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.

Wet (effusive) FIP causes fluid accumulation in the abdomen or chest cavity, producing visible swelling and laboured breathing. Progresses rapidly — most untreated cats survive only days to weeks.

Nucleoside analogue antiviral that blocks FIPV RNA replication inside cat's cells. Most extensively studied FIP treatment, with >80% clinical recovery rates across multiple independent studies.

Persistent fever unresponsive to antibiotics, weight loss, loss of appetite, lethargy, dull coat. Later: swollen abdomen (wet) or neurological/eye changes (dry).

Wet: distended abdomen, laboured breathing. Dry: neurological signs (tremors, seizures), eye changes, jaundice. Both: fever, weight loss, lethargy, appetite loss.

Wet FIP = fluid accumulation, rapid progression. Dry FIP = organ lesions, slower. Both treated with GS-441524; neurological/ocular cases require injections only.

FIP itself is NOT contagious between cats. Underlying FCoV is contagious via litter boxes and close contact. Standard hygiene precautions are sufficient during treatment.

No single definitive test. Combination: A/G ratio blood test, Rivalta test (wet FIP), FIPV immunofluorescence, AGP, RT-PCR on effusion fluid, or tissue biopsy.