Feline Infectious Peritonitis (FIP): Signs, Diagnosis and Treatment

By Tailwisp Editorial TeamUpdated October 20, 20265 min read

For decades, a diagnosis of feline infectious peritonitis meant a death sentence. That has changed. Antiviral treatments now cure a large share of cats, though diagnosis, access and cost still pose real challenges. This guide explains what FIP is, how it appears and what happens next.

Coronavirus and the mutation

Feline coronavirus is extremely common, especially in shelters and multi-cat homes. Most strains infect the gut, and cause mild or no illness. In a small share of infected cats, the virus mutates, gains the ability to infect white blood cells called macrophages and spreads around the body. The immune system's response causes inflammation of blood vessels and the lining of the abdomen and chest. That condition is FIP. The cat that develops FIP is not usually infectious in the same way, because the mutated virus is not readily passed to other cats. Most cats exposed to coronavirus never develop FIP.

Who is at risk

FIP is most common in cats under two years old, particularly kittens between about three months and one year. Stress, crowded conditions, other illness and genetic factors may increase risk. Purebred cats and males appear to be affected somewhat more often. Older cats can develop it too, if their immune system weakens.

Forms of FIP

• Wet (effusive) FIP. Fluid accumulates in the abdomen, causing a swollen belly, or in the chest, causing breathing difficulty. This form progresses quickly.
• Dry (non-effusive) FIP. Granulomas form in organs such as the kidneys, liver, lymph nodes, eyes and brain. Signs depend on the organs affected and can develop more slowly.
• Some cats have a mix of both.

Signs

• A fever that does not respond to antibiotics, coming and going.
• Loss of appetite, weight loss and lethargy.
• A dull coat.
• A pot-bellied appearance from fluid.
• Fast or labored breathing. See cat panting and open-mouth breathing.
• Jaundice, a yellow tint to the skin, gums or eyes.
• Changes in the eyes, such as cloudiness, color change, inflammation or bleeding. See cat eye problems.
• Neurological signs, such as wobbling, seizures, behavior change or paralysis. See seizures in cats.
• Swollen lymph nodes, and enlarged kidneys.

Diagnosis

FIP is hard to diagnose, because the signs overlap with many other illnesses and there is no single test that confirms it in every case. Vets combine the history, exam, bloodwork, which may show high globulins and low albumin, imaging and analysis of any fluid. A fluid sample, tested for protein and cells, and PCR or special staining for coronavirus, can be very helpful. In dry FIP, tests on tissue samples or fluid from the eye or brain may be needed. A coronavirus antibody test only shows exposure, and cannot diagnose FIP.

Treatment

Antiviral drugs that block the virus from replicating, such as GS-441524 and related compounds, have transformed outcomes. Studies and large clinical experiences report that a majority of treated cats go into remission, with courses usually lasting around twelve weeks. The legal status, availability and cost of these drugs vary between countries. In some places they are approved, and in others they are obtained through compounding pharmacies or special programs. Ask your vet about legal options, and be cautious about unregulated sources, because quality can vary and treatment needs veterinary monitoring.

Supportive care includes fluids, nutrition, anti-nausea drugs and drainage of fluid. Steroids were used before antivirals, and slow progression, but do not cure the disease.

During and after treatment

Cats are rechecked regularly, with weight, bloodwork and sometimes imaging. Doses are adjusted as the cat grows. After the treatment course, there is an observation period. Relapses can occur, particularly if the cat had neurological or eye disease, and may need further treatment. Many cats return to normal life.

Prevention

Preventing FIP means reducing coronavirus exposure and stress:

• Keep litter boxes clean, and separate from food. See how often to clean a litter box.
• Avoid overcrowding, and provide enough resources. See setting up a multi-cat home.
• Quarantine new cats and kittens.
• Reduce stress. See signs of stress in cats.
• Buy kittens from breeders and sources that screen and keep healthy conditions.
• Keep vaccinations for other diseases current. See cat vaccination schedule.

Living with other cats

If one cat in a home has FIP, others are not generally at high risk from that cat, though they may carry coronavirus. Ask your vet for guidance on hygiene and monitoring.

Costs, access and practical questions

Treatment is a significant commitment. A full course usually runs about twelve weeks, with daily injections or tablets, and the cost depends on the country, the product and the cat's weight. Insurance may not cover it. Ask your vet about legal sources, dosing, how often to recheck and what signs to watch for. Many owners find support in groups run by veterinary teams, though you should always check advice with your own vet. Do not delay: the sooner antivirals begin, the better the chance of success, particularly in cats with neurological or eye disease.

Differences between FIP and other illnesses

Fever, weight loss and a swollen belly can also come from heart disease, liver disease, cancer, bacterial infections or pancreatitis. That is why diagnostic testing matters. A confident diagnosis lets treatment begin early and avoids unnecessary antibiotics. If your vet is unsure, ask about a second opinion or referral to an internal medicine specialist.

Caring for a sick kitten at home

Keep your kitten warm, offer tempting foods and encourage water. Keep other pets in the household away until a diagnosis is made. Give all medicines on time. Watch for changes in breathing, walking or behavior, and call the clinic promptly if you notice any. Offer quiet comfort. Many kittens regain energy within days of starting treatment, which is a joyful moment for families.

When to see a vet

See a vet if a young cat has a persistent fever, swollen belly, weight loss or breathing trouble. Use the US vet directory to find a clinic.

This article is general educational information, not veterinary advice. Every cat is different. If your cat is unwell, or you are unsure about a symptom, diet change or medication, contact your veterinarian.