FIP (Feline Infectious Peritonitis) — what every cat owner should know

What is FIP?

FIP is a progressive disease caused by a mutated feline coronavirus. Most cats carry the coronavirus harmlessly; in a small share of infected cats the virus mutates and becomes invasive, causing a wet form (fluid in the abdomen or chest) or a dry form (nerves, eyes, kidneys).

Symptoms to watch

• Persistent fever that does not respond to treatment
• Lethargy, loss of appetite, weight loss
• Abdominal swelling (wet form)
• Vision problems, seizures, weakness in one or more legs (dry form)

Diagnosis

There is no single test that confirms FIP — the diagnosis is built from the picture: blood work (globulin pattern), effusion analysis, and virus PCR on the effusion or tissue.

Treatment — the reality by country

Antiviral therapy (84 days or more, every day, no missed doses) cures the majority of cats. Availability depends on where you live:

• **US:** for years the main route was compounded GS-441524 from US compounding pharmacies (permitted under FDA guidance GFI #256, May 2024) — a compounded, *unapproved* product, not an approved medicine. In 2025 **fosbunivir (VetFosb)** became the first FDA-approved FIP drug.
• **Serbia and Croatia: there is no approved or authorized FIP treatment.** Neither fosbunivir nor compounded GS-441524 is on the market here, and a vet in these countries cannot prescribe them. Privately importing an unapproved compounded product is a risk the owner takes on their own — it is not something a clinic can recommend.
• **Costs:** US list prices (2026) run about $9–12 per tablet or capsule; a full 84-day-plus course typically totals several thousand dollars and scales with the cat's weight. Fosbunivir is no cheaper, and private import adds shipping cost and the risk of losing the medication.
• **What to do:** ask your vet for FIP testing when the picture fits, and treat early — outcomes improve sharply. Current overviews are maintained at felinevma.org and aafp.org.
• **Discuss treatment with your vet — do not buy or start antiviral therapy on your own.**

Prevention

There is no vaccine and the virus cannot be prevented; reducing stress and avoiding mixing cats with unknown health status lowers the risk of the initial coronavirus infection.