What is FIP in cats? Causes, symptoms, wet vs dry, and whether it's curable now

Short answer: FIP (feline infectious peritonitis) is a disease caused by a mutated form of a common feline coronavirus. Until recently it killed nearly every cat it touched. It is now treatable — antiviral therapy built on GS-441524 puts a large majority of cats into lasting remission — but it is still serious, the supply of medicine is largely unregulated, and diagnosis is genuinely hard. None of what follows is a substitute for a veterinarian.

If you have just heard the letters "FIP" from a vet, you are probably reading through fear. The single most important thing to know up front is the thing that was not true a decade ago: FIP is no longer a death sentence. The rest of this page is the honest detail behind that sentence.

What FIP actually is

Most cats carry feline coronavirus (FCoV) at some point. In the vast majority it lives quietly in the gut and causes nothing worse than mild, passing diarrhoea. In a small minority of cats, the virus mutates inside the body and changes behaviour — it starts infecting the immune system's own macrophages and spreads through the body. That mutated form is feline infectious peritonitis virus (FIPV), and the disease it causes is FIP.

Two consequences follow from that biology, and both matter:

• FIP is not "caught" cat-to-cat the way a cold is. Cats catch the harmless gut coronavirus from each other (shared litter trays, dense housing). The dangerous mutation happens within an individual cat. A cat with FIP is not a direct FIP risk to your other cats the way you might fear.
• It is an immune-driven disease. Much of the damage is the body's own inflammatory response to the infected cells — which is why FIP causes fever, fluid, and organ inflammation rather than a simple infection you'd shrug off.

Young cats are most at risk — the majority of cases are in cats under two — and crowded, high-turnover environments (shelters, catteries, multi-cat homes) see more of it. Some pedigree lines appear predisposed.

The forms of FIP

FIP is usually described in forms, though many cats have a mix, and the form can shift during illness:

• Effusive ("wet") FIP — inflammation makes fluid accumulate in the abdomen (a swollen belly) or chest (laboured breathing). This is the faster-moving form.
• Non-effusive ("dry") FIP — instead of fluid, inflammatory masses (granulomas) form in organs: kidneys, liver, intestines, lymph nodes. Slower, and harder to diagnose.
• Ocular FIP — the eye is involved: a change in iris colour, cloudiness, inflammation inside the eye (uveitis).
• Neurological FIP — the virus reaches the brain and spinal cord, causing wobbliness (ataxia), tremors, seizures, or behaviour change.

Ocular and neurological FIP matter enormously for treatment, because the drug has to reach places — the eye, the brain — that many drugs struggle to penetrate. That is why those forms are dosed differently, a point we come back to in the treatment evidence.

Symptoms: what sends people looking

There is no single tell-tale sign, but the pattern that most often leads to an FIP diagnosis is:

• A persistent fever that does not respond to antibiotics — often the first clue.
• Weight loss and a fading appetite, sometimes despite eating.
• Lethargy and a general "fading" that owners describe as the cat not being themselves.
• A swollen, fluid-filled abdomen (wet form) or laboured breathing (chest fluid).
• Jaundice — a yellow tinge to the gums, eyes, or skin.
• Eye changes or neurological signs (wobbliness, seizures) in the ocular/neuro forms.

Any of these deserve a vet, quickly. FIP moves, and time matters.

Why diagnosis is hard

This is the uncomfortable part owners are rarely told plainly: there is no single, perfect antemortem test for FIP. A diagnosis is built from the whole picture:

• Signalment — a young cat from a multi-cat background raises suspicion.
• Bloodwork patterns — high globulin and a low albumin-to-globulin (A:G) ratio, high bilirubin, and often lymphopenia and anaemia. None is proof alone; together they build a case.
• Effusion analysis — if there's fluid, it's typically thick, straw-yellow, protein-rich; the Rivalta test and cell analysis support the diagnosis.
• RT-PCR and immunostaining — detecting viral RNA or antigen in effusion or tissue is the closest thing to confirmation, and increasingly available.
• Imaging — ultrasound and X-rays to find fluid or organ changes.

A careful vet weighs all of it. Be wary of anyone — including any seller of medicine — who declares certainty from a single test.

Is FIP curable now? The honest answer

Yes, for most cats — and this is the genuine revolution. Antiviral treatment centred on GS-441524 (and its prodrug remdesivir, and the oral alternative molnupiravir) has turned a near-universally fatal disease into one where the large majority of treated cats reach lasting remission. Published cure and survival figures now sit above 80%, and in some cohorts above 90% — a scale of change almost unheard of in medicine. We lay out those numbers, with their sources, in the treatment evidence.

Three honest caveats belong right next to that hope:

1. Treatment is a commitment — typically an 84-day course of daily medicine, plus monitoring, plus a watch period after.
2. The medicine supply is largely unregulated in much of the world. Independent testing has found unlicensed products containing very different amounts of drug than their labels claim — a real risk to getting the dose right. We cover what that means, and how to lower the risk, in the treatment pages.
3. It is still a decision for a veterinarian. Diagnosis is hard, the forms differ, and dosing is not a number to take off a website.

If your cat has just been diagnosed, the next thing worth reading is what the treatment actually involves and how well it works: FIP treatment: what the current evidence shows. There is a great deal of reason for hope.

Not veterinary advice. FIP is serious and time-sensitive; diagnosis and treatment are decisions for a veterinarian who has examined your cat.

References

1. Pedersen N.C. An update on feline infectious peritonitis: virology and immunopathogenesis. The Veterinary Journal, 2014. Find on PubMed
2. Taylor S.S. et al. Efficacy of GS-441524 for feline infectious peritonitis: a systematic review (2018–2024). Read on PMC
3. Cornell Feline Health Center — Feline Infectious Peritonitis. vet.cornell.edu