FIP in Cats: Signs, Diagnosis & New Treatment
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Feline infectious peritonitis (FIP) is a serious disease caused when a common, usually harmless feline coronavirus mutates inside a cat and triggers widespread inflammation. It mainly affects young cats and cats in multi-cat environments. Once considered almost always fatal, FIP now has effective antiviral treatments that save many cats — making prompt diagnosis and veterinary care more important than ever.
Act quickly — FIP is time-sensitive
If a young cat has a persistent fever that doesn't respond to antibiotics, a swollen belly, or is losing weight and fading, see your vet promptly. FIP progresses and can become life-threatening, but antiviral treatment now saves many cats — and the sooner it starts, the better the odds.
FIP can be hard to diagnose and moves faster in some cats than others. Don't wait to see if a lingering fever or a listless kitten improves on its own.
What FIP is
Most cats carry feline coronavirus (an intestinal virus, unrelated to the human COVID coronavirus) at some point, usually with no more than mild or no signs. In a small number of cats — especially young ones and those in shelters or catteries — the virus mutates and provokes an intense, damaging immune reaction throughout the body. That reaction is FIP.
It's most common in cats under two years old and in crowded, multi-cat settings where the underlying coronavirus spreads easily through shared litter boxes. It is not simply "caught" from another cat as FIP itself; it develops from the mutation within an individual cat.
The two forms: wet and dry
FIP classically shows up in two overlapping forms:
| Form | Hallmark signs |
|---|---|
| Wet (effusive) | Fluid buildup in the abdomen (a swollen, pot-bellied look) or chest (causing labored breathing). Tends to progress faster. |
| Dry (non-effusive) | Little or no fluid; inflammation targets organs, eyes and the nervous system, causing weight loss, eye changes, wobbliness or seizures. |
Both forms usually share a persistent fever that doesn't respond to antibiotics, loss of appetite, weight loss and lethargy. Many cats show a mix of features.
How it's diagnosed
FIP is notoriously tricky to confirm because no single test is perfect. Your vet builds the diagnosis from the whole picture: the cat's age and history, clinical signs, and a combination of tests such as bloodwork changes, analysis of any abdominal or chest fluid, and specialized coronavirus testing (including PCR on fluid or tissue). Imaging and eye exams can add supporting evidence.
Practical take: Because diagnosis is a judgment built from many clues, a detailed history — when the fever started, appetite and weight changes, energy level over time — genuinely helps your vet reach the right answer faster.
Treatment, outlook and prevention
The biggest change in feline medicine here is treatment. Antiviral drugs (in the GS-441524 / remdesivir family) now cure or achieve long-term remission in many cats that would previously have been lost — treatment typically runs for several weeks under veterinary supervision. Availability, protocols and cost vary, so work closely with your vet or a specialist.
Because FIP arises from a mutation, it's hard to prevent directly, but reducing feline-coronavirus load helps: keep litter boxes clean and separated from food, avoid overcrowding, and keep group-housed cats as low-stress and healthy as possible. Tracking a recovering cat's weight, appetite, energy and temperature during and after treatment gives your vet a clear read on whether the antiviral is working.
Track a cat through FIP treatment
Tavi logs your cat's weight, appetite, energy and symptoms and charts them over time — so during weeks of FIP antiviral treatment you and your vet can see recovery taking hold, or catch a stumble early. Free for your first pet.
Frequently asked questions
Is FIP in cats always fatal?
No longer. FIP was once considered almost always fatal, but antiviral drugs in the GS-441524/remdesivir family now cure or achieve long-term remission in many cats. Prompt diagnosis and treatment under veterinary supervision give the best chance, so don't wait on a young cat with a persistent fever.
What are the first signs of FIP in cats?
Early signs are often vague: a persistent fever that doesn't respond to antibiotics, reduced appetite, weight loss and lethargy in a young cat. From there it may develop into the wet form (a swollen belly or breathing trouble from fluid) or the dry form (eye and neurological changes). Any of these warrants a prompt vet visit.
Is FIP contagious to other cats or people?
FIP itself develops from a mutation inside an individual cat and isn't spread cat-to-cat as FIP. The underlying feline coronavirus is contagious between cats, mainly through shared litter boxes, but most infected cats never develop FIP. The virus does not infect humans.
How is FIP diagnosed?
There's no single definitive test, so vets combine the cat's age and history, clinical signs, bloodwork, analysis of any chest or abdominal fluid, and coronavirus PCR testing. Imaging and eye exams can add support. Because it's a judgment built from many clues, a detailed symptom history helps.
Which cats are most at risk of FIP?
FIP most often affects cats under about two years old and those from multi-cat environments like shelters and catteries, where the underlying coronavirus spreads easily. Stress and crowding increase risk. Keeping litter boxes clean and avoiding overcrowding help reduce the viral load cats are exposed to.
Tavi's guides are informational and not a substitute for professional veterinary advice, diagnosis or treatment. When in doubt, call your veterinarian. Sources: Cornell University Feline Health Center; American Association of Feline Practitioners (AAFP); VCA Animal Hospitals; Merck Veterinary Manual