Feline infectious peritonitis (FIP) is one of the leading infectious causes of mortality in young cats.1 In this article, we review the clinical signs, diagnostic options, and treatments for this complex disorder.
How it all begins
Feline coronavirus is a single-stranded RNA virus that is endemic in many multicat households, shelters, and catteries.2 In the common enteric form, feline coronavirus replicates in enterocytes and causes mild, self-limiting diarrhea. The virus
may be shed in feces for several months after infection and is spread to other cats through oral ingestion. It survives in
the environment for several weeks, and transmission may occur through shared litter pans, mutual grooming, or fomites. Young
cats are more vulnerable to infection than adults are.2
Mutations routinely occur within the feline coronavirus genome through nucleotide substitution, deletion, and recombination.3 Certain mutations transform the virus into a virulent biotype, which is able to replicate in monocytes and macrophages.
This mutated form is called FIP virus and causes a fatal systemic inflammatory disease. Estimates vary, but fewer than 10% of cats infected with feline coronavirus
will develop FIP.2 Why the disease arises in some cats but not in others is not completely understood.2 Purebreed cats appear to be predisposed, although the incidence within specific breeds varies among countries.2
The cat's immune system appears to play a crucial role. Those with a prompt and robust cell-mediated immune response are able
to destroy the FIP virus and are unlikely to develop clinical disease.2
Effusive vs. noneffusive
There are two main forms of FIP: effusive (wet) and noneffusive or granulomatous (dry). Some patients will switch between
the two forms over the course of their illness. The development of one type vs. the other depends on the balance between the
host's humoral (antibody-related) and cell-mediated immune responses. Cats with strong humoral responses develop effusive
FIP, while those with mixed responses manifest the dry form.2
How FIP takes hold
The FIP virus infects mononuclear phagocytes and spreads hematogenously through infected monocytes. These may attach to the
vascular endothelium or migrate into tissues. The virus replicates within macrophages, which die as the virus is released.
This death of the cell and viral release triggers a marked inflammatory reaction, with the recruitment of more inflammatory
cells and the release of cytokines and the activation of complement.
Vasoactive agents result in vasculitis and effusion in the chest, abdomen, or pericardial sac.4 Perivascular pyogranulomatous lesions develop on serosal surfaces and within solid organs and are characterized by an infiltrate
of macrophages and neutrophils.
Because the FIP virus is strongly cell-bound and tissue-bound, shedding is unlikely unless there is effacement of the renal
tubules or intestinal mucosa. Consequently, FIP is not regarded as a contagious disease, and the risk of horizontal transmission
to other cats is minimal.2