Feline Infectious Peritonitis (FIP) represents the worst outcome of infection with a common group of viruses: the feline coronaviruses
(FCoVs). Coronaviruses are common pathogens found in many mammals and birds. They have the largest RNA genome of any RNA virus.
Because of this, errors in replication of genetic material will occur more commonly than in other smaller RNA viruses. Ultimately,
this means that no two coronaviruses are identical! This realization has revolutionized our thinking about the spread of FIP.
Quoting from Dr. Marian Horzinek: "Although generally associated with acute, self-limiting enteric and respiratory infections,
coronaviruses can persist in infected animals. We have shown this to be true for feline coronavirus, which normally cause
only mild enteric infections, and which occur in almost any cattery in Western Europe and America. The low-virulence "enteric"
FCoVs and the disease-causing FIPVs are closely related genetically; we think that the latter are virulent variants of the
former, which arise in individual FCoV-infected hosts. This means that no two cases of FIP are caused by the same virus, and that horizontal transmission, i.e., cat-to-cat transfer
is rather the exception than the rule.
Supporting this we see that serologic studies in the United States have shown that between 75 – 100% of purebred cats in catteries
have FCoV antibodies. This compares to only 30% of non-purebreds living in pet households. Notably, despite these staggering
statistics, only approximately 7-8% will develop FIP.
How do cats get it?
Available data supports a model in which chronic carriers maintain endemic infections in cat societies. Virtually every kitten
born in a breeding facility becomes infected, probably from its queen, as soon as its maternal protection wanes. Once infected,
the cats appear to resist superinfection by closely related FCoVs, with every cat carrying its private, harmless clan of variants.
FIP is a mutation of the benign, non-life threatening enteric coronavirus that causes a self-limiting diarrhea in most purebred
1. Almost all kittens will become infected with FCoV. Within 24 hours of ingesting FCoV particles, they can be found in tonsil
and small intestinal tissues. Within the next 14 days, the cecum, colon, mesenteric lymph nodes and liver will become infected.
2. They will develop antibodies to FCoV within two to four weeks. But they may start to shed FCoV in their feces in as short
a time as two days.
3. They develop diarrhea, which resolves.
4. In most cats, the FCoV remains in the enterocytes without causing further illness. These cats will continue to shed FCoV
in feces, which can infect other cats and kittens. Virus is spread between cats from the feces or saliva to the oral or nasal
tissues. In some cats FCoV strays from the enterocytes infecting circulating monocytes and tissue macrophages. Macrophages
and monocytes cannot rid themselves of coronaviral infection.
5. In some cats, the virus will mutate and gradually become a unique virulent virus that can cause FIP.
6. In some of these cats, there will be no cell-mediated immunity (CMI) and the cat will develop "wet" FIP.
7. In some of these cats there will be a partially protective cell mediated immune response which will eventually allow the
mutated virus to cause the "dry" form of the disease.
8. In some of these cats, it will be held in check by solid cell mediated immunity until either the CMI declines (e.g. old
age, FeLV infection, FIV infection, chemotherapy, etc.)
9. In some of these cats, it may become a latent infection but these "immune" cats continue to shed FCoV to other cats.
10. Feral cats are at low risk for infection as roaming behaviour precludes fecal-oral transmission.
11. At highest risk are kittens raised "under-foot". Kittens raised in isolation with their queens are at next greatest risk.
Early weaned and isolated kittens are at the lowest risk for coronaviral infection.