For decades, feline infectious peritonitis was one of the hardest diagnoses a cat owner could hear. The disease, known as FIP, develops from a feline coronavirus infection, and it was considered fatal.
That has changed. Updated treatment guidelines from the European Advisory Board on Cat Diseases (ABCD), published April 9, 2026 in the peer-reviewed journal Viruses, put it plainly: “It used to be regarded a fatal disease, with euthanasia commonly recommended following diagnosis due to its very poor prognosis.” And now: “FIP is now a treatable and frequently curable disease.”
What changed for FIP in cats
Antiviral drugs. The most studied is GS-441524, given by mouth, with success rates the guidelines describe as often exceeding 90 percent. Remdesivir, given by injection, is mostly used for cats too sick to take oral medication. Molnupiravir is a second-line option with more side effects.
The guidelines also report that a 42-day course of treatment can work as well as the traditional 84-day course. The American Veterinary Medical Association covered the update in June and noted a 2025 study in the Journal of Feline Medicine and Surgery that found 77 percent survival at six months.
Not a guarantee
Some cats relapse. Dr. Petra Cerna of the University of Georgia College of Veterinary Medicine told AVMA News that studies show roughly 10 to 15 percent of cats treated with GS-441524 relapse.
In the United States, GS-441524 is not an FDA-approved drug. In May 2024, the FDA said it does not intend to take enforcement action against compounded GS-441524 when a veterinarian prescribes it for an individual cat with FIP. That gives U.S. vets a legal path to treatment that did not exist a few years ago.
Owners with questions about FIP should talk with their veterinarian, who can confirm a diagnosis and discuss which treatments are available.
Sources
- Viruses, Update on Treatment of Feline Infectious Peritonitis: European Advisory Board on Cat Diseases (ABCD) Guidelines (Apr. 9, 2026)
- AVMA News, New FIP guidelines highlight available antivirals, shorter treatment course (June 23, 2026)
- U.S. Food and Drug Administration, FDA Announces Position on Use of Compounded GS-441524 to Treat FIP (May 10, 2024)
