Feline Infectious Peritonitis (FIP) was until recently a uniformly fatal disease in cats, causing immense suffering and devastating for cat owners and welfare professionals. The recent development of effective antiviral treatments has transformed FIP from a death sentence to a treatable condition, representing one of the most significant recent advances in feline welfare.
FIP is caused by a mutated form of feline coronavirus (FCoV). Most cats exposed to FCoV experience mild intestinal disease or no clinical signs. In a minority of infected cats (particularly young cats under 2 years and immunocompromised individuals), the virus mutates and spreads systemically in macrophages, triggering a dysregulated immune response. Two forms occur: wet (effusive) FIP with fluid accumulation in body cavities, and dry (non-effusive) FIP with granulomatous lesions in organs.
Clinical FIP causes profound suffering: weight loss, fever, lethargy, abdominal distension (wet form), neurological signs (ataxia, behavioural changes, seizures), ocular changes (uveitis, retinal disease), and jaundice. Without treatment, progression to death typically occurs within days to weeks of diagnosis. The disease most commonly affects kittens and young adult cats, making it particularly heartbreaking.
The development of nucleoside analogue antivirals—initially GS-441524 (a remdesivir metabolite) and subsequently licensed veterinary formulations—has transformed FIP outcomes. Clinical remission rates of 80-90% have been reported in studies and clinical practice. Treatment typically involves 84 days of daily injections or oral medication, followed by monitoring for 3 months. Most cats in remission remain disease-free long-term. Licensed medications (Lynzee/GS-441524 equivalents) are now available in several countries following regulatory approval.
Treatment costs are high—a full treatment course can cost £2,000-5,000 in the UK—creating welfare and equity challenges. Before licensing, many owners accessed unlicensed GS-441524 through online suppliers, with variable quality control. Welfare organisations have supported treatment access for cats in rescue or owned by low-income households. The availability of licensed treatments with appropriate veterinary oversight is a significant welfare advance, though cost remains a barrier.
FIP diagnosis combines clinical signs, laboratory findings (lymphopenia, elevated serum proteins, high alpha-1-AGP), imaging (effusions, lymphadenopathy), and fluid analysis (high protein, characteristic biochemistry). PCR testing of effusions can confirm coronavirus presence. Response to treatment is monitored through clinical improvement, body weight, and normalisation of laboratory parameters. Regular veterinary monitoring throughout treatment ensures appropriate dosing and identifies treatment failure or relapse early.
FIP cannot be reliably prevented, but risk reduction strategies include: avoiding overcrowding and stress in multi-cat households (which increase FCoV transmission and mutation rate), careful management of FCoV-positive catteries, and maintaining excellent overall health to reduce immunosuppression risk. A nasal vaccine exists but its efficacy is disputed. The most important welfare advance remains the availability of effective treatment that prevents the suffering previously inevitable in diagnosed cases.