As long as we've known about, tried to diagnose, and attempted to treat feline infectious peritonitis (FIP), it still eludes us! This complex (....and getting more complex all the time) infection of kittens and adult cats is caused by a feline Coronavirus (FCoV). But that's the easy part...everything about this disease, from pathogenesis to transmissibility, is tough to understand...and the more we learn about it, the tougher it's getting!

The first references to the fact that cats infected by this virus developed disease were described as early as 1960. However, it was not until 1966 that FIP was described as a distinct clinical entity and the infectious nature of the disease was described. Although the initial name, "feline infectious peritonitis," has remained the popular name for this disease, the virus by no means is restricted to the peritoneum.

In fact, coronaviruses are a widely distributed group of viruses capable of infecting several species of birds and mammals. They can cause upper respiratory and gastrointestinal disease, hepatitis, vasculitis, peritonitis, pleuritis, and encephalitis. Perhaps the best known of these viruses are the FIP virus in cats, canine coronavirus in dogs, and transmissible gastroenteritis virus of swine.

Epizootiology

The overall prevalence of FIP is not precisely known. In the general population, it has been reported by some sources to be less than 1% of all cats presented to university teaching hospitals. In multiple cat households and catteries, the prevalence is probably considerably higher. Nonetheless, deaths appear to be sporadic and unpredictable. Under the worst conditions, the morbidity (clinical illness) due to FIP is typically around 3-4% in cluster households. (NOTE: that compares to 28%-30% for FeLV endemic households).

Clinical FIP is seen primarily in cats between six months and five years of age, with the highest incidence occurring between six months and two years. In our experience, however, we have diagnosed fatalities caused by FIP in cats as young as two months of age. FIP infection in adult cats must be regarded as a chronic infection that has persisted for months or years. This may account for the fact that clinical signs attributed to FIP virus are occasionally recognized in adult cats 10 years of age and older despite an excellent history that the cat has lived indoors as the lone cat in the household for all of its life! Don't disregard the fact that the infection was likely acquired from the queen and coronavirus transmission occurred during the first several weeks of life.

The Clinical Disease

Generally speaking, FIP occurs in two distinct forms: an effusive form characterized by peritonitis or pleuritis, or both, and a non-effusive, or dry, form that causes granulomatous lesions in major organs, such as lymph nodes, kidneys, the eyes, and the central nervous system (CNS).

Effusive FIP is characterized by a widespread vasculitis that is responsible for the outpouring of protein and fibrinrich fluid. Although antibody titers do not correlate with immunity, titers will rise simultaneously with the development of lesions of effusive FIP. Cellmediated immunity is probably the only beneficial protective response in this disease, since antibody actually appears to enhance virus uptake by phagocytic cells, a preferred site for virus replication.

The non-effusive form of FIP, clearly the most difficult to diagnose, is characterized by a dramatic granulomatous reaction in localized tissues, such as the nervous system or the eye. Again, antibody is not protective. Cellmediated immunity, if the response is strong enough, will prevent the development of signs of illness in the infected cat.

Cellmediated immunity does not always lead to complete elimination of the virus. Apparently, virus can persist in the body of some cats for an indefinite period of time. With advancing age or druginduced immunosuppression (FeLV infection or steroids), the FIP infection may again become active.