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. The feline enteric coronavirus (FECV), another very common virus known to infect the GI tract of kittens-especially
those living in multiple-cat households, has "traditionally" been described as causing mild, transient diarrhea in kittens.
In other kittens, infection causes no clinical signs at all…but…THAT'S WHAT WE USED TO THINK! Some compelling evidence about
the role of the FECV in the pathogenesis of FIP has recently been published and sheds quite a different picture of this so-called
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 fibrin-rich
fluid. Although antibody titers do not correlate with immunity, titers will rise simultaneously with the development of lesions
of effusive FIP. Cell-mediated 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. Cell-mediated immunity,
if the response is strong enough, will prevent the development of signs of illness in the infected cat.
Cell-mediated 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 drug-induced immunosuppression (FeLV infection or steroids),
the FIP infection may again become active.