FIP in Cats: The Death Sentence That Stopped Being One

For decades, feline infectious peritonitis was the diagnosis veterinarians dreaded giving most. The disease was close to uniformly fatal, and the numbers were brutal: in Merck's veterinary manual, the median survival for the untreated effusive form is nine days.

That era is over, and this page exists because the ending changed. In the largest real-world study to date, 307 cats treated legally in the UK, 88.6% were alive at the end of their initial treatment and 84.4% at the longest follow-up. The disease did not get milder. The treatment arrived, and then, country by country, it became legal. Where you live still decides how you get it and what it costs, and that map is the second half of this page.

What FIP actually is

FIP begins with a virus most cats meet anyway: feline coronavirus, an ordinary gut virus of cats that has nothing to do with COVID-19 in people. In single-cat households roughly 15% of cats have met it; in dense multi-cat settings such as shelters, catteries and rescues, 50 to 90% have. For most cats it causes, at worst, brief diarrhea.

The disease happens when the virus changes inside one unlucky cat. In Cornell's summary, in approximately 10% of infected cats a mutation alters the virus's behavior so that it infects white blood cells and spreads through the body; Merck's range for how often infection progresses to FIP is 5 to 12%. The immune system's own intense inflammatory response around blood vessels, in the abdomen, kidney or brain, is what does the damage. The exact mutation is still not pinned down, and researchers still debate the details of how outbreaks happen, so we state the mechanism the way the manuals do: mostly understood, not fully settled.

The practical upshot: FIP is not really "caught" the way flu is. The common coronavirus spreads easily between cats; the disease itself develops from within.

Who gets it

The risk profile is sharp and worth knowing. Approximately 70% of cases are diagnosed in cats under a year and a half old, and half of all cases in kittens under seven months. High-density housing raises risk. So does being male, and so does pedigree: Merck's list of predisposed breeds includes the Abyssinian, Bengal, Birman, Himalayan, Ragdoll, and the Cornish and Devon Rex. There is a second, smaller risk peak in elderly cats.

Two forms, one disease

FIP wears two faces. The effusive (wet) form fills body cavities with fluid: a swollen, pot-bellied abdomen or labored breathing from fluid around the lungs, usually fast-moving. The non-effusive (dry) form builds inflammatory masses in organs instead, and can show up as eye disease or neurological signs, on a slower clock. Untreated, Merck's course for the wet form is days to weeks, and weeks to months for the dry.

The early signs, though, are the unspecific kind this site keeps warning about: a kitten or young cat that stops growing, loses appetite, runs a fever that antibiotics do not touch, and fades. That pattern belongs at the vet, quickly.

Why diagnosis is genuinely hard

There is no single definitive test for FIP in a living cat, and both Cornell and Merck say so plainly. Coronavirus antibody tests mostly measure past exposure, which most cats have; PCR generally cannot tell the harmless biotype from the dangerous one. Vets assemble a diagnosis from the signalment, the fluid, the bloodwork and imaging, and increasingly from one more clue: a cat with FIP that starts antiviral treatment usually responds fast, and that response itself supports the diagnosis.

The turn: 31 cats in California

The modern story starts with a 2019 trial at UC Davis led by Niels Pedersen, testing an antiviral called GS-441524, a close chemical relative of remdesivir. We re-read the published trial in full. Of 31 cats with naturally occurring FIP, five that arrived with severe disease died early. The 26 others completed at least 12 weeks of treatment; a few relapsed and were re-treated at a higher dose. The final count: 25 long-term survivors, from a disease with a nine-day untreated median.

What followed was five strange years. The drug worked, but no licensed veterinary product existed, so a worldwide owner-to-owner network sourced unregulated Chinese-made GS-441524 for desperate households. That black-market era saved cats and hurt some too: the acidic unregulated injections burned, quality control did not exist, and owners paid thousands of dollars for vials handed over in parking lots. We mention it because millions of search results still lead there, and because everywhere in our four markets there is now a legal path.

What legal treatment actually achieves

The best real-world answer comes from the UK, where legal veterinary supply arrived first at scale. A 2023 retrospective study, which we also re-read in full, followed 307 cats treated with legally compounded remdesivir and GS-441524 between 2020 and 2022. Results: 88.6% of cats alive at the end of the initial treatment period, 84.4% alive at the longest follow-up, and 33 of 307 cats (10.8%) relapsing along the way, most of them treatable again. The authors' own words: all three protocols were "highly effective in treating this previously fatal disease."

A standard course is long, typically twelve weeks, and a 2024 randomized trial found a 42-day course as effective as the 84-day standard for cats with the effusive form, which matters for cost. In Australia, a 2025 study of 92 cats across 35 practices found molnupiravir, a cheaper oral antiviral, achieved survival comparable to remdesivir and GS-441524. None of this makes FIP trivial: cats still die of it, especially those arriving very sick. It makes FIP treatable, which is a different disease than the one older websites describe.

The legal map: how you get treatment, market by market

This is where your passport matters, and every line below is dated, because this landscape has moved every year since 2019. As of August 2026:

Costs move too fast to promise, so treat published figures as snapshots: an independent US guide put a full course for a small cat anywhere from about $200 to $1,200 depending on form and dose as of 2024-2025, UK owners reported totals around £5,000 to £6,000 before the March 2023 price drop with costs falling since, and Canada's quoted all-in figure is above. A twelve-week antiviral course is exactly the kind of bill our cat insurance guide exists for; whether a policy covers it depends on the policy, and FIP's youth skew makes the kitten insurance timing question real.

What to take away

• FIP develops when a common, usually harmless feline coronavirus mutates inside a small minority of infected cats; it is not caught the way flu is.
• FIP is a young cat's disease: about 70% of cases are diagnosed before a year and a half of age, half before seven months.
• Untreated, the effusive form has a median survival of nine days (Merck); the disease was historically close to uniformly fatal.
• The treatment era is real: in the largest real-world study, 307 UK cats on legal antivirals, 88.6% were alive at the end of initial treatment and 84.4% at longest follow-up.
• All four of our markets now have a legal route to the antivirals: Australia since 2020, the UK since 2021, Canada since February 2024, the US since June 2024.
• A young cat with a stubborn fever, a swelling belly, or that is simply fading needs a vet now: for the first time in this disease's history, speed buys something.