For years, feline infectious peritonitis, or FIP, was one of the most devastating diagnoses a cat owner could hear. The disease was considered nearly always fatal, with few options available once a cat became sick. Today, however, that conversation can look very different.
“Survival rates for the wet form of FIP with medication are now as high as 80% to 90%,” says Dr. Jennifer Reinhart, a small animal internal medicine specialist at the University of Illinois Veterinary Teaching Hospital in Urbana.
What Is FIP?
FIP develops from feline coronavirus, a virus that normally remains in a cat’s gastrointestinal tract. Many cats infected with this form of coronavirus show no signs at all, while others may have mild vomiting or diarrhea. FIP occurs when the virus escapes the gut and spreads elsewhere in the body.
“When the virus escapes from the gut and gets into the rest of the body, it causes massive inflammation, illness, and potentially death,” Dr. Reinhart explains.
For owners, one of the challenges is that FIP may not look alarming at first. Early signs can include lethargy, weight loss, and fever, changes that could easily be blamed on many other illnesses. This is one reason a cat that simply seems “off” for an extended period may need further evaluation rather than waiting for a more obvious sign to appear. As the disease progresses, some cats develop fluid in the abdomen or chest, which can make the condition more noticeable.
FIP can also look different from one cat to another. Cats with “wet” FIP develop fluid in the abdomen, chest, or around the heart. Cats without this fluid accumulation are considered to have “dry” FIP. The disease can also affect the eyes or brain, leading to ocular or neurological disease, and some cats may initially have the dry form before later developing fluid.
How Is FIP Diagnosed?
Even after FIP is suspected, getting a clear answer is not always simple.
A definitive diagnosis requires a tissue sample from an affected organ, which can be difficult and invasive to obtain. Veterinarians therefore often rely on the cat’s overall clinical picture along with other testing, including PCR.
Importantly, a positive PCR result does not automatically mean a cat has FIP.
“The problem is that these tests do not reliably distinguish coronavirus from the gut and coronavirus that has spread to the rest of the body,” Dr. Reinhart says.
Testing fluid from the chest or abdomen of a cat with wet FIP tends to be more useful. Dry FIP can be harder to diagnose because there may be no fluid available to test. For owners, this means diagnosis sometimes requires putting several pieces of information together rather than relying on one simple blood test.
FIP occurs most commonly in older kittens and young adult cats up to about 2 to 3 years old, with another increase seen in cats around 10 years of age. Purebred cats and cats living in large-group environments may also be at greater risk.
Treatment for FIP
Antiviral medications, including GS-441524 and molnupiravir, became legally available in the United States in 2024. Treatment generally involves daily or twice-daily oral medication for about 12 weeks. Cats typically need veterinary visits at least every two weeks early in treatment so their response can be monitored and their dose adjusted as they gain weight.
Most cats that respond positively appear to do very well after treatment, although long-term data are still being collected. Relapse can occur, particularly soon after medication is stopped, and some cats may need additional treatment.
For owners, perhaps the most important thing to understand is that FIP now comes with real reason for hope, although not a guarantee.
“The success rate is really high now, which is wonderful,” Dr. Reinhart says, “but some cats will still die of FIP, even if they get treatment.”
While still serious, an FIP diagnosis no longer automatically means there is nothing that can be done. By working closely with a veterinarian to reach the best possible diagnosis and treatment plan, owners can give many affected cats a chance that simply did not exist a few years ago.
By Julia Bellefontaine
