FIP in Cats: Symptoms, Diagnosis, Treatment and Recovery

FIP in cats, short for feline infectious peritonitis, is a serious disease caused by a mutated form of feline coronavirus, a very common and usually harmless virus. It mostly affects young cats and typically causes a fever that keeps coming back, tiredness, poor appetite and weight loss, often with fluid building up in the belly or chest. For decades FIP was regarded as almost always fatal. That has changed: antiviral treatment, mainly a drug called GS-441524, has led to recovery in most treated cats in published studies, although results vary between cats, and legal, vet-prescribed versions are available in a growing number of countries.

This guide explains how FIP develops, the signs of wet and dry FIP, how vets reach a diagnosis without a single definitive test, what treatment involves week by week, the legal and safety questions owners face, and what happens after treatment ends. It is based on the 2026 European Advisory Board on Cat Diseases (ABCD) treatment update, the FelineVMA 2025 treatment guide, the AAFP/EveryCat diagnosis guidelines, the Merck Veterinary Manual and International Cat Care. It is general information and not a substitute for advice from your own veterinarian.

FIP in cats at a glance

• Cause: certain strains of feline coronavirus (FCoV) that change inside an individual cat. The Merck Veterinary Manual cites an older estimate that about 5 to 12 percent of FCoV-infected cats go on to develop FIP.
• Who gets it: mainly young cats. International Cat Care reports that about 80 percent of diagnosed cases are in cats under 2 years old, but a cat of any age can be affected.
• Main signs: fever that does not respond to antibiotics, lethargy, poor appetite and weight loss, plus a swollen belly or breathing difficulty in wet FIP, or eye and neurological signs in dry FIP.
• Diagnosis: there is no single simple test. Vets combine the cat’s history, an examination and several tests.
• Treatment: a course of antiviral medication, usually lasting 6 to 12 weeks, prescribed and monitored by a vet. Without effective treatment, FIP is almost always fatal.
• People: feline coronavirus infects cats only and is not passed to humans.

What is FIP?

FIP is caused by feline coronavirus. FCoV spreads easily between cats through feces, most commonly from mother cats to their kittens, and through shared litter trays and contaminated objects. According to the Merck Veterinary Manual, antibodies to FCoV are found in about 15 percent of cats in single-cat homes and free-roaming cats, and in about 50 to 90 percent of cats in crowded settings such as catteries and shelters. In most cats the virus causes no signs at all, or only mild diarrhea that clears up on its own.

Feline coronavirus versus FIP

In a small proportion of infected cats, the virus changes (mutates) inside the body. The changed virus starts to infect immune cells called macrophages and spreads around the body inside them. The cat’s own immune reaction then causes inflammation of blood vessels and tissues, which leads to the fluid build-up and the inflammatory lumps (granulomas) typical of FIP. Merck notes that the exact mutation responsible is still not known.

This is why a cat that tests positive for coronavirus does not necessarily have FIP, and why most coronavirus-positive cats stay healthy. Feline coronavirus is also a different virus from the one that causes COVID-19 in people.

Is FIP contagious?

Feline coronavirus is contagious between cats, but FIP itself is not considered to be. Merck explains that the FIP-causing form of the virus is thought to arise within each affected cat rather than being passed directly from one cat to another, and the PDSA describes direct cat-to-cat spread of FIP as rare. Cats that live together usually share the same coronavirus, however, and related kittens may share a genetic susceptibility, so more than one case in a household or litter is possible. The section on other cats in the home explains what this means in practice.

Which cats get FIP?

Any cat can develop FIP, but the risk is higher in some groups. Merck lists the following risk factors:

• age under 2 years or over 10 years, with many cases in kittens aged about 4 to 12 months, according to International Cat Care
• living in a group, particularly in catteries, shelters and breeding households
• being male, and not being neutered
• pedigree breeds; Merck names Abyssinian, Bengal, Birman, Himalayan, Ragdoll and Rex cats, and International Cat Care adds British Shorthairs and Maine Coons
• recent stress, such as rehoming, surgery including neutering, illness or boarding
• infection with a retrovirus such as FIV or feline leukemia virus

International Cat Care points out that many cats with FIP now live in single-cat homes, but they often came from a multi-cat environment such as a breeder or shelter, where they were probably infected with coronavirus as kittens.

