Feline infectious peritonitis (FIP) is relatively rare. Among all cats exposed to the underlying virus, fewer than 5% ever develop FIP. The disease gets outsized attention because it primarily strikes kittens and young cats, and until recently it was almost always fatal. But most cat owners will never encounter it.
How Many Cats Actually Get FIP
To understand how common FIP is, you need to know how it starts. FIP doesn’t spread directly from cat to cat. Instead, it develops from a very common and usually harmless virus called feline coronavirus (FCoV). Somewhere between 50% and 90% of cats will test positive for FCoV antibodies at some point in their lives, meaning they’ve been exposed. In most cats, this virus causes mild diarrhea or no symptoms at all.
FIP happens when the virus mutates inside an individual cat’s body, transforming into a form that attacks the immune system. According to data from UC Davis School of Veterinary Medicine, fewer than 5% of FCoV-infected cats ever make that jump to clinical FIP. Other estimates put the conversion rate between 5% and 12%, depending on the population studied. A large meta-analysis covering cases in China between 2008 and 2023 found an overall FIP prevalence of about 2% across the cat populations studied.
In practical terms, this means FIP is uncommon in the general pet cat population but not vanishingly rare. Veterinarians in busy practices will see cases regularly, even though any individual cat’s risk is low.
Young Cats Are Hit Hardest
FIP has a strong age bias. Around 80% of diagnosed cases occur in cats under 2 years old, with a particular concentration in kittens between 4 and 12 months of age. This is the period when a kitten’s immune system is still developing and when they’re most likely to encounter feline coronavirus for the first time, often after weaning or when entering a new home.
Older cats can and do develop FIP, but it’s far less typical. When it does appear in senior cats, it’s sometimes linked to immune suppression from another illness or significant stress. The strong skew toward young cats is one reason FIP feels so devastating to owners. A kitten that seemed perfectly healthy a few weeks earlier can decline rapidly.
Where the Risk Is Higher
The biggest environmental risk factor is living with multiple cats. Feline coronavirus spreads through feces, so cats sharing litter boxes pass the virus back and forth easily. The more cats in a household, shelter, or cattery, the higher the viral load in the environment and the more opportunities the virus has to circulate and potentially mutate.
Catteries and breeding facilities tend to see more FIP cases than single-cat homes. Shelters are another high-risk setting, partly because of crowding and partly because the stress of shelter life can weaken a cat’s immune response. If you’re adopting a kitten from a multi-cat environment, the chance that your kitten has been exposed to FCoV is high, but remember that exposure alone rarely leads to FIP.
Breed and Genetics
FIP is not a breed-specific disease. It affects purebred and mixed-breed cats alike. However, genetics play a role at the family level. Niels Pedersen, a leading FIP researcher at UC Davis, has noted that the disease tends to follow certain bloodlines within breeds rather than affecting entire breeds uniformly. Certain matings are more likely to produce kittens that eventually develop FIP, which suggests a genetic component to immune susceptibility.
Breeds that have been flagged in clinical observations include Burmese and Birmans, though cases cluster in specific bloodlines rather than across the breed as a whole. If a breeder has had multiple FIP losses from the same pairing, that’s a meaningful red flag worth asking about.
Wet Form vs. Dry Form
FIP shows up in two main forms, and recognizing them matters because they look very different. The “wet” or effusive form causes fluid to accumulate in the abdomen or chest. A kitten with wet FIP may develop a visibly swollen belly, have difficulty breathing, or lose weight despite the bloating. This form tends to progress faster and is often easier to diagnose because the fluid buildup is distinctive.
The “dry” or non-effusive form is subtler. Instead of fluid, it causes inflammatory lesions in organs like the kidneys, liver, eyes, or brain. Symptoms depend on which organs are affected and can include fever that doesn’t respond to antibiotics, eye inflammation, seizures, or a slow decline in energy and appetite. Dry FIP is harder to pin down because its symptoms overlap with many other conditions. Some cats develop a mix of both forms.
Treatment Has Changed the Outlook
FIP was considered a death sentence for decades. Cats diagnosed with it were typically euthanized or given supportive care while they declined. That changed dramatically with the development of an antiviral compound called GS-441524.
UC Davis researchers now estimate the cure rate for FIP treated with GS-441524 at over 80%. Treatment failures tend to result from misdiagnosis, doses that are too low, drug resistance, or complicating health conditions rather than the drug itself being ineffective. Treatment typically involves daily injections or oral medication for 12 weeks, and many cats recover fully, returning to normal energy levels and weight.
Access has been a complicated issue. For years, GS-441524 was only available through underground markets because it wasn’t formally approved. Regulatory approvals have been expanding in several countries, making legal treatment increasingly accessible through veterinarians. If your cat is diagnosed with FIP, effective treatment now exists, which is a sea change from even five years ago.
What This Means for Cat Owners
If you have a single indoor cat, the odds of FIP are very low. Your cat may have been exposed to feline coronavirus at some point, but the vast majority of exposed cats never develop the disease. Testing for FCoV antibodies in a healthy cat isn’t particularly useful, because a positive result just means exposure, not that FIP is coming.
The cats at highest practical risk are kittens under a year old, especially those recently adopted from shelters, catteries, or multi-cat foster homes. Watch for persistent fever, weight loss, a swollen abdomen, eye changes, or neurological symptoms like wobbliness or seizures in the weeks and months after bringing a kitten home. These symptoms aren’t specific to FIP, but they warrant a veterinary visit, and FIP should be on the radar for any young cat that’s failing to thrive without an obvious explanation.