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Feline infectious peritonitis is a relatively uncommon viral disease of cats caused by a feline Coronavirus. The virus is acquired through contact with feces or eye/nasal discharge from infected cats. The virus can live in bodily secretions in the environment for weeks, but can be inactivated by many disinfectants and cleaners. Most affected cats are less than five years or greater than 10 years of age and from multi-cat households. Pure-bred cats have an increased risk for FIP. Infection may be more likely in the presence of other viral infections, such as with feline leukemia (FeLV), immunodeficiency (FIV) and herpes viruses. The feline Coronavirus may stay localized within the gastrointestinal tract and cause self-limiting fever, lethargy, vomiting and diarrhea. Very serious disease develops when the virus gains access to the blood stream and circulates, disseminating around the body. Two distinct forms of disease can develop in this situation – the effusive, or “wet”, form or the non-effusive, or “dry”, form.

The effusive form develops from interaction of the virus and antibodies and consequent inflammation of blood vessels (vasculitis). The vasculitis can occur anywhere in the body, affecting any organ or system. Thick, protein-rich fluid oozes from the damaged blood vessels, filling various body cavities/spaces. The dry form develops when the virus incites a specific type of inflammatory reaction called granulomatous inflammation. Essentially, a ball of inflammation develops around the virus and damages the surrounding tissue. Any organ or body system can be affected; however, the central nervous system and eye are most often involved.

Clinical signs and physical examination findings are referable to the organ(s)/system(s) affected and overlap is seen between the two forms of FIP. Commonly the only signs noted are vague, including lethargy, decreased appetite and weight loss. Many patients present with a substantial fever and jaundice/icterus. The effusive form often causes abdominal distension, respiratory distress and/or muffled heart/lung sounds. Animals affected by the dry form may present seizing, comatose and/or blind. Liver, kidney and/or splenic enlargement/irregularity can be palpated in a patient with either form. Overall, it is important to realize that any combination of signs can be seen due to the often diffuse nature of this disease.

A presumptive diagnosis is made based on the clinical signs, breed/age of the cat and fitting history. Supportive laboratory findings include anemia, increased neutrophil (a type of white blood cell) counts, increased blood proteins (especially immunoglobulins), chest or abdominal fluid that is very high in protein and relatively low in cells, and a positive antibody titer. The antibody titer is unfortunately not specific to the FIP Coronavirus because of cross-reaction with the enteric Coronavirus and so, cannot be used for definitive diagnosis. Cerebrospinal fluid analysis that reveals increased protein and white blood cell numbers is also supportive, but not definitive. X-rays and ultrasound may show bodily effusions and enlarged organs. A definitive diagnosis requires a biopsy sample of affected tissue and more specific laboratory analyses; however, supportive data is often convincing enough to base critical decisions on.

The prognosis for FIP varies somewhat based on the form, but is overall not favorable. The effusive form carries a particularly grave prognosis and most cats are euthanized within days of presentation. Supportive and symptomatic treatments are the mainstay of therapy, if it is to be pursued. No specific anti-viral treatments have yet been shown to be successful against the FIP Coronavirus. The dry form may be successfully treated when localized within the eye. Central nervous system involvement worsens the prognosis to poor although, palliation with immunosuppressive drugs for varying periods of time may be achievable.

An intranasal FIP vaccine exists and is variably successful. Whether it should be administered or not, is usually determined on a case-to-case basis. Cats in catteries or multi-cat households may make better candidates for receiving this vaccine.