Feline Coronavirus/Feline Infectious Peritonitis Antibody Test (IFA)
Feline Coronavirus (FCoV) is an extremely infectious virus of cats with a high morbidity but relatively low mortality rate. In a specific pathogen free cat colony the introduction of one cat infected with FCoV will result in 85% of the colony infected within a month. A serious disease associated with FCoV infection is Feline Infectious Peritonitis (FIP). This disease is characterized by the formation of pyogranulamatious lesions that lead to either an effusive (wet FIP), or non-effusive (dry FIP).
FIP is an autoimmune disease that is caused when FCoV is processed in the cat’s protesome producing altered peptide fragments that are presented to the immune system. These altered peptides mimic a component of the cat’s normal protein structure and the immune cells that are generated to dispose of these viral peptides then attack normal cat tissue structures causing damage. The more tissue damage that the immune cells do the more peptides are generated and more immune cells are produced forming a vicious cycle that is very difficult to stop. Cats with full-blown FIP do not survive for more than a few months even with the most vigorous treatment.
Our Test for FCoV antibodies is an indirect fluorescent antibody (IFA) procedure which is carried out on Teflon matted glass slides that contain fixed virus infected cells (TN406 virus in CRFK cells). This method requires the use of diluted patient serum being placed on the slide and incubated for 30-minutes at which point the slide is washed and a fluorescein conjugated anti-cat globulin is placed on the slide. If any antibodies to FCoV are present in the patient serum they will combine with the FCoV antigen fixed to the slide surface. The fluorescent antibody conjugate will then be bound to the FCoV antibodies and the resulting antibody-conjugate complex viewed with an ultraviolet microscope are seen as bright areas of fluorescence in the infected cells which contain multi-nucleated syncytium and do not exceed 40% of the cells on the slide. The negative cells enhance contrast.
Cats with FCoV antibody titers are at risk for developing FIP disease but the incidence of FIP is low and is likely dictated by genetics. Clinical signs of FIP are fever of unknown origin, total protein above 8g/dl with globulin component elevated, white blood count over 20,000, anterior uveitis, effussive abdominal fluid, and lumpy/bumpy kidney on palpation. Seroconverted but asymptomatic cats should be watched for signs of disease.
Much emphasis has been placed on titer levels in FCV/FIP testing. In our opinion the titer level is not diagnostic without clinical signs. We have seen thousands of normal cats with elevated FCV titers who never show any signs of FIP. FCV titers should be used as one part of determining the definitive diagnosis of FIP.