January 2024

New Diagnostic Option for Feline Infectious Peritonitis

By Drs. Katherine Klenda and Roman Pogranichniy

Feline Infectious Peritonitis (FIP) is a viral disease of cats resulting from a combination of a mutation of Feline Coronavirus (FCoV) and an abnormal immune response. Most strains of FCoV have low pathogenicity and show mild enteritis and/or upper respiratory signs or no clinical signs of disease. Due to the ubiquitous nature of the virus, nearly all cats have a positive coronavirus serum antibody test result, making interpretation of the titer difficult.

A small percentage of cats will develop FIP due to a mutation in the FCoV ā€œSā€ gene allowing the virus to enter macrophages and spread throughout the body. The resulting inflammatory immune response causes systemic vasculitis. Two forms of the disease exist including the effusive or wet form and non-effusive or dry form. Typically seen in young cats, clinical signs of FIP can vary and may include non-specific signs such as anorexia, weight-loss, and fever, as well as body cavitary effusions, dyspnea, abdominal distension, abdominal lymphadenopathy, irregular renomegaly, uveitis, and neurologic signs. With no FDA approved treatments available, once FIP develops, it is almost always fatal.

At the KSVDL, we offer three test options for FIP:

  • ***New***Feline Infectious Peritonitis (FIP) Virus Antigen Detection (VIF-2013)
    • As our newest and most inexpensive test, this virology assay detects viral antigen present in the macrophages of body cavitary effusions. The result is reported as either positive or negative. This test is specific to the mutated virus present within the macrophages and therefore, exhibits increased specificity to FIP.
  • Feline Coronavirus/FIP IFA (SIF-1064)
    • This serological test detects antibodies to FCoV via indirect fluorescent antibody and is reported as a titer (level of antibodies in the sample). Specimens include serum or peritoneal fluid. If the titer is <50, antibodies to FIP are not detected. Due to most cats having a positive coronavirus antibody test result, interpretation can be difficult. A positive titer only indicates prior exposure to a coronavirus (FCoV), not necessarily a mutated strain that may cause FIP. Titers must be interpreted with consideration for the level of antibodies (titer), clinical signs and other diagnostic testing performed for this animal. The detection of feline coronavirus antibodies does not mean the FIP virus is present in the sample and causing disease.
  • Feline Infectious Peritonitis Virus (FIP) IHC (IHC-1025)
    • This immunohistochemistry test is considered the gold standard. Requiring fixed tissue, this test confirms the presence of the mutated FCoV in the tissue of cats with lesions seen in histology. The result is reported as either positive or negative for FIP.

FIP can be difficult to diagnose by serological test only due to the seroprevalence of FCoV in cats. The addition of our Virus Antigen Detection Test provides increased specificity to FIP and decreases the ambiguity of the FCoV titer.

We encourage you to request VIF-2013 if you are submitting feline body cavitary effusions for FIP testing.

For more information, please reach out to our client care department at 866-512-5650 or clientcare@vet.k-state.edu.


Katherine Klenda, DVM is the KSVDL Section Head for Client Services and a member of the KSVDL Outreach team.

Roman Pogranichniy, DVM, MS, PhD is the KSVDL Section Head for Serology and Virology.



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