What is feline infectious peritonitis (FIP)?
Feline infectious peritonitis (FIP) is a disease caused by a mutated (changed) strain of feline coronavirus. This mutation allows the virus to spread throughout the body in white blood cells called macrophages. The virus targets organs and tissues that normally contain large numbers of macrophages (e.g., liver, spleen, and lymph nodes), resulting in a variety of clinical signs.
When is FIP testing recommended?
FIP is one of the most challenging diseases to diagnose because feline coronaviruses are commonly found in the intestinal tract of many healthy cats. When this virus mutates, clinical disease occurs. Many cats are exposed to feline coronavirus and therefore carry antibodies (up to 30% of the general cat population and up to 80% within catteries), but only a small proportion of those cats develop FIP.
Unfortunately, routine blood testing for feline coronavirus is not clinically useful. Instead, testing is restricted to those cats in which a diagnosis of FIP is strongly suspected due to clinical signs and other supportive laboratory data. Occasionally, catteries and multi-cat households that want to maintain a coronavirus-free status may routinely test for the presence of feline coronaviruses.
What blood tests are available to detect FIP?
FIP testing is somewhat problematic. A working diagnosis of FIP is typically made based on the cat’s clinical history and supportive laboratory data. Routine blood testing may indicate evidence of kidney or liver damage, because the FIP coronavirus can invade many different tissues. Protein levels in the blood are often increased, due to the inflammatory nature of FIP. The combination of these findings adds extra evidence to support a FIP diagnosis, but there is no blood test that confirms it.
Some cats with FIP develop effusions (fluid accumulations) in the abdominal or chest cavities. Your veterinarian may collect a sample of this fluid and submit it to a veterinary laboratory for evaluation by a pathologist. FIP-related effusions have a characteristic protein content and appearance when examined microscopically. The identification of such fluid supports a diagnosis of FIP but is not absolutely diagnostic for the disease on its own.
Many laboratories provide feline coronavirus antibody tests, but these tests alone cannot be used to diagnose FIP. If a cat has clinical signs consistent with FIP, then a positive antibody test supports the diagnosis but is not conclusive. There is currently no blood test that distinguishes between antibodies to a non-FIP strain of coronavirus and antibodies to a FIP-causing strain of coronavirus. Likewise, a negative coronavirus test, in the presence of advanced signs, does not rule out a FIP diagnosis.
Similarly, polymerase chain reaction (PCR) tests have been designed to detect viral genetic material (DNA), but these tests cannot accurately distinguish between the different strains of coronavirus. PCR testing of the blood of a suspected FIP cat tends to be less helpful in diagnosing FIP than testing on fluid from the abdomen or chest. No unique genetic sequence associated with FIP has been identified.
The best way to diagnose FIP in a living cat is through histopathology (the microscopic evaluation of tissue samples).
