Feline infectious peritonitis (FIP) is a highly fatal infection of cats and is the leading infectious cause of mortality in young cats (~3 months–3 years) from pedigreed catteries and cat shelters, although cats of all ages can be affected.

Recent research indicates that the viruses that cause FIP (feline infectious peritonitis viruses; FIPV) arise as relatively simple mutations from feline enteric coronaviruses (FECV) endemic in the same environment. While FIPV may not be transmitted from cat-to-cat, the majroity of cats probably become infected with benign FECV, which subsequently mutates into virulent FIPV. So, it is apparently the spread of FECV amongst cats that results in most cases of FIP, rather than primarily by the spread of FIPV per se.

FECV is very prevalent, with ~30% of pet cat households (with less than 5-7 cats) and ~90% of multiple cat households, catteries, and cat shelters having serologic evidence of FECV infection. FECV is highly contagious and is transmitted primarily by the fecal-oral route, although oral-oral and oral-nasal transmission may also occur. Cat-to-cat contact and environmental contamination (fomites, dust) with feces are important sources of infection. Between 30-80% of cats in coronavirus-infected households will shed FECV and multiple cat environments are most likely to include cats shedding FECV. Both healthy cats and those with FIP can be shedders of FECV. Cats may shed FECV persistently, transiently, or intermittently. Some cats shed large numbers of virus while others shed small numbers. Cats also can become re-infected with FECV and start to shed virus again. FECV can remain infective for 6 weeks at room temperature. Accurate identification of cats shedding FECV can be difficult, and requires multiple fecal FECV PCR tests.

Most cats are infected with FECV at a young age. In FECV-contaminated environments, kittens are infected, often by 3–6 weeks old, even in the presence of maternal immunity. Strict isolation of queens with their litter, early weaning (3-6 weeks), and isolation of kittens may prevent their FECV infection. Recurrent FECV infections can occur subsequent to waning humoral immunity.

The key step in the transformation of benign FECV into virulent FIPV is a change that enables the virus to replicate in macrophages. Mutations that cause this transformation mostly involve the 3c gene and have been postulated to enable expression of the coronavirus 7b gene and subsequent synthesis of the 7b protein. This is the rationale behind the FIP-7b ELISA test (which detects antibodies to the 7b protein). However, there could also be other mutations that confer virulence on FECV.

The likelihood and rate of development of FIPV mutants is probably related to the strain of FECV and also the rapidity with which it replicates. FIP may be more likely to occur when FECV replication is rapid, as expected in kittens or immuno-compromised cats infected with FECV, and in environments where there is frequent and high level spread of FECV between cats (overcrowding, poor husbandry).

The outcome in cats that develop the FIPV mutation depends upon their cell-mediated immune (CMI) response, as the humoral immune response is largely ineffective in preventing the development of FIP. In cats with a weak or no CMI response, FIPV replication in macrophages proceeds swiftly, the virus spreads systemically, and the characteristic wet form of the FIP develops subsequent to an intense pyogra nuloma tou s vasculitis. In cats that mount a partial CMI, viral replication is slowed, with consequential granuloma formation (the dry form of FIP). Cats that mount a strong CMI response generally do not develop FIP, although they may harbor latent FIPV and have the potential to reactivate their infection and develop disease if they are stressed. The majority of cats show clinical signs of FIP within 18 months of the occurrence of FIPV mutation, although there are anecdotal reports of latency as long as 10 years.

Although any cat that has been infected with FECV is potentially at risk for developing FIP, the vast majority (>99%) never develop FIP.