FIP is caused by a mutation of FECV. Although the mutation of FECV to FIPV is common, it is fortunate that only a small percentage of cats exposed to this mutant virus will get FIP. FECV is undergoing continuous mutation and several genetic forms of the virus may co-exist in the same animal at the same time. Most of these mutations have very little effect on the behavior of the virus and merely serve to genetically reflect the region from which the virus originated. However, certain have a pronounced effect on the biologic behavior of the virus One study indicated that 20% of the kittens infected with FECV will produce an FIP mutant. Of course, only a fraction of the mutants will go on to produce FIP, depending on host resistance factors (genetic or non-genetic).
This FECV to FIPV genetic change is referred to as the internal mutation theory. The internal mutation theory has two corollaries: 1) that each cat that develops FIP, even if it is a littermate, closely related or commonly housed, has a unique mutation, and 2) that horizontal (cat-to-cat) transmission of the FIPV mutant is uncommon. Researchers at UC Davis have reconfirmed corollary 1, and have confirmed corollary 2 in concept but not in fact. Reconfirmation for the internal mutation theory came from a recent outbreak in three kittens in a litter of Scottish Folds and in a half-sibling from a second litter. All four FIPVs had very similar yet genetically unique gene mutations. These mutations were not present in a fecal form of the virus that was being shed by a healthy contact cat.