Feline infectious peritonitis (FIP) is a viral disease of cats that is almost invariably fatal. FIP is caused by a coronavirus, which can invade certain white blood cells called macrophages and other body tissues. The virus is similar to other members of the same virus family that infects pigs, dogs, and humans, but the FIP virus itself can cause disease only in cats.
Problems with FIP are found primarily in cats who live in or grew up in multicat environments. Many catteries raising purebreds produce kittens who later develop FIP, and it is a more frequent problem among cats who live in crowded, stressful surroundings where there are concurrent problems with other infections (e.g. feline respiratory disease complex (FRDC)), feline leukemia virus (FeLV) infections) than in more well-managed environments. Some genetic strains of cats may even be more susceptible than others to disease caused by FIP virus.
FIP infection is transmitted from cat to cat by close contact. Virus is shed in oral and respiratory secretions, feces, and urine, so it may contaminate food and water bowls, litter pans, clothing and bedding. Spread via the feces is most common. The virus does not, however, survive longer than a few weeks in the environment and it is easily killed by common household detergents, disinfectants, and bleach. Therefore, problems with FIP infections only persist in households with actively infected cats. In some cases, queens transmit the virus to their kittens while they are still in the uterus. This can result in a disease that occurs within a few days of birth.
A cat may develop signs within a few weeks or many years after infection with FIP virus. Why there is so much variation in the onset of signs is not understood. One theory is that related but harmless coronavirus that infect the bowel (feline enteric coronavirus) must mutate to the illness-causing form in an infected cat. It is known, however, that most often affected cats are between 6 months to 2 years of age. The most characteristic sign of infection is a persistent fever that does not respond to antibiotic treatment. Some cats lose weight, have poor appetites, and become lethargic and depressed. They may also be anemic. Some cats remain alert and eat well until more specific signs of illness apear and become severe, but all cats sick with FIP have a fever that may fluctuate from just slightly above normal to very high.
Other signs of FIP are classically grouped and used to cetegorize the illness into "wet" (effusive) or "dry" (noneffusive) forms. In effusive FIP fluid accumulates in the abdomen and/or chest cavities resulting in abdominal distention and/or respiratory disease. In noneffusive FIP signs of almost any organ involvement may be seen. Common signs include those caused by kidney failure (increased water intake and urination), liver disease, vomiting, and signs of diabetes mellitus. There may be eye inflammation that may lead to blindness and involvement of the nervous system causing a wide variety of signs ranging from hindleg weakness and loss of balance, to seizures and behavior changes. Some cats may have signs found in both wet and dry FIP at once. All these signs are caused by antibody-mediated damage to the small blood vessels (vasculitis) as the body's own immune system tries to eliminate the virus.
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Analysis of fluids from the chest and/or abdomen, biopsy of affected organs, and blood and other laboratory tests are all used to aid diagnosis. Because FIP antibodies in the blood cross-react with other common but harmless coronaviruses found in healthy cats, and because cats not made ill by FIP virus may still harbor antibodies formed during previous exposure, it is not possible to diagnose FIP on the basis of blood tests alone. Many cats have been euthanized unnecessrily on the basis of inaccurately interpreted blood tests. Lab tests that can detect the damage caused by the virus or the FIP virus itself will make diagnosis easier. Currently veterinarians are able to avoid problems in diagnosis only by thoroughly understanding the disease and by carefully evaluating all available information before making the treatment decisions for their patients.
There is no specific treatment for FIP. Cats who have not become too debilitated or who have signs limited to one organ system such as the eyes may live comfortably for weeks or months with adequate supportive care. It is up to you and your vet to decide together what treatment plan is best to follow if your cat is diagnosed with FIP.