Symptoms of FIP in cats

Early signs are vague and easy to mistake for other illnesses. They often come and go, and they include a fever that does not improve with antibiotics, lethargy, a reduced appetite, weight loss and sometimes vomiting or diarrhea. Our guides on how to tell if your cat is sick and a cat not eating cover these general warning signs.

As FIP progresses, it usually takes one of two forms, often called wet and dry FIP. The two overlap, and a cat can show features of both or move from one form to the other.

Wet (effusive) FIP

In wet FIP, inflamed blood vessels leak protein-rich fluid into body cavities. Fluid in the abdomen gives a swollen, pot-bellied look, while fluid in the chest causes fast or labored breathing. The fluid is typically thick, sticky and straw-yellow. Cats may also develop jaundice, a yellow tinge to the gums, skin or whites of the eyes.

Dry (non-effusive) FIP

In dry FIP, inflammatory lumps form in organs such as the kidneys, liver, intestines, lymph nodes, brain and eyes. Signs depend on which organs are affected and may include weight loss, vomiting or diarrhea, jaundice, a mass a vet can feel in the abdomen, and eye or neurological problems.

Eye and neurological signs

International Cat Care estimates that the eyes are affected in around 30 percent of FIP cases and the brain in around 30 percent. Eye signs include cloudiness, inflammation inside the eye (uveitis), bleeding inside the eye and loss of vision. Neurological signs include a wobbly or uncoordinated walk, tremors, abnormal eye movements and seizures. These forms are important to recognize because they usually need higher medication doses and closer follow-up, and short treatment courses have mainly been studied in cats with wet FIP.

Feature	Wet (effusive) FIP	Dry (non-effusive) FIP
What happens	Fluid leaks into the abdomen or chest	Inflammatory lumps form in organs
Typical signs	Swollen belly, breathing difficulty, jaundice, fever	Weight loss, fever, eye changes, wobbliness, seizures, jaundice
Course without treatment	Usually days to weeks, according to Merck	Usually weeks to months, according to Merck
Key diagnostic sample	A sample of the fluid	Tissue samples, eye examination, imaging or spinal fluid

When to see a vet urgently

Contact your vet the same day, or an emergency clinic, if your cat:

• breathes fast or with effort while resting, or breathes with an open mouth
• has a seizure, collapses, or suddenly becomes wobbly or unable to stand
• has a belly that is visibly swollen or getting bigger
• has yellow gums, skin or eyes
• develops sudden eye changes, such as cloudiness, blood inside the eye or loss of vision
• has stopped eating, especially a kitten or a cat that is already unwell

If your cat is already on FIP treatment, the FelineVMA guide advises contacting your vet promptly if they stop eating, if you cannot give the medication, or if new signs appear. For a young cat with a fever, low energy and weight loss that develop over days or weeks, book an appointment soon rather than waiting to see whether it passes.

How vets diagnose FIP

There is no single, simple test that confirms FIP in a living cat. The 2022 AAFP/EveryCat FIP diagnosis guidelines describe building up an index of suspicion step by step, combining the cat’s age and background, history, physical examination and test results. The more findings fit, the more confident the diagnosis becomes. Fluid, when present, is the most useful sample: Merck notes that tests on effusion fluid have higher predictive value than blood tests.

