Last updated: 28 September 2026
FIP (feline infectious peritonitis) is a rare but serious complication of feline coronavirus (FCoV): according to ABCD it usually develops in fewer than 10% of infected cats, most often in cats under 2 years of age. Without treatment it is almost always fatal, but in published studies 81–95% of cats that received antiviral treatment survived. Diagnosis is a job for the vet; a rapid test alone cannot detect FIP.
We summarise how FIP develops, which symptoms to watch for, what a coronavirus rapid test does and does not show, and what we know about treatment today.
What is FIP and how does it develop?
FIP develops when the feline coronavirus, which lives in the intestine and is usually harmless, mutates inside a cat's body and starts to multiply outside the intestine.
- Feline coronavirus is very widespread: according to ABCD (European Advisory Board on Cat Diseases), wherever it is present, most cats are antibody-positive.
- FIP develops in a small proportion of infected cats – according to ABCD usually fewer than 10%; according to AAFP/EveryCat this can be as high as 12% in multi-cat environments.
- Cats under 2 years of age are most affected, and it is more common where many cats live together: shelters, catteries and multi-cat households.
The intestinal coronavirus is shed in the faeces and spreads easily via shared litter trays. The mutated virus that causes FIP inside the cat, however, rarely spreads between cats.
What are the symptoms of FIP?
The most common sign is a persistent fever that does not improve with antibiotics, loss of appetite and weight loss in a young cat; this is often accompanied by fluid build-up, eye or neurological symptoms.
| Form | Typical symptoms | What to watch out for? |
|---|---|---|
| Wet (effusive) FIP | Fluid in the abdomen (swollen, "pear-shaped" belly) or in the chest (difficulty breathing, panting), fever, weight loss | According to data cited by ABCD, 78% of 224 cats with FIP had fluid build-up. |
| Dry (non-effusive) FIP | Persistent fever, weight loss, eye inflammation, cloudy eyes, unsteady gait, seizures | Harder to recognise; symptoms develop more slowly. |
| Both forms | Loss of appetite, lethargy, jaundice, stunted growth in kittens | The two forms often overlap and can change from one to the other. |
How is FIP diagnosed?
There is no single test that can confirm FIP on its own: the vet decides based on the history, symptoms, blood tests and, where possible, analysis of any fluid build-up.
- Blood tests: blood count, total protein, albumin-to-globulin ratio and other abnormalities.
- Fluid analysis: according to ABCD, detecting coronavirus in the fluid (by RT-PCR or antigen staining) is extremely useful for diagnosis.
- Imaging: ultrasound to find fluid and organ changes.
- Coronavirus antibodies: according to ABCD, a positive antibody test does not prove FIP, but if there are no antibodies, FIP is less likely.
What do the FIPV Ag and FIPV Ab rapid tests show?
Neither rapid test is an FIP test: the antigen test shows whether the cat is shedding coronavirus in its faeces, and the antibody test shows whether it has ever been exposed to the virus.
| Test | Sample | What does it detect? | What is it useful for? | Manufacturer's validation |
|---|---|---|---|---|
| FIPV Ag | Fresh faeces or rectal swab | Coronavirus antigen in the faeces (virus shedding) | Finding out which cat is shedding the virus in multi-cat households or catteries | Sensitivity 96.67%, specificity 97.33% (PCR, 300 samples) |
| FIPV Ab | Blood, serum or plasma | Antibodies against coronavirus | Shows whether the cat has been exposed to coronavirus; if FIP is suspected, the vet interprets it together with other findings | Sensitivity 95.88%, specificity 96.25% (ELISA, 330 samples) |
In multi-cat environments many healthy cats are antibody-positive, so a positive result on its own is no reason to panic. If your cat shows symptoms suggestive of FIP, do not wait for a test – take it to the vet.
How is FIP treated today?
Today FIP is treated with antiviral nucleoside analogue drugs: oral GS-441524 and remdesivir, which is mainly given by injection and is also converted into GS-441524 in the body.
| Study | Cats | Treatment | Outcome |
|---|---|---|---|
| Pedersen et al., 2019 (USA) | 31 cats | GS-441524, at least 12 weeks | 25 cats became symptom-free long-term; 8 needed a repeat course because of relapse |
| Taylor et al., 2023 (United Kingdom) | 307 cats | Remdesivir and/or GS-441524, legally sourced compounded preparation | 84.4% were alive at the longest follow-up; 10.8% relapsed |
| Zuzzi-Krebitz et al., 2024 (Germany) | 40 cats, wet FIP | Oral GS-441524 for 42 or 84 days | 38 cats recovered (19 in each group); the 42-day course was just as effective |
The classic course lasts 84 days (12 weeks); for wet FIP, a 42-day treatment has also proven effective. With eye or neurological symptoms, vets often choose a higher dose or longer treatment, and regular check-ups are needed during treatment.
