Feline infectious peritonitis

What it is

Feline infectious peritonitis (FIP) is caused by feline coronavirus (FCoV). Infection with the enteric form is very common: the virus replicates in the intestinal epithelium, is shed in faeces and spreads through the litter box, usually causing nothing more than mild diarrhoea. In a small proportion of infected cats the virus mutates within the cat and gains the ability to replicate in monocytes and macrophages, which spread it throughout the body. The cat's immune response shapes the disease: infected macrophages accumulate around venules and cause pyogranulomatous vasculitis. The mutated FIP-causing virus does not usually spread from cat to cat.

Two presentations are described, which often overlap:

• Effusive ("wet") form: vasculitis increases vascular permeability and protein-rich fluid accumulates in the abdomen, thorax or pericardium.
• Non-effusive ("dry") form: pyogranulomas form in organs such as the kidneys, liver, mesenteric lymph nodes, intestinal wall, eyes and central nervous system.

Who it affects

Mainly young cats, usually under 2 years old, from catteries, shelters or multi-cat households, where exposure to coronavirus is high. Stress (rehoming, neutering, vaccination, other illness) often precedes the disease. Some pedigree breeds appear more predisposed. It is also seen in older cats.

Anatomy involved

• Peritoneum and pleura: serosal surfaces with fibrinous plaques and pyogranulomas; yellow, viscous, protein-rich effusion.
• Liver and kidneys: whitish pyogranulomatous nodules on the surface and in the parenchyma; the kidneys may feel large and irregular.
• Small and large intestine: focal granulomatous thickening of the wall, often at the ileocaecocolic junction, and enlarged mesenteric lymph nodes.
• Eyes: anterior or posterior uveitis, with keratic precipitates, iris colour changes, hyphaema, chorioretinitis and retinal vasculitis.
• Central nervous system: meningoencephalitis and ependymitis, with secondary hydrocephalus in some cases.

Clinical signs

Persistent fever that does not respond to antibiotics, lethargy, loss of appetite and weight, stunted growth in kittens and jaundice. In the effusive form, abdominal distension with a fluid wave, or dyspnoea from pleural effusion. In the non-effusive form, signs depend on the organ affected: abdominal masses, uveitis, ataxia, seizures, nystagmus or behavioural changes.

Diagnosis

There is no single definitive test short of histology; the findings are combined:

• Compatible history and signs in a young cat from a multi-cat environment.
• Blood work: lymphopenia, mild non-regenerative anaemia, hyperglobulinaemia with a low albumin:globulin ratio, hyperbilirubinaemia.
• Effusion analysis: yellow, viscous fluid with high protein and low to moderate cellularity (macrophages and non-degenerate neutrophils). Detection of FCoV RNA by RT-PCR or immunostaining of antigen in effusion macrophages strongly supports the diagnosis.
• Ultrasound: effusion, enlarged mesenteric lymph nodes, kidneys with irregular margins, intestinal thickening.
• Serum coronavirus antibody titres only indicate exposure and cannot diagnose FIP on their own.
• Histopathology with immunohistochemistry of lesions confirms the diagnosis. In neurological or ocular forms, cerebrospinal fluid or aqueous humour is analysed.

Treatment

For decades FIP was considered fatal. Today nucleoside analogue antivirals such as GS-441524 (oral) and its prodrug remdesivir (injectable) achieve lasting remission in most treated cats, with courses that usually last about 12 weeks; doses are increased for ocular and neurological forms. Molnupiravir has also been used, mainly in resistant cases. The legal availability of these drugs varies by country. Supportive care is added: drainage of effusions that compromise breathing, fluids, nutrition and topical treatment of uveitis. Response is monitored with body weight, temperature, the albumin:globulin ratio and the effusions. Relapses are possible, especially in neurological forms. In multi-cat settings the coronavirus load is reduced with enough clean litter boxes and small groups.

Key studies

• AAFP/EveryCat guidelines (Thayer et al., 2022): the reference guide for diagnosing FIP. No sign or laboratory change is pathognomonic, especially when there is no effusion, so the diagnosis is built by combining history, examination and tests whose sensitivity and specificity are understood.
• Ritz et al. (2007), placebo-controlled, double-blind trial in 37 cats with confirmed FIP, before the antiviral era. Feline interferon-omega did not prolong survival or improve quality of life; median survival was 9 days and only 1 cat lived longer than 3 months. It shows what the disease was like without specific treatment.
• Pedersen et al. (2019), field trial of GS-441524 in 31 cats (26 with effusive disease; severe neurological and ocular forms were excluded). Four cats died in the first 2–5 days and another at 26 days. Of the 26 that completed at least 12 weeks, 18 stayed healthy after one course and 8 relapsed; retreatment at the higher dose of 4 mg/kg a day brought the total to 25 long-term survivors. There was no control group, and two authors worked for the drug's manufacturer.
• Zuzzi-Krebitz et al. (2024), randomised trial in 40 cats with effusive FIP given oral GS-441524 (15 mg/kg a day) for 42 or 84 days. Two cats died during treatment; the other 38 (19 per group) recovered, and every cat that completed the short course was still in remission at day 168. Only the effusive form was studied.
• Brostoff et al. (2026), randomised, double-blind non-inferiority trial in 20 cats with non-effusive FIP treated for 84 days with oral remdesivir or oral GS-441524. At week 16, 9 of 10 and 7 of 10 cats were alive and in remission; all deaths occurred in the first 11 days, and remdesivir met the criteria for non-inferiority. The groups were small, and four cats showed signs suggesting a late relapse 1.5–2 years later.
• ABCD guidelines (Tasker et al., 2026): updated treatment recommendations. GS-441524 is the most studied antiviral, with success rates often above 90%. Injectable remdesivir is mainly reserved for cats too ill for oral medication and replaced by oral treatment as soon as possible, and 42-day courses appear to be as effective as the classic 84-day ones.