Tests your vet may use

Test	What it looks for	What it can and cannot tell you
Blood tests	High globulin protein, a low albumin to globulin (A:G) ratio, low lymphocyte count, anemia, high bilirubin and raised inflammation markers	Supportive but not specific; many other diseases cause similar changes
Fluid analysis	Merck lists high protein (above 35 g/L), an A:G ratio below 0.4 and a low to moderate cell count as typical	One of the most useful tests when fluid is present, but these findings are typical of FIP rather than proof of it
Rivalta test	A simple in-clinic test on the fluid	A positive result raises suspicion but is not specific; a negative result makes FIP very unlikely
RT-PCR	Coronavirus genetic material in fluid, tissue, eye fluid or spinal fluid	Can support FIP alongside other findings, but it cannot reliably tell FIP strains apart, and virus in the blood alone does not confirm FIP
Immunostaining	Coronavirus inside macrophages in fluid or tissue samples	The basis of a definitive diagnosis, but it needs fluid or a tissue sample
Imaging and specialist tests	Ultrasound, X-rays, eye examination, and MRI or spinal fluid testing for neurological cases	Finds fluid, enlarged lymph nodes, organ changes and inflammation in the eyes or brain
Antibody test	Antibodies to feline coronavirus	Often unhelpful, because most cats have been exposed and some cats with FIP test negative

Why a positive coronavirus test does not mean FIP

Because feline coronavirus is so common, a positive antibody test only shows that a cat has been exposed to the virus. Standard PCR tests also cannot reliably tell the FIP-causing form apart from the ordinary form. A coronavirus result therefore has to be interpreted alongside everything else. The reverse also applies: a single normal or negative result does not always rule FIP out, and a single abnormal result, such as a low A:G ratio, supports FIP without proving it.

Sometimes a cat is too ill for extensive testing and FIP looks highly likely. Vets may then discuss starting treatment while monitoring the response closely. International Cat Care stresses that vets should be confident FIP is likely before starting antivirals, because using them in cats without FIP could encourage coronavirus resistance and may delay treatment of the real problem. The FelineVMA guide adds that the response to treatment should not be the only basis for a diagnosis, and that a cat that fails to improve on an adequate dose should be investigated for another cause.

Is FIP curable? Treatment in 2026

Until a few years ago, treatment could only ease signs for a short time, and euthanasia was commonly recommended soon after diagnosis. Antiviral drugs have transformed the outlook. The 2026 ABCD treatment update reports success rates of 80 to 100 percent across published studies of GS-441524, the most studied antiviral. The FelineVMA guide describes response rates above 85 percent and survival often above 90 percent, and International Cat Care states that antivirals can cure FIP in most cats (more than 85 percent) in studies.

These figures are not directly comparable and are not a guarantee for an individual cat. Studies differed in size, drug product, length of follow-up and how they defined success, for example survival, remission or clinical response, and some excluded the most severely ill cats. The ABCD guidelines list jaundice, high bilirubin, anemia, low platelet counts and a very high amount of virus as signs of a poorer outlook. Cats with eye or neurological disease need higher doses and closer monitoring.

GS-441524 and other antivirals

• GS-441524: a nucleoside analogue that stops the virus copying itself. Remdesivir, an antiviral used in people, is converted into GS-441524 in the body. Oral GS-441524 is the preferred first-line treatment, and tablets or capsules are now favored over injections, which the ABCD notes are often painful and can cause skin sores.
• Remdesivir: an injectable antiviral mainly used for cats that are too ill to take oral medication, with a switch to oral GS-441524 as soon as possible.
• Molnupiravir: an alternative antiviral, mainly used where GS-441524 is unavailable or has not worked. The ABCD reports first-line success rates of 72 to 91 percent in most studies, but the response is slower, and side effects include low white blood cell counts, stomach upset, folded ears and broken whiskers. Because of concerns that it can damage genetic material, the ABCD prefers GS-441524 where it is available, and the FelineVMA guide recommends reserving molnupiravir as a second-line option. In the European Union, molnupiravir is reserved for human use.
• Steroids: do not cure FIP. In an older placebo-controlled study cited by the ABCD, in which cats received steroids and supportive care but no effective antiviral, median survival was only 8 days. Steroids still have specific roles, for example steroid eye drops for eye inflammation, or treating an immune-mediated anemia linked to FIP.
• Supportive care: may include draining fluid from the chest to ease breathing, fluid therapy, appetite support, pain relief and anti-seizure medication.

