Risk Factors for FIP
Experts agree that feline infectious peritonitis occurs more often in young kittens (3 months to 5 years) and older cats (10 to 14 years). Kittens may be most vulnerable to FIP, particularly those under 16 weeks of age, as they have incompletely developed immune systems. Males and females are affected equally.
Purebred cats in general are regarded as being at higher risk, and certain breeds, notably Persians and Burmese, seem to be most susceptible to FIP. One study of pedigree and health data from 10 generations of cats in purebred catteries suggests the existence of a genetic predisposition to the development of FIP. Researchers found inherited susceptibility to FIP may be as high as 50%.
Outdoor cats and cats that live in catteries and multi-cat households are at greater risk for FIP than solitary, indoor animals. Exchanging animals, especially kittens and young cats, increases the risk. Cats in actively breeding households are also considered to be at risk for FIP.
Cats in poor physical condition and those under stress are more susceptible to FIP. The presence of feline leukemia virus (FeLV) or feline immunodeficiency virus (FIV) may predispose a cat to developing FIP. Poor nutrition and husbandry practices that produce highly inbred cats increase the likelihood of infection. The stress associated with neutering, vaccination, or moving to a new home may be risk factors as well.
About 95 percent of cats with FECV remain healthy. In some cats, the infection begins a cascade of genetic mutations within the feline enteric coronavirus (FECV), and the cat's immune system response climaxes in FIP. Most of these mutations are harmless, but some have the effect of causing disease. These mutant FECV strains are called FIPV. Another theory holds that FIP may be caused by FECV and other viruses combined.
Research suggests that some cats have a preexisting antibody to FIPV, which sensitizes the animal to the mutant virus. Instead of killing the virus, the FIPV antibodies seem to accelerate and actually promote the disease process. This phenomenon is called antibody-dependent enhancement (ADE), in which the sensitized cat experiences a more serious disease than a cat that lacks the preexisting antibody.
This vulnerability may be exacerbated by an undeveloped immune system in a kitten or young cat or by weakening the immune system in an older cat. This strongly suggests that most cats do not "catch" FIP, but develop it from their own mutant FECV.