Feline Infectious Peritonitis in Cats

Feline infectious peritonitis, usually called FIP, is a serious inflammatory disease associated with feline coronavirus. Most cats exposed to feline coronavirus do not develop FIP. In the small number that do, the virus changes in a way that allows it to spread in immune cells and trigger widespread inflammation in the body. This can affect the abdomen, chest, eyes, brain, spinal cord, kidneys, liver, and other organs.

FIP has traditionally been divided into effusive or “wet” FIP and non-effusive or “dry” FIP, although many cats show features of both. Wet FIP often causes fluid buildup in the belly or chest. Dry FIP is more likely to cause weight loss, fever, eye inflammation, neurologic changes, or organ-specific disease without large fluid accumulation. Young cats, cats from crowded multi-cat settings, and some purebred cats are at higher risk.

Years ago, FIP was considered almost uniformly fatal once signs appeared. That has changed. Antiviral treatment, especially GS-441524-based therapy prescribed by your vet, has improved survival and remission rates in many cats. Even so, diagnosis can still be challenging, and treatment plans vary based on the form of disease, the cat’s overall condition, and what is realistically available for the family.

Because FIP can worsen quickly, early veterinary evaluation matters. Cats with breathing trouble, a rapidly enlarging abdomen, collapse, seizures, or sudden vision changes need urgent care. Even when signs seem vague at first, such as fever, poor appetite, and weight loss, prompt testing can help your vet sort out whether FIP or another serious condition is involved.

Diagnosing FIP usually takes a step-by-step approach rather than one single test. Your vet will consider your cat’s age, history, exam findings, and pattern of illness. Bloodwork may show inflammation, anemia, high globulins, low albumin, or other changes that raise suspicion. Imaging such as chest X-rays or abdominal ultrasound may help identify fluid buildup or affected organs.

If fluid is present in the chest or abdomen, testing that fluid can be very helpful. Your vet may look at protein levels, cell counts, and appearance, and may submit samples for PCR testing for feline coronavirus RNA. PCR on effusion is generally more useful than blood PCR in suspected FIP cases. In cats without fluid, fine-needle aspirates, tissue samples, eye fluid, cerebrospinal fluid, or other targeted samples may be considered depending on the signs.

A coronavirus antibody test alone does not diagnose FIP, because many healthy cats have been exposed to feline coronavirus. Likewise, a positive fecal or blood coronavirus result does not automatically mean a cat has FIP. Definitive diagnosis may require histopathology with coronavirus antigen detection in affected tissues, but in real-world practice many cats are diagnosed based on a combination of clinical findings and supportive test results.

Because other diseases can look similar, your vet may also test for FeLV, FIV, toxoplasmosis, lymphoma, bacterial infections, liver disease, or other inflammatory conditions. This is especially important in cats with neurologic or eye signs, where the list of possible causes is broader.

FIP is linked to feline coronavirus, a very common virus in cats. The enteric form of feline coronavirus usually infects the intestinal tract and often causes no signs or only mild digestive upset, especially in kittens. In a small percentage of cats, the virus changes in a way that allows it to survive in macrophages, a type of immune cell. That change is what is associated with FIP.

Transmission of feline coronavirus is mainly fecal-oral. Cats are commonly exposed through shared litter boxes, contaminated surfaces, and close contact in multi-cat homes, shelters, rescues, and breeding environments. The form associated with FIP is generally thought to arise within an individual cat rather than spread efficiently from cat to cat in the same way the common enteric coronavirus does.

Risk is higher in young cats, especially those under 2 years old, and may also increase again in older cats. Crowded housing, stress, recent rehoming, surgery, concurrent illness, and immune system strain may all play a role. Merck notes increased risk in multi-cat settings and in some purebred lines, including Abyssinian, Bengal, Birman, Himalayan, Ragdoll, and Rex breeds.

It is important for pet parents to know that FIP is not caused by poor care. Many well-cared-for indoor cats develop it, and many cats exposed to coronavirus never do. The disease reflects a complicated interaction between the virus, the cat’s immune response, age, genetics, and environment.

There is no guaranteed way to prevent FIP, because the underlying feline coronavirus is very common. Prevention focuses on lowering coronavirus exposure and reducing stress, especially in kittens and multi-cat homes. Good litter box hygiene, frequent scooping, regular cleaning of food and water bowls, and avoiding overcrowding can help reduce fecal-oral spread.

When bringing a new cat home, a gradual introduction and a clean quarantine space are sensible steps. Breeders, rescues, and shelters often face the greatest challenge because coronavirus spreads more easily where many cats live together. Keeping kittens healthy, minimizing stress, and controlling other infectious diseases may lower overall risk.

Routine wellness care also matters. Cornell notes that keeping cats healthy and preventing other viral infections where indicated may help decrease the likelihood of FIP. Vaccination against common feline diseases, parasite control, good nutrition, and prompt care for illness all support the immune system.

An intranasal FIP vaccine exists in some settings, but major feline vaccine guidance does not routinely recommend it because effectiveness is questionable. If you are worried about FIP risk in your household, ask your vet about practical prevention steps tailored to your cat’s age, environment, and health history.

The outlook for cats with FIP has improved dramatically, but prognosis still depends on the form of disease, how early treatment starts, and whether the cat has severe neurologic or eye involvement. Without effective treatment, clinical FIP is usually progressive and often fatal. With antiviral therapy, many cats now achieve remission, especially when treatment begins before the disease becomes advanced.

Recovery is rarely immediate, even when a cat responds well. Some cats improve within days, with better appetite, energy, and breathing. Others recover more gradually over weeks. Your vet will usually recommend repeat exams and lab work during treatment to track weight, blood proteins, red blood cells, organ values, and overall response.

Neurologic and ocular FIP can be more complicated. These cats may need longer treatment courses, higher-intensity protocols, or specialty input. Some cats are left with residual neurologic or eye changes even after the infection is controlled. Relapse is possible, so ongoing monitoring matters.

For pet parents, the most helpful mindset is to think in stages. First comes stabilization and diagnosis. Next comes choosing a treatment path that fits the cat and the family. Then comes monitoring for response, side effects, and quality of life. Your vet can help you balance medical goals, home care demands, and cost range at each step.