FIP in cats: symptoms, treatment and life expectancy

FIP, short for feline infectious peritonitis, is a serious disease that develops when a common and usually harmless cat virus, feline coronavirus, changes inside a cat's body and sets off a damaging immune reaction. For decades it was almost always fatal. That has changed: with antiviral medicines prescribed by a vet, a large majority of treated cats in published reports have gone into remission.

Untreated FIP gets worse, treatment works best when it starts early, and the diagnosis is hard to pin down. If your cat has a fever that will not go away, a swelling belly or steady weight loss, call your vet today and ask whether FIP is on the list.

What is FIP?

Feline coronavirus is a gut virus that passes between cats through shared litter boxes. It is very common wherever cats live together, such as shelters, breeding homes and multi-cat households. Most infected cats show nothing at all, or a few days of soft stools.

In a small minority of cats the virus mutates inside the body. The changed virus can live in a type of white blood cell and travel everywhere those cells go. The cat's immune system attacks it and, in doing so, inflames blood vessels and organs. That inflammation is the disease.

Feline coronavirus is not the virus that causes COVID-19 in people, and you cannot catch FIP from your cat.

Is FIP contagious to other cats?

The ordinary gut coronavirus spreads easily. The mutated form that causes FIP is generally thought to arise within the individual cat and not to pass directly to housemates. Cats who live with an FIP cat have very likely met the ordinary virus already, and most never develop FIP. Ask your vet whether anything needs doing for them.

Wet FIP and dry FIP

Vets describe two main forms. Many cats have features of both, and one form can turn into the other.

Symptoms of FIP in cats

The early signs are vague and shared with many other illnesses. None of them confirms FIP.

• A fever that comes and goes and does not respond to antibiotics
• Low energy and less interest in food
• Weight loss, or failure to grow in a kitten
• A swollen, fluid-filled belly that is not painful to touch
• Fast, shallow or effortful breathing when fluid collects in the chest
• Yellow tint to the gums, the whites of the eyes or the skin inside the ears
• Eye changes: a cloudy eye, a change in the color of the iris, an oddly shaped pupil
• Nervous system signs: a wobbly walk, head tilt, tremors, seizures, a change in behavior

These signs have many other causes, including heart disease and cancer. Our pages on heavy breathing in old cats and cloudy eyes in old cats list the others.

FIP in older cats

FIP is mainly a disease of young cats, with most cases in kittens and cats under about two years old. There is a second, smaller rise in senior cats, thought to reflect an aging immune system that is less able to keep the virus in check.

In an old cat it is easy to miss. The dry form is more typical, and its signs are the same ones that older cats show for far more common reasons: gradual weight loss, a dull coat, a poor appetite, enlarged or lumpy kidneys, swollen lymph nodes in the belly. On an exam or an ultrasound that picture can look like lymphoma, another cancer, bowel disease or chronic kidney disease.

Most older cats with these signs do not have FIP. But if your senior cat is being worked up for weight loss or a persistent fever and nothing fits, it is reasonable to ask the vet whether FIP has been considered, because the treatments are completely different.

Why FIP is hard to diagnose

There is no single quick test that says yes or no. A blood test for coronavirus antibodies only shows that a cat has met the ordinary virus at some point, which is true of a great many healthy cats. A positive result does not mean FIP, and a negative one does not fully rule it out.

Vets build the diagnosis from several pieces:

• History and exam. Age, background, fever, fluid, eye and nerve signs.
• Blood tests. Certain patterns are suggestive, such as high blood proteins called globulins, low numbers of one type of white cell, mild anemia and raised bilirubin.
• Fluid analysis. If there is fluid in the belly or chest, a sample is the most useful test. FIP fluid is typically straw-yellow, sticky and high in protein, and the lab can look for the virus in it.
• Imaging. X-rays and ultrasound to find fluid, enlarged lymph nodes and organ changes.
• Tissue samples. Finding the virus inside inflamed tissue under the microscope is the most definite proof, but it means a needle sample or biopsy, which is not always possible in a sick cat.

In dry FIP there is often no fluid to sample, so the vet may be working from probabilities, and the response to antiviral treatment sometimes becomes part of the evidence.

How FIP is treated

The treatment that changed the outlook is a group of antiviral medicines that stop the virus copying itself. The best known is GS-441524. Remdesivir, a closely related drug, and molnupiravir are also used. They are prescription medicines, given as injections or by mouth.

Which antiviral, in what form, at what dose and for how long is set by the prescribing vet and depends on the cat's weight, the form of FIP and how the cat responds. Eye and nervous system disease generally needs a different plan from simple wet FIP. It is a long course, measured in weeks and not days, and stopping early is linked to relapse.

Legal access has improved but varies by country. In the United States, vets have been able to prescribe a compounded oral antiviral for FIP since 2024. Do not buy antivirals online yourself. Unlicensed products vary in what they contain, and a cat on treatment needs monitoring that only a vet can provide.

Alongside the antiviral, a sick cat may need supportive care:

• Draining chest fluid so the cat can breathe
• Fluids and anti-nausea medicine
• Appetite support and assisted feeding
• Eye drops for inflammation inside the eye
• Repeat blood tests during treatment and through an observation period afterward

Many cats brighten, eat and lose their fever within the first days of treatment. A cat who does not improve needs a recheck, because the diagnosis or the plan may need to change.

The amount of medicine depends on the cat's weight and the course is long, so the total cost can be substantial. Ask for a written estimate early, and if you have pet insurance, ask the insurer how FIP treatment is handled.

FIP life expectancy

Without antiviral treatment, FIP is almost always fatal. Cats with the wet form commonly live days to a few weeks after signs become obvious. Cats with the dry form may have weeks to a few months.

With treatment, published reports from several countries describe a large majority of cats responding and staying well after the course ends. Cats who stay healthy through the observation period are generally expected to go on to an ordinary life, though long-term follow-up is still being gathered.

The outlook is more guarded when a cat is already very weak at the start, when the brain or eyes are involved, or when an older cat has other illness such as kidney disease alongside. Some cats relapse and need a second course. Your own vet, who knows your cat's results, is the one to give a prognosis.

If treatment is not possible or is not working, comfort becomes the goal. The quality-of-life scale for senior cats can help you and your vet judge how your cat is coping.

Can FIP be prevented?

There is no reliable way to stop the mutation. What helps is less exposure to the gut virus and less stress: enough litter boxes, scooped daily and kept away from food, and no overcrowding. A vaccine exists in some countries, but feline expert groups do not generally advise it.

Call the vet today if

• Your cat is breathing fast, with effort or with an open mouth. This is an emergency at any hour.
• The belly has become round or tight over a few days
• Your cat has not eaten for a day
• The gums, eyes or ears look yellow
• An eye has turned cloudy, changed color or looks painful
• Your cat is wobbly, has a seizure or seems suddenly confused
• A fever keeps coming back after antibiotics
• A cat on FIP treatment gets worse, stops eating or reacts badly to a medicine

This article is general educational information about feline infectious peritonitis and is not veterinary advice. The signs described can point to FIP but are shared with many other diseases, and only a veterinarian can diagnose it. Antiviral treatment, including the drug, the dose and the length of the course, must be prescribed and monitored by your veterinarian. Decisions about your cat's care should be made with them.