as ataxia or seizures. Bloods commonly show lymphopenia, neutrophilia, mild anaemia, high globulins with a low albumin to globulin ratio, and raised bilirubin. Effusions are clear, sticky, straw-yellow, protein-rich and relatively cell-poor.

Feline infectious peritonitis is caused by feline coronavirus (FCoV), an enveloped positive-sense RNA virus of the genus Alphacoronavirus. After faecal-oral infection the virus replicates in small intestinal and colonic epithelium, usually without signs or with mild enteritis, then may reach mesenteric lymph nodes and blood. In a small proportion of cats, mutations arising within that cat, together with the nature of its immune response, allow efficient replication in activated monocytes and macrophages; no single critical mutation has been identified. Infected monocytes interact with vessel walls, migrate out and become macrophages, producing a perivascular phlebitis that can affect any organ. Loss of endothelial tight junctions lets protein-rich fluid leak into body cavities; more chronic cases form larger perivascular pyogranulomas.

FCoV is very common worldwide, especially in multi-cat households, catteries and shelters; published antibody surveys include 7% of 248 pet cats presented to a referral hospital in Iran (2012) and, in a 1999 Israeli study, 60% of 68 feral, 83% of 54 shelter and 21% of 33 pet cats. Faeces in the litter tray are the main source of infection, and repeated cycles of infection and reinfection maintain the virus in a household. Only a small proportion of infected cats develop FIP, up to 12% in multi-cat environments. Young cats (especially under two years), pedigree cats and males are at higher risk, and recent stress such as rehoming, shelter stays or neutering is common in the history. Cat-to-cat spread of FIP itself is considered very unlikely.

Septic peritonitis or pleuritis gives cell-rich effusions with degenerate neutrophils and bacteria. Lymphoma, which also affects young cats, shows neoplastic cells on cytology. Congestive heart failure gives low-cell effusions without fever, often with a murmur or gallop. Pancreatitis, lymphocytic cholangitis (higher liver enzymes, more chronic), toxoplasmosis, mycobacteriosis (without marked hyperglobulinaemia) and traumatic haemorrhagic effusions are also considered. Definitive diagnosis needs consistent histopathology with FCoV antigen immunostaining; FCoV RNA or antigen in effusions or aspirates strongly supports it.

Blood vessels (perivascular phlebitis), peritoneum, pleura, pericardium, mesenteric lymph nodes, liver, kidneys, spleen, intestine, pancreas, eyes (uvea), central nervous system

Hygiene to cut faecal-oral FCoV spread: no more than three cats per room, outdoor access, one more litter tray than cats, away from food, cleaned twice daily; separate shedders. ABCD does not recommend the intranasal FIP vaccine.

Without effective antiviral treatment FIP has a very poor prognosis and short survival. Nucleoside analogues such as GS-441524 now cure many cats, with reported survival of 81-94%, but they are costly and not legally available everywhere.