FIP in Cats: Symptoms, Diagnosis and Antiviral Treatment

For decades, a diagnosis of feline infectious peritonitis (FIP) brought devastating news. Today, antiviral treatment has changed the outlook for many cats. FIP is still a serious, fast-moving illness, but prompt veterinary care now gives affected cats real hope.

What Causes FIP?

FIP develops after a common feline coronavirus (FCoV) changes inside an individual cat. FCoV is widespread, particularly in multi-cat environments. Most infected cats show no signs or only temporary mild digestive illness. Only a small percentage develop FIP. Age, immune response, stress, and exposure in high-density environments can influence risk.

FIP itself is not considered directly contagious from cat to cat in the way many respiratory infections are. Cats can spread the underlying feline coronavirus, however, mainly through fecal contamination and shared litter areas. Careful litter-box hygiene and avoiding overcrowding help reduce exposure.

Signs That Need Prompt Veterinary Attention

FIP can affect many parts of the body, so symptoms vary. Contact a veterinarian promptly if a cat—especially a kitten or young adult—has persistent fever, poor appetite, weight loss, lethargy, a swollen abdomen, breathing difficulty, eye changes, weakness, poor coordination, tremors, or seizures.

Veterinarians often describe FIP as effusive (“wet”) or non-effusive (“dry”), but these forms can overlap. Some cats also have ocular or neurologic involvement.

How FIP Is Diagnosed

There is no single routine blood test that confirms every case of FIP. A veterinarian combines the cat’s history and physical examination with bloodwork, imaging, analysis of abdominal or chest fluid when present, and sometimes specialized testing of fluid or tissue. Because diagnosis can be complex, evaluation by a veterinarian familiar with FIP is especially valuable.

The Antiviral Breakthrough

GS-441524 is an antiviral medication that has transformed the prognosis for FIP. Published research and veterinary experience show that many appropriately treated cats can recover and achieve sustained remission. Results vary with the form and severity of disease, how quickly treatment begins, and whether the cat has neurologic or eye involvement.

In the United States, GS-441524 is not an FDA-approved animal drug. However, veterinarians can prescribe compounded oral GS-441524 under the FDA’s current enforcement-discretion framework. Treatment should always be obtained and monitored through a licensed veterinarian; unverified products may have uncertain strength, purity, or safety.

What Treatment Involves

The dose, formulation, treatment length, and monitoring plan must be tailored by the veterinarian. Many protocols last several weeks, but there is no universal treatment promise. Follow-up examinations and laboratory testing help the veterinarian judge response, adjust treatment, and monitor for relapse or adverse effects.

Never delay veterinary care while trying to obtain medication independently. Cats with breathing problems, neurologic signs, severe weakness, dehydration, or inability to eat may need urgent supportive care as well as antiviral treatment.

A Hopeful—and Responsible—Message

FIP is no longer automatically hopeless. Early recognition, accurate veterinary evaluation, reliable medication, and careful monitoring have allowed many cats to return to happy, healthy lives. If you suspect FIP, contact your veterinarian immediately and ask about current antiviral treatment options.

For additional veterinary information, see the Cornell Feline Health Center’s FIP overview, Cornell’s guide to GS-441524 availability in the United States, and the FDA statement on compounded GS-441524.

This article is for educational purposes and is not a substitute for veterinary diagnosis or treatment.

Explore more health and care guidance in our Maine Coon blog and kitten care essentials.