Your vet will calculate the dose from your cat’s weight and the form of FIP, and will adjust it during treatment. This guide deliberately does not give doses. Never change the dose or stop treatment without talking to your vet.

How long treatment lasts

Most published studies have used a 12-week (84-day) course, and the Merck Veterinary Manual still recommends at least 84 days for all cases. Newer evidence shows that a shorter course can work for some cats. In a randomized controlled study published in 2024, 40 cats with wet FIP were treated with quality-controlled oral GS-441524 for either 42 or 84 days. In each group, 19 of 20 cats reached complete remission, and the cats in the 42-day group were still in remission at the last check on day 168. The study was small and included only cats with fluid build-up, so it does not show that 42 days is enough for every cat, particularly cats with eye or neurological disease. The ABCD 2026 update and the FelineVMA 2025 treatment guide accept shorter courses for selected cats, mainly those with wet FIP that respond quickly, and both stress that the length should be decided by the vet based on how each cat responds. Shorter courses cost less and are easier on both cat and owner, but they are not suitable for every cat.

Is FIP treatment legal?

This depends on where you live, and the rules have been changing quickly. No antiviral is licensed for FIP in cats, but legal, quality-controlled options now exist in many countries. Rules on prescribing, compounding and importing differ between countries and can change, so this is a summary rather than legal advice:

• United States: on May 10, 2024, the FDA stated that no FDA-approved drug is available to treat FIP, but that it would not take enforcement action against compounded GS-441524 when a veterinarian prescribes it for a specific cat and it is compounded under the agency’s guidance for compounding from bulk drug substances (GFI #256). This is enforcement discretion, not FDA approval: compounded GS-441524 remains an unapproved drug.
• United Kingdom and Australia: legal, quality-assured antivirals have been available from vets for several years; International Cat Care dates this from 2021 and the FelineVMA guide from 2020.
• European Union: the ABCD notes that remdesivir is permitted for treating FIP. Legal compounded GS-441524 products exist in many, but not all, countries.
• Elsewhere: the FelineVMA guide lists a growing number of other countries with legal compounded products, but notes that some parts of the world still have no quality-assured, legal supply. Ask your vet what is available and permitted where you live.

Why to avoid black-market products

Many owners have bought unregulated antivirals online, often illegally. Their contents and strength are unknown and vary between batches. The ABCD cites a study in which none of five sources of one such antiviral, GC376, actually contained GC376, and links unregulated injectable products to bladder stones. The FelineVMA guide does not recommend black-market products and advises using legal, quality-assured preparations. Because the true content is uncertain, a reliable dose cannot be guaranteed, and a poor response is harder for your vet to interpret. Where no legal antiviral exists, vets can usually still provide supportive care.

Side effects to watch for

Side effects of GS-441524 reported in the ABCD update and in a 2025 follow-up of the 42-versus-84-day trial include diarrhea, temporary rises in liver enzymes, raised lymphocyte and eosinophil counts, and changes in kidney blood values; a mild anemia is rare. The FelineVMA guide notes that injections of remdesivir are painful in about half of cats. Bladder stones are rare and mostly linked to unregulated products.

A 2026 case series described large-cell lymphoma, a type of cancer, in 4 of 202 cats (2 percent) diagnosed between about 3 and 20 months after successful treatment with legally sourced oral GS-441524. The authors did not show that the drug caused the cancer. They suggest it could be linked to FIP itself, to long-term immune stimulation, to the treatment, or to the fact that treated cats now live long enough for other diseases to appear, and they recommend continued monitoring after treatment, especially for weight loss, vomiting or a reduced appetite. Report any new sign to your vet; our cat cancer guide explains lymphoma in more detail.

Caring for a cat during FIP treatment

Treatment involves daily medication for several weeks, regular weighing and a series of vet checks. Good organization at home makes a real difference to success.

