Serious severity

Feline Infectious Peritonitis (FIP)

A viral disease triggered by a mutated feline coronavirus. Once almost always fatal, it is now treatable in the great majority of cats thanks to a breakthrough in antiviral medicine.

⚠️ Common symptoms

Persistent fever that does not respond to antibioticsLethargy, weight loss and failure to thrive (especially in kittens)Swollen, fluid-filled abdomen ("wet" form)Laboured breathing if fluid collects in the chestJaundice (yellow gums or eyes)Neurological signs — wobbliness, seizures — or eye changes in the "dry" form

Feline infectious peritonitis is one of the most feared diagnoses in cat medicine — and one of the most hopeful turnaround stories in recent veterinary history. It is caused not by a single 'FIP virus' but by feline coronavirus (FCoV), a germ so common that a large share of cats, especially those from multi-cat homes, have been exposed to it. In the overwhelming majority, FCoV is a harmless gut bug. FIP develops only when the virus undergoes specific mutations within an individual cat and the immune system mounts a damaging inflammatory reaction to the changed virus. This is why FIP is not truly 'contagious' in the way a cold is — cats spread the harmless coronavirus to one another, but the deadly mutation arises inside each affected cat.

The disease classically takes two forms. The 'wet' (effusive) form causes inflammation of blood vessels that leaks protein-rich fluid into body cavities, giving the cat a characteristically swollen, pot-bellied abdomen or, if the chest fills, difficulty breathing. The 'dry' (non-effusive) form produces little or no fluid but instead causes inflammatory lesions in organs such as the eyes, brain, liver and kidneys, leading to neurological signs, vision changes and organ dysfunction. Many cats show a mixture of both. Diagnosis is notoriously tricky: there is no single, simple, definitive test, so vets piece together the clinical picture, blood changes, fluid analysis and sometimes advanced imaging or tissue sampling.

For most of the breed's history, an FIP diagnosis meant inevitable decline and euthanasia. The transformation came with the antiviral compound GS-441524 (closely related to remdesivir), which trials and large real-world treatment programmes have shown can cure the great majority of cats over a roughly 84-day course. Initially these drugs were available only through an unregulated grey market, putting desperate owners in a difficult position, but legitimate, vet-supervised access has expanded enormously and continues to improve. The result is a complete shift in outlook: FIP, once a near-certain killer of young cats, has become — with prompt diagnosis and committed treatment — a disease most cats survive.

🔍 Causes

FIP begins with feline coronavirus (FCoV), an extremely common and usually harmless intestinal virus that spreads between cats through shared litter trays and faeces. In most cats it causes only mild, transient diarrhoea or no signs at all. In a minority — perhaps around one in ten infected cats — the virus mutates inside the body and the cat's own immune response to it turns destructive, producing FIP. Young cats (under two), pedigree kittens, and cats living in crowded multi-cat environments such as shelters and catteries are at highest risk.

🛡 Prevention

Because the underlying coronavirus is so widespread, FIP itself cannot be reliably vaccinated against, and prevention focuses on reducing coronavirus pressure: keeping multi-cat households and catteries clean, providing plenty of litter trays (one per cat plus one spare) sited away from food, scooping daily, and avoiding overcrowding and stress. Reputable breeders manage coronavirus levels in their colonies and avoid breeding from lines that have repeatedly produced FIP. New cats are best introduced gradually into low-density, low-stress homes.

💊 Treatment

For decades FIP was considered untreatable and almost universally fatal. That has changed dramatically. Antiviral drugs — most notably GS-441524 (the active form of remdesivir) — now cure well over 80% of treated cats in published studies, typically over a roughly 12-week course of daily oral or injectable medication. Once available only through unofficial channels, legitimate veterinary access to these antivirals has expanded rapidly, and early treatment by a vet experienced with FIP gives the best chance of a full recovery.

When to see a vet: FIP is difficult to diagnose because its early signs — a vague fever, a kitten that simply will not grow, mild lethargy — mimic many other illnesses. Any young cat with a persistent fever that does not respond to antibiotics, unexplained weight loss, a swelling abdomen, jaundice, or sudden neurological or eye changes should be seen promptly. Crucially, the old advice that "FIP is a death sentence" is out of date: with modern antivirals, an early diagnosis and referral to a vet experienced in FIP treatment can now save the cat's life.

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⚕️ Medical disclaimer

  • This information is educational only and is not a substitute for veterinary advice.
  • Symptoms overlap between conditions — only a vet can diagnose accurately.
  • Never withhold treatment or self-medicate based on information found online.
  • When in doubt, call your vet. Early intervention almost always leads to better outcomes.

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