Feline Infectious Peritonitis (FIP)

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In memory of Sully

Feline Infectious Peritonitis (FIP) is a severe multi-organ-system viral disease that affects domestic cats as well as other felines [2, 5]. Until very recently, a diagnosis of FIP was a death sentence for a cat. It is still often fatal. However, knowledge is power. Knowing when to go to the vet and what your options are can save a life.

What Causes FIP?

FIP starts with an intestinal virus called feline enteric coronavirus (FeCV or sometimes FCoV). You can’t catch this coronavirus; only felines can [1, 4, 5, 9]. FeCV is very common among cats, especially in places where many cats live close together like shelters and catteries [1, 2, 5, 8]. Most cats infected with FeCV experience no symptoms, mild diarrhea, or mild respiratory illness before defeating the infection on their own [1, 4, 5, 8, 9]. In the meantime, they can easily spread the infection through their fecal matter or, less often, saliva [1, 2, 4-6]. Cats usually become infected by getting the virus in their mouths [1, 5, 6]. Shared litter boxes are a typical culprit [1, 2, 4-6]. An uninfected cat needs only to step in the litterbox after an infected cat has used it, and then when they clean their paws later, they become infected themself.

But what does this benign virus have to do with FIP? It isn’t well understood, but in about 10% of cats a FeCV infection turns into FIP [1, 2]. This is believed to occur as a combination of two phenomena. First, FeCV undergoes a spontaneous mutation that enables it to infect macrophages, a type of white blood cell [1, 2, 5, 8, 9]. This is the feline infectious peritonitis virus (FIPV). Second, the immune systems of certain cats overreact to the mutated virus, causing out-of-control inflammation and damage [1, 4, 6]. Incidentally, it is an overblown immune reaction to the coronavirus of the moment, COVID-19, that makes it so dangerous for a proportion of the population.

In the bodies of cats where both of these things happen, FIP develops. Some cats are more prone to FIP than others. Although the specific genetic risk factors remain unclear, specific breeds are at greater risk: Abyssinians, Bengals, Birmans, Himalayans, Ragdolls, Cornish Rexes, and Devon Rexes [6, 8]. Younger cats and most especially kittens make up the majority of FIP cases [1, 4, 6]. Their weaker immune systems are less able to fight off a FeCV infection, giving it more time to mutate [5]. Unneutered cats also seem to be at greater risk, for unknown reasons [8].

Symptoms and Subtypes

The most common and earliest symptoms of FIP are, frustratingly, not very specific. They may include fever, change in appetite, weight loss, and depression [1, 5, 6]. As FIP advances, it develops into one of two general forms: wet/effusive or dry/non-effusive. In the wet form, fluid builds up in the abdominal and chest cavities and may cause breathing difficulty [1, 4-6, 8]. The dry form causes neurological and eye symptoms such as seizures and uveitis (inflammation of the outer layers of the eye) in addition to the non-specific symptoms [1, 4-6, 8].

Diagnosis

Diagnosing FIP is difficult and complicated. The symptoms can all be signs of other diseases. Once it is suspected, there is no definitive, foolproof test for FIP. At right is a diagnostic scheme for FIP from one scientific article as an example of the mental gymnastics veterinarians have to do when deciding how to pursue FIP testing.

The options include blood and fluid tests, genetic tests for FIPV, and the veterinarian’s observations during an exam [1, 2, 4-6]. Many of the tests are complicated by the fact that so many cats have been infected by FeCV [1, 2, 5]. A cat’s veterinarian has to determine the best course of testing based on their individual case.

Treatment

Until just a couple years ago, nothing could be done for cats with FIP except supportive care to ease their suffering and extend their life a short time. For many cats, that is still the case, for reasons I will get to shortly, or simply because their disease is too severe. Supportive care consists of subcutaneous or intravenous fluids, draining fluid from body cavities if applicable, steroids to control inflammation, and immunosuppressive medication [1, 2, 4-6].

GS-441524

GS-441524 is an antiviral drug that prevents viruses from synthesizing RNA, a molecule which many viruses, including FeCV/FIPV, use instead of DNA [3, 9]. It was created by the pharmaceutical company Gilead, which specializes in antivirals [9]. Gilead first tested GS-441524 against Ebola but was ineffective [9]. However, FIP researcher Niels Pedersen knew someone at Gilead and asked if they could send him anything to test [9]. His friend at Gilead obliged, and Pedersen found two promising candidates that killed FIP in cultured cat cells [9]. He moved on with testing one of them, GS-441524, in cats with FIP [9].

