FIP: feline infectious peritonitis
Once almost always fatal, now treatable in many countries.
Key points. Most cats that meet feline coronavirus never develop FIP. FIP is not usually passed from cat to cat: it develops when the virus mutates inside one cat. Antiviral treatment with GS-441524 has changed the outlook, but access and legal status differ widely between countries.
What it is
FIP is a disease caused by certain mutated strains of feline coronavirus (FCoV). Ordinary FCoV lives in the gut and usually causes no signs or mild diarrhoea. In a minority of infected cats (usually under 10 percent, per the ABCD) the virus mutates so that it infects white blood cells and spreads around the body. The resulting immune reaction causes the disease.
A positive coronavirus antibody test shows only exposure to the common virus. It does not mean a cat has, or will get, FIP.
How it spreads and how contagious it is
- Ordinary FCoV spreads by the faecal-oral route, mostly through shared litter trays. Kittens are commonly infected from the mother at 5 to 8 weeks of age. Re-infection is common.
- FIP itself arises from a change within the individual cat. The ABCD says that spread of the FIP-causing virus between cats is believed to occur only very rarely, although outbreaks in multi-cat settings have been reported.
- Risk factors include young age (about 70 percent of cases are under 18 months old), being a pedigree or male cat, and crowded housing or stress. Studies differ on how much household size matters.
Forms and signs
- Effusive ("wet") FIP: fluid in the abdomen and/or chest, causing a pot-bellied look or breathing difficulty. Progression is often rapid.
- Non-effusive ("dry") FIP: inflammatory lesions in organs, eyes or brain. Signs vary: persistent fever, weight loss, poor appetite, eye changes, wobbliness, seizures. It is often slower.
- The two forms can convert into each other. A cat, especially a young one, with fever not responding to antibiotics should be seen by a vet.
Diagnosis
Diagnosis is difficult because no single simple test confirms FIP in a living cat.
- Antibody (titre) tests cannot distinguish harmless coronavirus from FIP.
- Vets build the case from history, examination, blood tests (for example raised globulins and a low albumin-to-globulin ratio) and, where there is fluid, analysis of the fluid and detection of coronavirus in it by PCR or staining. If there is no fluid, samples from affected organs may be needed.
- Only examination of tissue with coronavirus staining is truly confirmatory. A rapid response to antiviral treatment also supports the diagnosis (ABCD).
- The ABCD publishes a diagnostic tool for veterinarians.
Treatment
- Antiviral drugs, first remdesivir by injection and then GS-441524 as tablets or liquid, have transformed treatment. Legal veterinary antivirals have been available in the UK and Australia since 2021 (International Cat Care); in the US an oral compounded formulation is available with a veterinary prescription (Cornell).
- Treatment lasts several weeks. Published protocols often use around 12 weeks, but dose, duration and monitoring are decided by the treating vet and research is ongoing.
- Unregulated "black-market" products may vary in strength and purity (Cornell). Use a veterinarian-supervised source where one is legally available.
- Supportive care may include fluids, drainage of effusions, appetite stimulants and pain relief.
- Availability, licensing and cost differ greatly by country. Where antivirals are unavailable, FIP remains very serious.
Prognosis
Without treatment, clinical FIP is almost always fatal. With oral GS-441524 and supportive care, the ABCD reports recovery in over 85 percent of cases, and International Cat Care cites studies from 2022 and 2023 finding more than 85 percent of treated cats cured. Improvement is typically seen within days. Outcomes are less certain in cats that are very ill at the start or that have neurological or eye involvement; discuss this with your vet.
Care and prevention
- There is no reliable way to prevent coronavirus infection, but good hygiene lowers risk: clean litter trays (scooped daily), trays away from food and water, one tray per cat plus one extra, and avoiding crowding.
- One FIP vaccine is licensed in some countries, but its effectiveness is questioned and it is not routinely recommended (Cornell).
- Vaccinate against the other core diseases. See vaccination.
When to call a vet urgently: breathing difficulty, collapse, a cat that is straining to pass urine or producing none, repeated vomiting, not eating for 24 hours or more, or sudden weakness in the back legs. See signs that need a vet urgently.
Living with other cats
Because the FIP-causing virus rarely spreads directly, other cats in the home are generally not considered at high risk from a cat with FIP. However, they may already carry ordinary coronavirus. Ask your vet about hygiene during treatment, especially for litter trays, and about any kittens or vulnerable cats in the household.
People and dogs
Feline coronavirus infects only cats and cannot be passed to humans (Cornell; International Cat Care).