The Silent Killer
What is FIP and why should you care?
Feline Infectious Peritonitis (FIP) is a serious disease in cats. It can be extremely tricky to diagnose, fatal if untreated, and doesn’t have FDA-approved treatment options. Moreover, the virus that (kind of, possibly, eventually) causes the disease is everywhere — more than half of the cats in shelters and commercial catteries carry feline coronavirus (FCoV), the virus that can mutate into a strain that causes FIP. Confused yet? Then read on!
FCoV what?
Feline Coronavirus exists as two distinct varieties: a relatively harmless Feline Enteric Coronavirus (FECV) that lives and replicates in small intestine of cats, and is shed in feces. The viral particles can enter uninfected cats (and especially kittens) if they share letterboxes, and the virus extremely common: no shelter or cattery was found to be free of FECV, and in some prevalence was as high as 100%. Most cats do not have any clinical signs, and the infection goes away on its own, as most cats’ immune system is perfectly capable of dealing with this weakling little virus. However, some cats become “persistent shedders”, meaning that they continue to harbor and shed the virus for weeks or months, becoming the source of infection in others. Kittens are especially vulnerable, and can get infected as early as 2 weeks of age.
FECV usually produces no symptoms, but some cats and kittens develop diarrhea. Sometimes, we see cases of persistent diarrhea that would just not go away, in cats and kittens who tested negative for intestinal parasites. It is likely that FECV causes those mystery diarrhea cases, and an expense of a PCR test may be a good investment in those cases!
And then some cats develop peritonitis, right?
Not quite! The FECV, like most viruses (and especially corona viruses) mutates a lot! So each cat who is shedding the virus often sheds multiple variants (think of them as sentences in the game of telephone — with each replication, errors creep up, creating slightly different versions of the virus. Some of those become capable of infecting white blood cells, not just cells of the small intestine. Simply put, these versions can reach other organs and systems — but are no longer shedding in the feces! With some additional mutations, these viruses can start causing the disease (FIP), and the virus at this stage is called FIPV (Feline Infectious Peritonitis Virus).
So cats with FIP can infect other cats?
No! Remember, this version is no longer shedding in the feces, so the cycle of mutations have to start anew in each new case — from FECV to FIPV. FIPV is not passed along directly.
You said it was common! Why is so difficult to diagnose then?
Part of it is because the symptoms are often so vague — loss of appetite, listlessness, vomiting… these symptoms can describe any disease under the sun! The most definitive diagnostic tool for FIP is fine needle aspiration in cats that develop abdominal swelling as a result of FIP. Ascitis (fluid accumulation in the abdomen) is often the first sign anything is wrong, and if the fluid is yellow and has other characteristics, FIP can be diagnosed definitively. But not all cases have this fluid accumulation, and are thus often overlooked or misdiagnosed. Sometimes FIPV affects the brain (white blood cells can travel anywhere in the body, including the brain), and neurological symptoms can develop too.
To make things even more complicated, FIPV effects are extremely variable from one individual to the next, since the immune system and other physiological characteristics of the cats determines whether FIP develops, and in what form.
So is it fatal or can it be treated?
Yes and yes! FIP is a serious condition, fatal if left untreated. While there is no approved cure, some antivirals have shown to be effective against FIP. These drugs can be purchased from Canada and China, and while it is not technically approved in the US, some vets may choose to recommend these treatments “off-label”. Nucleoside analog GS-441524 is more effective than previously studied viral protease inhibitor GC376 in some cats, but both are used, and both can be obtained via gray market. Facebook groups, such as FIP 5.0 warriors, assist owners of cats with FIP to obtain this lifesaving treatment. It is expensive ($5,000-10,000), but currently it is the only option available. A vaccine against FECV is currently being developed.
FIP is a very serious condition, but there is hope! As trials continue, FDA is expected to approve antiviral GS-441524 following the clinical trials currently underway in UC Davis. Meanwhile, there are unapproved options!
References and further readings:
https://catvets.com/guidelines/practice-guidelines/fip-guidelines
https://www.avma.org/javma-news/2020-01-15/fip-drugs-continue-show-promise-while-being-sold-black-market
https://ccah.vetmed.ucdavis.edu/cats/resources/general-feline-infectious-peritonitis-resources