FIP in Cats: Feline Infectious Peritonitis — Causes & New Treatments

Feline infectious peritonitis, or FIP, is a serious disease linked to feline coronavirus (FCoV). Most cats exposed to feline coronavirus never develop FIP. In some cats, though, the virus changes inside the body and begins infecting immune cells, which can lead to widespread inflammation in the abdomen, chest, eyes, brain, liver, kidneys, and other tissues.

Vets usually describe FIP in two broad forms. Wet (effusive) FIP causes fluid buildup in the belly or chest. Dry (non-effusive) FIP causes inflammatory lesions in organs and may lead to eye disease or neurologic signs. Some cats have features of both forms, so the disease does not always fit neatly into one category.

FIP is seen most often in kittens and cats under 2 years old, especially those from multi-cat settings or stressful environments. The good news is that treatment has changed dramatically. What was once almost always fatal is now often treatable with antiviral medication, especially when your vet can diagnose it early and start care promptly.

FIP begins with infection by feline enteric coronavirus, a very common virus spread mainly through fecal-oral exposure, especially in multi-cat homes, shelters, and breeding settings. In most cats, this virus causes no signs or only mild digestive upset. In a smaller group, the virus mutates inside that individual cat and gains the ability to survive in certain white blood cells, which helps it spread through the body and trigger intense inflammation.

That is why FIP itself is generally not considered directly contagious in the way a cold is contagious. Cats spread the common enteric coronavirus, not the disease form itself. Risk appears to be higher in young cats, cats under stress, cats living in crowded environments, and cats with immune challenges. Some bloodlines may also be more susceptible, which is why breeders and rescue groups often pay close attention to litter history.

Stress does not cause FIP by itself, but it may play a role in which cats become sick. Recent rehoming, surgery, weaning, illness, or major household change can overlap with the time symptoms appear. If your cat has risk factors and starts losing weight, running a fever, or developing fluid buildup, your vet may recommend testing for FIP early rather than waiting for the picture to become more severe.

Diagnosing FIP is often a process of putting several clues together. There is no single blood test that proves every case. Your vet usually combines your cat's age, history, exam findings, blood work, imaging, and sometimes fluid or tissue testing to decide how likely FIP is.

Common supportive findings include high globulins, low albumin-to-globulin ratio, anemia, increased bilirubin, and inflammatory changes on blood work. In wet FIP, fluid from the abdomen or chest is especially helpful. That fluid is often yellow to straw-colored, sticky, and high in protein. A Rivalta test may be used as a screening tool on effusion, and PCR testing on fluid or tissue can add support.

The most definitive diagnosis comes from finding coronavirus antigen in affected tissue, often through immunohistochemistry or similar tissue-based testing, but that is not always practical or safe in a sick cat. In real-world practice, many cats start treatment based on a strong presumptive diagnosis, especially when the clinical picture and test results fit well and delaying care could worsen the outcome.

There is no guaranteed way to prevent FIP, because the disease depends on how a common feline coronavirus behaves inside an individual cat. Prevention focuses on lowering coronavirus exposure and reducing stress, especially in kittens and multi-cat homes. Good litter box hygiene, prompt feces removal, avoiding overcrowding, and separating age groups when possible may help reduce viral load in the environment.

Stress reduction also matters. Gradual introductions, predictable routines, good nutrition, parasite control, and prompt care for other illnesses may support overall health during high-risk periods like adoption, weaning, transport, or surgery. Breeders and rescue groups may also work with their vets on sanitation plans and kitten management strategies.

An intranasal FIP vaccine exists, but it is not routinely recommended by many feline experts because evidence for consistent benefit is limited. The most practical message for pet parents is this: while prevention is imperfect, early recognition and fast veterinary care now make a major difference, because effective antiviral treatment is available through veterinary prescribing channels in the United States.