Feline Infectious Peritonitis (FIP) is one of the leading infectious cause of cat mortality. FIP is an unusual consequence – aberrant immune response – to infection with Feline Coronavirus (FCoV).  The majority of cats who encounter this virus are asymptomatic. The reason why some cats develop FIP is not fully understood. Soon after the discovery of FIP in 1963, it was recognized that many more cats were infected with FCoV than developed FIP and the hypothesis that there were two coronaviruses of the cat was offered, meaning it was previously thought that cats were infected with two similar but distinctive feline coronaviruses. The hypothesis was that there is an avirulent or enteric feline coronavirus (FECV) which differs from a virulent feline infectious peritonitis virus (FIPV). However, these are now considered phenotypic variants (biotypes) of the same virus, and it is recognized that wherever there is feline coronavirus, there is the potential for the development of FIP. In this website, the virus, for the most part, will be called FCoV and the disease FIP.
Most cats with FIP come from breeding catteries and shelters.
Whenever FCoV is present in a cat, there is the potential for FIP to develop. There is no such thing as a “safe” coronavirus.
Cats of any age can develop FIP, most cats with FIP are younger than two years old. FIP typically occurs some weeks to months after a stressful event in the cat’s life and can appear as effusive (or “wet”) or non-effusive (or “dry”). In the former, fluids collect in the abdomen and/or thorax, in the latter, there is no effusion but the cat loses weight, is anorexic, pyrexic, lymphopenic and shows clinical signs according to which organs are affected: the eyes, liver, kidneys, and brain being the most commonly affected.
Cats from multicat environments – for example, pedigree and rescue cats – are most at risk of developing FIP for several reasons:
Increased chance of becoming infected with feline coronavirus.
Increased dose of FCoV.
Increased stress (cats are naturally solitary or from a small, family, group).
Increased probability of concurrent disease, lowering immune function.
Possibly poorer nutrition: cereal based cat food is pro-inflammatory.
In brief, feline infectious peritonitis (FIP) is the result of an excessive inflammatory response to infection with feline coronavirus (FC0V) and presents as a progressive and distinctly fatal systemic disease. Regardless of its name, the lesions of FIP are widespread and not restricted to the peritoneum.
Since first recognized – more than five decades ago, FIP has been one of the most studied feline diseases; however, effective treatments and fully reliable vaccine are still lacking.