Feline infectious peritonitis virus

Feline infectious peritonitis (FIP) is the name given to an uncommon, but usually fatal, aberrant immune response to infection with feline coronavirus (FCoV).

FCoV is a virus of the gastrointestinal tract: most infections are either asymptomatic, or cause diarrhea, especially in kittens, as maternally derived antibody wanes at between 5 and 7 weeks of age. The virus is often referred to as feline enteric coronavirus (FECV). From the gut, the virus very briefly undergoes a systemic phase, returning to the gut, from where it is shed in the feces. The pathogenesis of FIP is complicated: the reductionist view is that it is entirely due to mutation of the virus, enabling it to enter, or replicate more successfully in, monocytes. The holistic approach is that FIP occurs as a result of a number of factors, including viral virulence (some strains are undoubtedly more virulent than others), and the immune status and general health of the host.

FCoV is common in places where large groups of cats are housed together indoors (e.g. breeding catteries, animal shelters, etc.). The virus is shed in feces and cats become infected by ingesting or inhaling the virus, usually by sharing cat litter trays, or by the use of contaminated litter scoops or brushes transmitting infected microscopic cat litter particles to uninfected kittens and cats. Direct, cat-to-cat, virus transmission does not commonly occur.

There are two main forms of FIP: effusive (wet) and non-effusive (dry). While both types are fatal, the effusive form is more common (60–70% of all cases are wet) and progresses more rapidly than the non-effusive form.

The hallmark clinical sign of effusive FIP is the accumulation of fluid within the abdomen or chest, which can cause breathing difficulties. Other symptoms include lack of appetite, fever, weight loss, jaundice, and diarrhea.

Dry FIP will also present with lack of appetite, fever, jaundice, diarrhea, and weight loss, but there will not be an accumulation of fluid. Typically a cat with dry FIP will show ocular or neurological signs. For example, the cat may develop difficulty in standing up or walking, becoming functionally paralyzed over time. Loss of vision is another possible outcome of the disease.

Diagnosis of effusive FIP has become more straightforward in recent years: detection of viral RNA in a sample of the effusion, by reverse-transcriptase polymerase chain reaction (RT-PCR) is diagnostic of effusive FIP. However, that does require that a sample be sent to an external veterinary laboratory. Within the veterinary hospital there are a number of tests which can rule out a diagnosis of effusive FIP within minutes:

...
Wikipedia