Treatment of FIP: Part 4 of 5

IMPORTANT UPDATE: Since this FIP blog series was written, advancements have been made in FIP treatment, including the legal availability of oral formulations of GS-441524 in the United States. Please visit Cornell Feline Health Center, EveryCat Health Foundation and University of California Davis Koret Shelter Medicine Program for the most up-to-date information. Always work with a veterinarian for treatment and support.

Dr. Niels Pedersen, Professor Emeritus at the University of California Davis School of Veterinary Medicine, has worked for 30 years to find a cure for FIP. In 2019, the results of his studies were published, coauthored by Gilead Sciences, the company that invented and patented GS-441524 (GS). The studies showed that GS blocks the viral replication of FIP and has a cure rate of approximately 85%.

However, Gilead has not licensed GS for animal use in the United States. This has led desperate cat parents to obtain GS from unverified sources in China. It’s important to be cautious when seeking information about FIP treatment and products.

FIP treatment with GS, in injectable or oral form, usually lasts 12 weeks/84 days. FIP symptoms of fever, loss of appetite and inactivity can improve or disappear within 12-36 hours; effusions (fluid) in the abdominal cavity within 10-14 days; and jaundice within 2-4 weeks.¹

When GS treatment was in its infancy, the injectable form was preferred. A major side effect of the injectable form is pain at the injection site; medications can be prescribed and given to minimize the pain prior to injecting.

Now that both forms have been used for some time with equal efficacy and the cost of oral GS has come down, more cats are being treated with oral GS. Side effects of oral GS haven’t been serious, but depending on the product have included lymphocytosis (an increase in disease-fighting white blood cells called lymphocytes), eosinophilia (an increase in disease-fighting white blood cells called eosinophils) and elevated liver enzyme activity.²

Cats with issues such as vomiting/regurgitation and diarrhea are not considered good candidates for starting with oral GS; they’re often started with GS injections until their gastrointestinal issues resolve at which time they may be switched to oral GS.

A cat is considered cured after a 12-week post-treatment observation period. A cat who is considered cured with GS treatment has no FIP symptoms and regains normal levels of activity and appetite, appropriate weight gain and/or growth and improved coat quality, as well as normal critical blood values.

Relapses that occur during the post-treatment observation period are usually due to infections that have escaped to the central nervous system during treatment for FIP which was not accompanied by neurological or ocular signs.

Treatment failures may occur due to incorrect diagnosis, poor quality GS, drug resistance, improper dose adjustment (once a cat gains weight), other diseases or medical conditions affecting the cat or a client’s inability to continue treatment due to financial constraints.

Whereas GS was only available initially in the United States from unregulated black market sources, it became legally available by veterinary prescription from compounding pharmacies in June 2024. Compounding pharmacies customize medications for people and pets. Although FIP compounded medications are not FDA-approved, compounding pharmacies may be accredited by the Pharmacy Compounding Accreditation Board, which indicates that they meet or exceed national quality standards for compounding medicines.

You/your veterinarian can find providers of FIP medications by searching “FIP drug compounding pharmacies United States.” To find providers of FIP medications internationally, you/your veterinarian can search “FIP treatment providers + country; for example, “FIP treatment providers Australia.”

GS medication may be available in liquid and tablet/capsule form. Compounding pharmacies can flavor oral medications with tastes, such as tuna or chicken, that appeal to cats, which may make treatment less stressful for everyone.

In part 5 of this series, we’ll discuss control and prevention of FIP.