Monitoring and vet rechecks

• The FelineVMA guide says owners should expect an improvement in energy and temperature within about 48 hours, and that fluid build-up usually resolves within about 2 weeks.
• A first vet recheck is usually after 1 to 2 weeks, or sooner if your cat is getting worse. For a 12-week course, the guide describes a re-examination at 6 weeks and tests at 10 and 12 weeks.
• Weigh your cat every week on accurate baby or pet scales. Growing kittens and recovering cats gain weight, and the dose usually needs to increase to avoid underdosing. MyyCat’s free Kitten Growth Tracker can help you record weights.
• Do not panic about a small early drop in weight. The ABCD notes that weight loss in the first two weeks, often as the fluid is reabsorbed, does not necessarily mean treatment is failing.
• Keep a simple diary of appetite, energy, breathing rate, litter box use and any new signs, such as changes in the eyes.

Giving medication every day

Give each dose at the same time every day, exactly as prescribed, and complete the course. The FelineVMA guide suggests giving tablets on an empty stomach or in a small treat, with a full meal at least 30 minutes later, rather than hiding medication in a main meal. If your cat spits out doses or stops eating, tell your vet straight away rather than skipping doses. MyyCat’s free Cat Medication Schedule Generator can help you keep track of daily doses over a long course.

Feed a complete, balanced and energy-dense diet; the FelineVMA guide says no special supplements are needed. Keep your cat’s routine calm and predictable, as stress is unhelpful for a recovering cat. Our cat anxiety guide has practical ideas.

Relapse and life after treatment

Your vet will decide when to stop treatment using a combination of findings, never a single number. The FelineVMA guide suggests that, ideally, the cat should be normal on examination and blood tests for about 2 weeks before stopping. Normal levels of an inflammation marker called alpha-1 acid glycoprotein (AGP) are an important part of this assessment, and the ABCD notes that two normal AGP results, a week or two apart, are encouraging. For shorter courses, the FelineVMA guide also looks for an albumin to globulin (A:G) ratio above 0.6, alongside these other findings. Mildly raised globulin, raised lymphocyte counts or enlarged lymph nodes in the abdomen can persist without meaning the disease has returned. Never stop treatment early on your own, even if your cat seems completely well.

Relapse is uncommon after GS-441524 treatment; the FelineVMA guide estimates it at under 10 percent, and probably under 5 percent, although published relapse data are still limited. It is most likely in the first month after treatment ends, so watch your cat closely during this time. A relapse can look different from the original illness; for example, a cat that first had wet FIP may relapse with neurological signs. Before treating a suspected relapse, vets usually re-check the diagnosis, sometimes with repeat fluid or tissue samples, because inflammation markers can stay normal. Relapses have been treated successfully with a further course of antivirals, usually at a higher dose, or with a different antiviral.

The FelineVMA guide also advises the following after recovery:

• Neutering and vaccination are ideally done from about a month after treatment ends, although either can be done during treatment if needed and the cat is well.
• Routine flea and worm treatment does not need to stop during treatment.
• Cats that have had FIP should not be used for breeding.

Treatment can be expensive, and costs vary widely by country, product and how long your cat is treated. Ask your vet for an estimate that includes the medication and the rechecks, and check what your pet insurance covers.

Other cats in the home

Feline coronavirus spreads between cats mainly through feces and shared litter trays, but FIP itself is not passed from cat to cat in the same way. The FelineVMA guide states that in-contact cats cannot catch FIP from an affected cat, that normal hygiene precautions are enough even in a veterinary hospital ward, and that trying to stop coronavirus spreading within a household is unlikely to reduce the risk of FIP. It recommends keeping household groups stable to limit stress, and advises against giving antivirals to in-contact cats unless they also have confirmed or highly likely FIP. International Cat Care likewise does not recommend routine antivirals to stop coronavirus shedding in cats without FIP. The PDSA takes a more cautious approach and suggests keeping an affected cat separate, with its own bowls and litter tray.

Discuss the best approach for your household with your vet. Good litter tray hygiene makes sense either way: scoop trays at least daily and keep them away from food and water. Keep an eye on the other cats, especially young ones, and contact your vet if any of them shows signs such as fever, lethargy or weight loss. If you are considering bringing a new cat home after an FIP diagnosis, discuss the timing with your vet; our guide on how to introduce cats explains how to keep the move low-stress.