The results were incredible. In a laboratory study, ten of ten infected cats recovered [9]. In a field trial with 31 pet cats with non-neurological FIP, 25 recovered, still an unheard-of success rate for this otherwise inevitably fatal disease [7, 9]. A more recent pilot study suggests that GS-441524 is effective in neurological and ocular FIP, too [2]. Side effects were not too bad. Cats experienced pain and skin reactions at the injection site, sometimes developing sores or scars [7]. This may be because the formula is very acidic [7].

Gilead, however, has declined to license GS-441524 for animal use [9]. GS-441524 is almost identical, molecularly, to another one of their drugs, remdesivir, which they are trying to get licensed for human use, and they believe that licensing GS-441524 for FIP would harm their chances with remdesivir [9]. Cat people and Chinese entrepreneurs have gotten creative, however.

GS-441524 is not a difficult molecule to synthesize, so numerous outfits in China have chosen to manufacture and sell it, filling the gap Gilead left open [9]. Because GS-441524 is not FDA-approved, veterinarians cannot prescribe it, and it’s illegal for cat parents to purchase and administer it [1, 9]. That has not stopped desperate people who want to save their cats. Because it is a black market, there is a risk that the product will not be as advertised [1, 9]. And it is incredibly expensive: it costs 7,000-12,000 USD for the 12-week course of medication [9]. When people can manage to acquire it, however, GS-441524 seems to be as good as the research suggested [1, 9].

If your cat has been diagnosed with FIP, and you would like help accessing GS-441524 treatment, contact FIP Warriors.

Preventing FIP

Because FeCV is so common and easily spread, it’s hard to prevent FIP entirely. There are some things you can do, however. Keep litter boxes clean and far away from food and water dishes [1, 6]. Cats are more vulnerable to developing FIP if they have another disease like FeLV or FIV, so get your cats vaccinated against all those easily preventable diseases [1, 6]. There is one FDA-approved vaccine against FeCV, but it does not seem to be all that effective [1, 5, 6]. You can always ask your veterinarian about the FeCV/FIP vaccine if you’re concerned, however. Keep in mind that cats often get FeCV infections at animal shelters and catteries, so you may want to isolate any new cats for a couple weeks [1, 5].

Works Cited

1. Cornell Feline Health Center. (2020). Feline infectious peritonitis. Cornell University College of Veterinary Medicine. Link
2. Diaz, J. V. & Poma, R. (2009). Diagnosis and clinical signs of feline infectious peritonitis in the central nervous system. The Canadian Veterinary Journal, 50(1), 1091-1093. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2748294/
3. Dickinson, P. J., Bannasch, M., Thomasy, S. M., Murthy, V. D., et al. (2020). Antiviral treatment using the adenosine nucleoside analogue GS-441524 in cats with clinically diagnosed neurological feline infectious peritonitis. Journal of Veterinary Internal Medicine, 34(4), 1587-1593. https://doi.org/10.1111/jvim.15780
4. Feline infectious peritonitis (FIP). (n.d.). Veterinary Specialty Center. https://www.vetspecialty.com/feline-infectious-peritonitis-fip/
5. Hartmann, K. (2005). Feline infectious peritonitis. Veterinary Clinics: Small Animals Practice, 35(1), 39-79. https://doi.org/10.1016/j.cvsm.2004.10.011
6. Levy, J. K. & Hutsell, S. (2020, October). Feline infectious peritonitis (FIP). Merck Veterinary Manual. https://www.merckvetmanual.com/cat-owners/disorders-affecting-multiple-body-systems-of-cats/feline-infectious-peritonitis-fip
7. Pedersen, N. C., Perron, M., Bannasch, M., Montgomery, E., et al. (2019). Efficacy and safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis. Journal of Feline Medicine and Surgery, 21(4), 271-281. https://doi.org/10.1177/1098612X19825701
8. Pesteanu-Somogyi, L. D., Radzai, C., & Pressler, B. M. (2006). Prevalence of feline infectious peritonitis in specific cat breeds. Journal of Feline Medicine and Surgery, 8(1), 1-5. https://doi.org/10.1016/j.jfms.2005.04.003
9. Zhang, S. (2020, May 8). A much-hyped COVID-19 drug Is almost identical to a black-market cat cure. The Atlantic. https://www.theatlantic.com/science/archive/2020/05/remdesivir-cats/611341/

Published June 15, 2022

Updated November 12, 2023