Chapter 10 Feline Infectious Peritonitis (Feline Coronavirus)
Feline infectious peritonitis (FIP) is a progressive and highly fatal systemic disease of cats caused by feline coronavirus. Feline coronavirus most frequently causes inapparent enteric infection with fecal shedding of virus. Mild enteritis and diarrhea are seen rarely (see Chapter 14). A mutation of feline coronavirus during intestinal replication enables it to infect macrophages and cause FIP. Despite its name, the lesions of FIP are widespread and not restricted to the peritoneum. Effusive and non-effusive forms of FIP occur. Since its recognition in the 1950s, FIP has been one of the most studied diseases of cats, yet a definitive diagnostic test, an effective treatment, and a reliable vaccine are lacking. With the decline in prevalence of feline leukemia virus from vaccination, FIP has become the deadliest infectious disease of cats.
Any cat with inapparent FCoV infection has the potential to develop FIP if the virus mutates during replication to allow the mutated FCoV variant to infect macrophages. Viral replication in macrophages is the defining event in FIP.
Cats with non-mutated FCoV infection of the intestinal tract infrequently develop clinical signs (see Chapter 14). This section describes the clinical manifestations of mutated, FIP-producing FCoV infection. Cats with FIP often present initially with nonspecific and non-localizing signs, such as fever, anorexia, inactivity, weight loss, vomiting, diarrhea, dehydration, and pallor (anemia). As the disease progresses, these signs worsen and additional clinical signs develop that indicate either body cavity effusions in the “wet” form of the disease or organ-specific abnormalities in the non-effusive or “dry” form (Table 10-1). Approximately 75% are effusive and 25% are non-effusive. Some cats manifest features of both effusive and non-effusive disease or change over time from one form to the other.
The non-effusive (dry) form of FIP is characterized by multifocal pyogranulomatous inflammation and necrotizing vasculitis in various organs, such as the abdominal viscera (e.g., liver, spleen, kidneys, pancreas, and intestines), eyes, central nervous system (CNS), and lungs.
Pyogranulomas appear as multiple discreet or coalescing gray-white nodular masses of variable size on the surface and within the parenchyma of affected organs. These are often mistaken for tumors. Effusion is often minimal or absent. The specific organs affected and the degree of resulting organ failure determine the presenting clinical signs.