Can FIP be prevented?

There is no reliable way to prevent FIP in an individual cat, but reducing exposure to coronavirus and reducing stress can lower the risk, especially in multi-cat homes, shelters and breeding catteries. International Cat Care recommends:

• keeping cats in small groups of fewer than five
• providing at least one litter tray per cat plus one extra, cleaned at least daily and kept away from food and water; MyyCat’s Cat Litter Box Calculator can help you check how many you need
• avoiding overcrowding and reducing stress, especially around rehoming and other big changes
• for breeders, keeping groups small, avoiding several litters at once, and stopping breeding from lines that produce kittens with FIP

A vaccine against FIP is available in some countries, given as nose drops to kittens from 16 weeks of age. The 2020 AAHA/AAFP Feline Vaccination Guidelines list it as not generally recommended. They note that evidence of real-world protection is insufficient, that the vaccine strain differs from the type of coronavirus most common in cats, and that only cats without coronavirus antibodies could potentially benefit, yet most kittens are already infected by 16 weeks of age. If you are considering it for a particular situation, discuss it with your vet. Our cat vaccination schedule guide covers the core vaccines. Merck adds that feline coronavirus is inactivated by dilute bleach and heat, which is useful when cleaning shared areas.

Frequently asked questions

How long can a cat live with FIP without treatment?

Not long, in most cases. In older studies from before effective antivirals, cats that received only steroids and supportive care survived a median of about 8 to 9 days after diagnosis, although a few lived for several months (range 3 to 200 days). Merck notes that cats with wet FIP usually survive days to weeks, and cats with dry FIP weeks to months. Modern antiviral treatment has changed this outlook, so see your vet promptly if FIP is suspected.

Can a cat fully recover from FIP?

Yes. Most cats treated with legal, good-quality antivirals in published studies have recovered and returned to normal life. Some may keep minor findings on blood tests, relapse is possible though uncommon, and long-term follow-up studies are still limited. It is not yet known whether every cat that appears cured stays well for life, so regular check-ups remain worthwhile.

Can humans or dogs catch FIP?

No. International Cat Care states that feline coronaviruses infect only cats and cannot be passed to humans, and the PDSA says there is no evidence FIP affects people or other pets. The PDSA notes it can, however, affect big cats.

Can an indoor cat get FIP?

Yes. FIP develops from coronavirus a cat already carries, often picked up as a kitten from its mother or littermates. An indoor-only cat living on its own can still develop FIP, although most cases occur in young cats.

Can older cats get FIP?

Yes, though it is less common. Merck lists cats over 10 years old among the higher-risk groups. In an older cat, fever, weight loss or fluid build-up have many other possible causes, so a full work-up is especially important.

Is there a vaccine for FIP?

A vaccine exists in some countries, but it is not generally recommended: evidence that it protects cats in practice is insufficient, and it can only be given from 16 weeks of age, by which time many kittens have already met the virus. Prevention relies mainly on hygiene, small groups and low stress.

Sources

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• Zuzzi-Krebitz AM, Buchta K, Bergmann M, et al. Short treatment of 42 days with oral GS-441524 results in equal efficacy as the recommended 84-day treatment in cats suffering from feline infectious peritonitis with effusion. Viruses. 2024;16(7):1144.
• Buchta K, Zuzzi-Krebitz AM, Bergmann M, et al. Unexpected clinical and laboratory observations during and after 42-day versus 84-day treatment with oral GS-441524 in cats with feline infectious peritonitis with effusion. Viruses. 2025;17(9):1181.
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• International Cat Care. Feline infectious peritonitis (FIP). Updated December 2025.
• U.S. Food and Drug Administration. FDA announces position on use of compounded GS-441524 to treat FIP. May 10, 2024.
• AAHA/AAFP. 2020 AAHA/AAFP Feline Vaccination Guidelines: not recommended vaccines. 2020.
• PDSA. Feline infectious peritonitis in cats. November 2023.