FIP (Feline Infectious Peritonitis) is a serious, progressive disease caused by a mutation of the feline coronavirus. In most cats, feline coronavirus causes no symptoms or only mild digestive upset. In a small percentage of cases, the virus mutates into FIPV — triggering a damaging immune response that attacks blood vessel walls and organs throughout the body. FIP presents in two main forms. Wet (effusive) FIP causes fluid accumulation in the abdomen or chest, producing visible swelling and breathing difficulty. Dry (non-effusive) FIP causes inflammatory lesions in organs such as the brain, eyes, kidneys, or liver, with a wider range of less obvious symptoms. Some cats develop both forms simultaneously or transition from one to the other. Until recently, FIP was considered almost universally fatal. The development of antiviral treatments — particularly GS-441524 — has fundamentally changed this, with recovery rates above 80% in cats that complete the full treatment protocol.

FIP stands for Feline Infectious Peritonitis. The name reflects the early characterisation of the disease — "peritonitis" refers to inflammation of the peritoneum, the membrane lining the abdominal cavity, which was the most visually obvious sign in the wet (effusive) form. The name has remained even though the disease's full scope extends well beyond the abdomen, affecting the brain, eyes, kidneys, and other organs in its dry form.

GS-441524 is a nucleoside analogue antiviral compound that works by blocking the replication of the feline infectious peritonitis virus (FIPV). When the virus attempts to copy its RNA inside your cat's cells, GS-441524 interferes with that process — stopping the disease at its source rather than just managing symptoms. It is the most extensively studied treatment for FIP, with clinical recovery rates consistently above 80% across multiple independent studies. Until recently, FIP was considered almost universally fatal. GS-441524 has changed that.

Across multiple independent clinical studies covering over 650 cats, the combined treatment success rate for GS-441524 is approximately 92%. Most cats that complete the full 84-day protocol reach sustained remission and return to a normal quality of life. Cats that begin treatment earlier in the disease course tend to have the best outcomes. Complicated forms — including neurological and ocular FIP — have a lower but still meaningful recovery rate when treated at higher doses with injections. The key variables are: starting treatment promptly, dosing correctly by weight, and completing the full protocol without interruption.

The earliest signs of FIP are often non-specific and easy to overlook. Persistent fever that does not respond to antibiotics is typically the first indicator. This is usually accompanied by loss of appetite, unexplained weight loss, lethargy, and a dull or unkempt coat. Some cats also develop mild digestive upset in the early stages. As the disease progresses, more specific signs emerge depending on the form. In wet FIP, a visibly swollen or distended abdomen develops as fluid accumulates. In dry FIP, signs vary more widely and may include neurological changes such as loss of coordination, tremors, or seizures, as well as eye abnormalities like cloudiness or uneven pupil size.

Early symptoms common to both wet and dry FIP include persistent fever that does not respond to antibiotics, unexplained weight loss, loss of appetite, lethargy, and dull coat condition. In wet FIP, additional signs include a visibly swollen or distended abdomen, laboured breathing (if fluid accumulates in the chest), and general weakness. In dry FIP, symptoms vary widely — neurological signs such as wobbling, tremors, or seizures; eye changes such as cloudiness, uneven pupil size, or inflammation; jaundice; or unexplained changes in behaviour.

Wet (effusive) FIP is characterised by fluid accumulation in the abdomen or chest cavity, and tends to progress rapidly. Dry (non-effusive) FIP involves inflammatory lesions in organs without significant fluid build-up, and typically progresses more slowly. Both forms are treated with GS-441524 using the same core protocol. What changes is the starting dose and urgency of treatment. Neurological and ocular FIP — which can occur in either form — require higher doses and must be treated with injections only.

FIP develops when the feline enteric coronavirus (FCoV) — a common intestinal virus that most cats carry without ever falling ill — undergoes a mutation inside the cat's body. This mutation transforms it into the feline infectious peritonitis virus (FIPV), which triggers a severe immune response and attacks the cat's own tissues. FCoV itself is contagious and spreads between cats through shared litter boxes, food and water bowls, and close contact — particularly in multi-cat households, shelters, and catteries. However, FIP itself does not spread directly from cat to cat. The mutation that causes FIP occurs randomly and cannot be predicted.

FIP can develop in indoor cats because the feline coronavirus (FCoV) that causes it is extremely widespread — many cats are exposed in early life, particularly if they came from a breeder, cattery, shelter, or multi-cat household before joining your home. FCoV can remain dormant in a cat's body for months or years before the random mutation event that triggers FIP. An indoor cat that has had no outside contact in years can still develop FIP from a coronavirus exposure that happened much earlier in its life. It is not a sign that anything went wrong in your care.

FIP itself is not directly contagious between cats. The disease cannot be passed from one cat to another. What is contagious is the underlying feline coronavirus (FCoV) — the common, usually harmless intestinal virus that, in rare cases, mutates inside an individual cat's body to cause FIP. FCoV spreads through shared litter boxes, food and water bowls, grooming, and close contact. During your cat's FIP treatment, standard hygiene precautions are sufficient: keep litter boxes clean and disinfect them regularly, wash food and water bowls separately, and monitor other cats in the household for changes in behaviour, appetite, or weight.