Feline Infectious Peritonitis Testing

By Malcolm Weir, DVM, MSc, MPH; Tammy Hunter, DVM; Kristiina Ruotsalo, DVM, DVSc, ACVP; Margo S. Tant, BSc, DVM, DVSc

What is feline infectious peritonitis (FIP)?

FIP is a disease caused by a mutated (changed) strain of feline coronavirus. This mutation allows the virus to spread throughout the body within specific white blood cells called macrophages. Organs and tissues that normally contain large numbers of macrophages, such as the liver, spleen, and lymph nodes, are targeted, resulting in a variety of clinical signs.

See the handout “Feline Infectious Peritonitis” for more information about this disease.

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 or changes, clinical disease occurs. The number of cats exposed to and therefore carrying antibodies to feline coronavirus is high (estimated to be up to 30% within the general cat population and up to 80% within catteries), but the proportion of cats developing FIP is small.

"FIP is one of the most challenging diseases to diagnose..."

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 wanting to maintain a coronavirus-free status may routinely test for the presence of feline coronaviruses.

What blood tests are available to detect FIP?

As mentioned, FIP testing is somewhat problematic. Although FIP is caused by a mutated strain of coronavirus, exposure to any strain of feline coronavirus will result in an immune response and the production of antibodies, therefore resulting in a positive blood test. There is currently no blood test that will distinguish between antibodies produced against a non-FIP strain of coronavirus and a FIP-causing strain of coronavirus.

To complicate the diagnosis even further, a negative blood test for coronavirus antibodies does not entirely rule out the possibility of FIP infection in a sick cat, as detectable antibody concentrations may be reduced in animals with the terminal form of FIP.

Even newer PCR (polymerase chain reaction) tests that have been designed to detect viral genetic material are unable to accurately distinguish between the different strains of coronavirus. Results of PCR testing on fluid from the abdomen of a suspected FIP cat can be helpful to rule in or rule out the disease. PCR testing of the blood from a suspected case tends to be less definitive for diagnosing FIP.

"A working diagnosis of FIP is typically made based on the cat's clinical history and supportive laboratory data."

A working diagnosis of FIP is typically made based on the cat's clinical history and supportive laboratory data. Because the FIP coronavirus can invade many different tissues, routine blood testing may indicate evidence of kidney or liver damage. Due to the inflammatory nature seen with FIP, protein levels in the blood are often increased. However, these tests can only provide additional evidence supporting the diagnosis of FIP.

Some cats with FIP develop effusions (fluid accumulations) within 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 is supportive of FIP but not absolutely diagnostic for the disease.

Many laboratories provide feline coronavirus antibody tests, but these alone cannot be used to diagnose FIP. If a cat has clinical signs consistent with a diagnosis of FIP then a positive antibody test is supportive of the diagnosis, but not conclusive. Likewise, a negative test in the presence of advanced signs does not rule out the diagnosis of FIP. Some laboratories provide tests such, as PCR tests, which can detect very small amounts of the virus in the effusions, but no unique genetic sequence associated with FIP has been identified.

Histopathology (the collection of tissue samples that are evaluated microscopically) remains the best way to diagnose FIP in the living cat.

"Histopathology remains the best way to diagnose FIP in the living cat."
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