During my veterinary medicine career, there’s been one cat disease I’ve hated diagnosing: feline infectious peritonitis (FIP). There wasn’t a straightforward way to make sure the diagnosis was correct nor was there a treatment. It’s been 100 percent fatal, and I dreaded telling a cat owner their kitten wasn’t going to make it. But because FIP is caused by a coronavirus, something good came out of COVID-19, a treatment for FIP that actually works.
Feline coronavirus (FCoV) is found everywhere in the world. It became a recognized disease in 1963, and the prevalence is up to 100 percent in shelters and catteries. FCoV is an RNA virus, which makes a lot of mistakes when it replicates and doesn’t bother to proofread for mutations. FCoV is highly contagious and spread through feces, which is why it’s much more prevalent in multi-cat shelters, catteries, and homes.
When a cat is infected, the virus replicates in the small intestine and, in the majority of cases, causes no noticeable disease; this biotype is also called feline enteric coronavirus (FECV). In some cats, the virus can cause diarrhea and vomiting for about a week, but the cat recovers on its own. Because these viruses mutate so readily, sometimes the virus leaves the small intestinal cells and enters white blood cells instead; this biotype of FCoV is called the FIP virus.
If the virus jumps from the small intestine into white blood cells, specifically macrophages, the virus starts replicating in the wall of blood vessels, leading to inflammation of the vessels and causing fluid to accumulate in body cavities, like the abdomen or chest. There can also be granulomas—clumps of tissue and white blood cells—that form on internal organs, such as the liver, spleen, brain, and eyes. FIP described as wet is when fluid is found somewhere in the body. When described as dry, there are just granulomas seen. It’s now thought that this is just a continuum of the disease and not separate diseases.
Though the vast majority of cats exposed to FCoV never show signs of illness, there are factors that increase a cat’s risk of getting FIP. Cats with immature or poor immune systems, such as kittens and elderly cats, are more susceptible to developing FIP. Purebred cats, such as Maine coons and British shorthairs, are also overrepresented, as are male cats in general. Cats that are stressed, such as those in multi-cat homes (homes with more than five cats), in overcrowded situations (such as in shelters), and those who have received recent vaccinations or surgery, are also more likely to develop FIP. About 10 percent of cats with FIP had a sibling or housemate that was also diagnosed with FIP.
Typically, a cat with FIP will have a fever, poor appetite, and weight loss and may have diarrhea, vomiting, or abdominal distention. But these are nonspecific signs, and you can see them with other diseases. Some cats will be jaundiced (yellow gums, skin, and whites of the eyes), some have cloudiness or white precipitates in their eyes, and some will walk like they’re drunk. Each clinical sign helps point more toward FIP.
There’s no specific diagnostic test for FIP. In addition to clinical signs, bloodwork will show a high globulin level and maybe an elevated bilirubin, which is what causes a cat to be jaundiced. If a cat has wet FIP, an ultrasound can show fluid in the abdomen or chest, and then a sample can be obtained and examined.
In the past, it was important that veterinarians had as many pieces to the FIP puzzle as possible because our recommendation for cats with FIP was humane euthanasia. We didn’t want to make a mistake in diagnosis. But now there is an effective treatment! While veterinarians don’t want to diagnose FIP with just one puzzle piece that fits, we just need enough pieces to have a high suspicion of the disease. Treatment isn’t inexpensive, so we have to balance expensive diagnostic tests with starting expensive treatment without much delay.
Remdesivir is a medication used to treat COVID-19 in humans. Remdesivir is broken down in the body to another drug, currently called GS-441524 (GS), an oral medication still in investigative stages in the United States (it’s approved in Australia and the United Kingdom). But recently, the FDA stopped blocking its use in cats with FIP, and it’s now available through a licensed compounding pharmacy—a game changer for cats with FIP.
Survival rates with GS are around 83 percent, but if the cat survives 48 hours, that increases to 92 percent—definitely better than the prior 100 percent fatality rate. Given by IV once daily, Remdesivir might be used initially for very sick FIP cats who are unable to swallow. Once a cat is able to take an oral medication, treatment for FIP is a tuna-flavored pill once or twice a day for 12 weeks. For a seven-pound cat, the cost is currently around $1,100.
I still don’t want to diagnose any cat with FIP, but with treatment now available, I don’t dread having the conversation about the cat’s prognosis anymore. If you have a sick kitty, please don’t wait to contact your vet. FIP or not, the sooner we can diagnose and treat your cat, the more likely they will live a long and healthy life.
