What is Feline Infectious Peritonitis (FIP)?

Feline infectious peritonitis, or FIP, is a serious disease caused by a mutated form of feline coronavirus. Most cats that encounter feline coronavirus never develop FIP. The initial infection is usually mild — it may cause temporary digestive upset or no symptoms at all — and clears without treatment. But in some cats, the virus mutates in a way that allows it to hijack the very white blood cells that should destroy it. Once that happens, the infection spreads and triggers a damaging inflammatory response the immune system cannot control on its own.

The two forms of FIP

FIP can present in two general ways. The “wet” or effusive form causes inflammatory fluid to accumulate in the chest, abdomen, or around the heart. Affected cats may have a swollen belly, difficulty breathing, or general lethargy and weight loss. The “dry” or non-effusive form is less obvious on the outside but causes granulomatous lesions to develop in organs throughout the body — including the eyes, kidneys, and nervous system.

Neurologic FIP, which falls under the dry form, is particularly complex. Signs depend on where in the nervous system the inflammation is most severe and can include seizures, changes in behavior or personality, eye abnormalities, weakness, wobbly gait, or loss of coordination. Neurologic cases tend to require more intensive management and careful monitoring over a longer treatment period.

Diagnosis is not always straightforward

No single test can definitively diagnose FIP in every case. Diagnosis typically involves a combination of bloodwork, protein levels, fluid analysis when fluid is present, imaging, and in some cases, biopsy or cerebrospinal fluid analysis. The clinical picture has to be interpreted as a whole — which is one reason early specialist involvement makes a real difference.

Treatment has changed dramatically

For decades, FIP was considered uniformly fatal. That is no longer true.

Antiviral medications — most notably a class of drugs called nucleoside analogs, including compounds related to GS-441524 — have transformed what is possible for cats with FIP. These medications can eliminate the active viral infection in many cases, including neurologic forms. Cats that complete a full treatment course can go on to live normal, healthy lives.

Treatment is typically administered daily for a minimum of 12 weeks, and neurologic cases often require longer courses. Some cats are monitored for relapse after completing treatment, and repeat courses are possible if needed.

Why a veterinary specialist matters

This is an area of veterinary medicine that is moving fast. Treatment protocols are still evolving, monitoring recommendations are in constant flux, and the right approach — including which medication formulation to use, what dose, for how long, and how to monitor for response and relapse — depends heavily on which form of FIP a cat has and how they are responding.

A veterinary specialist who is actively following the literature and treating FIP cases regularly is in the best position to guide these decisions. This is not a condition to navigate with general guidelines alone. The stakes are high, the variables matter, and access to current protocols can significantly affect outcomes.

If your cat has been diagnosed with FIP

Do not assume the worst — but do not wait, either. Reach out to a veterinary specialist as soon as possible. The earlier treatment begins, the better the response tends to be. With the right team and the right plan, many cats with FIP go on to do very well.