The legal background matters too: there is currently no FIP medicine authorised for cats in the European Union (Austrian medicines agency, BASG, 2025), and according to ABCD antiviral drugs are not yet legally available in every country. In some countries, such as the United Kingdom, a pharmacy-compounded preparation can be legally prescribed. Always decide on the product, dosage and how to obtain it together with your treating vet, and never give your cat a product from an unknown source.
When should you suspect FIP?
When a young cat has had a fever for days or weeks, is losing weight and not eating, and this does not improve with treatment. It is particularly suspicious if:
- the belly is noticeably large and swollen, or the cat breathes with difficulty or rapidly,
- its eyes are inflamed or cloudy, or the pupils are different sizes,
- it walks unsteadily, staggers or has seizures,
- its mucous membranes are yellowish,
- the symptoms started shortly after a stressful event (moving house, a new home, neutering, a stay in a shelter).
In this case, do not wait: ask for a veterinary work-up in which FIP is also considered. The earlier treatment starts, the better the chances.
How can you reduce the risk of FIP in a multi-cat household?
The risk can be reduced by lowering the coronavirus load and stress:
- provide enough litter trays (International Cat Care recommends one per cat plus one) and clean them daily,
- keep litter trays away from food bowls,
- keep a smaller, stable group of cats and avoid frequently introducing new cats,
- in catteries or shelters, the FIPV Ag rapid test can help identify which cat is shedding the virus.
We have summarised the general hygiene rules for multi-pet households here: Multi-pet households: how to stop infections spreading. All cat rapid tests in one place: rapid tests for cats.
Frequently asked questions about FIP
Is FIP contagious to other cats?
The mutated virus that causes FIP inside the cat rarely spreads further. The intestinal feline coronavirus, however, is shed in the faeces and spreads easily via shared litter trays, so hygiene is the most important thing in multi-cat households.
What are the chances that a kitten with FIP will recover?
In published studies, 81–95% of cats that received antiviral treatment survived the disease – for example 84.4% in a UK study of 307 cats. Without treatment, FIP is almost always fatal.
How long does FIP treatment take?
The classic course lasts 84 days. In a 2024 study, a 42-day treatment was just as effective for wet FIP. Eye and neurological symptoms often require longer or higher-dose treatment; the exact protocol is decided by the vet.
Can a rapid test detect FIP?
No. The FIPV Ag rapid test detects coronavirus shed in the faeces, and the FIPV Ab rapid test detects antibodies against coronavirus. Diagnosing FIP is a job for the vet, based on symptoms, blood tests and analysis of any fluid build-up.
Can humans or dogs catch FIP?
No. Feline coronavirus and FIP affect only cats and do not spread to humans or dogs.
Summary
FIP is now a treatable disease – but only if it is recognised in time. Take a young cat with persistent fever, weight loss, a swollen belly or eye and neurological symptoms to the vet straight away. A coronavirus rapid test helps with screening in multi-cat households and catteries, but it does not replace a diagnosis of FIP.
Sources
- ABCD – European Advisory Board on Cat Diseases: Guideline for Feline Infectious Peritonitis (2026). abcdcatsvets.org
- Thayer V, et al.: 2022 AAFP/EveryCat Feline Infectious Peritonitis Diagnosis Guidelines. Journal of Feline Medicine and Surgery, 2022;24(9):905–933. journals.sagepub.com
- Pedersen NC, et al.: Efficacy and safety of the nucleoside analog GS-441524 for treatment of cats with naturally occurring feline infectious peritonitis. Journal of Feline Medicine and Surgery, 2019;21(4):271–281. journals.sagepub.com
- Taylor SS, et al.: Retrospective study and outcome of 307 cats with feline infectious peritonitis treated with legally sourced veterinary compounded preparations of remdesivir and GS-441524. Journal of Feline Medicine and Surgery, 2023. journals.sagepub.com
- Zuzzi-Krebitz AM, 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. mdpi.com
- BASG (Austrian medicines agency): Feline infectious peritonitis – currently no approved treatment option available (2025). basg.gv.at
- International Cat Care: Introducing cats. icatcare.org
- Manufacturer's validation data (FIPV Ag and FIPV Ab rapid tests). product page
Important: this article provides general information and does not replace veterinary advice. Diagnosis, treatment and dosage must always be decided by a vet. MancsLab rapid tests are intended for preliminary screening at home.