Feline Infectious Peritonitis (FIP) was once considered a universally fatal disease in cats. Caused by a mutated feline coronavirus, FIP previously carried a 100% mortality rate within weeks to months of diagnosis. However, recent breakthroughs in antiviral therapy have transformed FIP from a death sentence into a treatable and in many cases curable disease. This guide explains what FIP is, how to recognize it, and what treatment options are now available.
What is FIP in Cats?
FIP is caused by a mutation of the feline enteric coronavirus (FCoV). FCoV itself is extremely common — an estimated 25-40% of cats carry it, often without symptoms. In most cats, FCoV causes mild gastrointestinal symptoms or none at all. However, in a small percentage of infected cats (roughly 5-10%), the virus mutates within the body into a form that attacks white blood cells and causes a systemic inflammatory disease — FIP. The mutated FIP virus cannot be spread directly between cats. Each case arises independently from the common FCoV already present in an individual cat.
Types of FIP: Wet, Dry, and Neurological
Wet (Effusive) FIP causes fluid to accumulate in the abdomen (causing a distended belly) or chest (causing breathing difficulty). This is the most common form and progresses most rapidly. Dry (Non-Effusive) FIP causes granulomatous lesions to form in organs including the kidneys, liver, eyes, or brain — it progresses more slowly. Neurological/Ocular FIP is a subtype of dry FIP causing neurological signs (wobbliness, seizures, behavioral changes) or eye inflammation. Mixed FIP involves elements of both wet and dry forms.
Symptoms of FIP in Cats
Early symptoms of FIP are often nonspecific: persistent fever unresponsive to antibiotics, weight loss and poor appetite, lethargy and depression, and jaundice in some cases. Wet FIP specific signs include a visibly distended abdomen and labored breathing if chest fluid is present. Dry/Neurological FIP causes wobbliness, ataxia, hind limb weakness, blindness, eye cloudiness, seizures, and behavioral changes. FIP can resemble many other diseases — definitive diagnosis requires specific testing and is best done by a specialist.
FIP Treatment: The GS-441524 Revolution
The antiviral compound GS-441524 has transformed FIP treatment. Clinical trials and real-world use have demonstrated remission rates of 85-90%+ in cats treated with GS-441524. Standard protocol involves 84 days of daily injections or oral medication at doses adjusted for the cat’s weight and FIP type. Neurological and ocular FIP require higher doses and longer treatment. After 84 days, most cats enter a 12-week observation period before being declared in sustained remission. Most cats in remission remain FIP-free permanently. GS-441524 is now available as a licensed veterinary product in many countries.
FIP Prognosis and Cost
Without treatment, FIP is fatal — usually within days to weeks for wet FIP and weeks to months for dry FIP. With GS-441524 treatment, approximately 85-90% of cats achieve sustained remission. Treatment cost typically ranges from $1,000 to $5,000+ for a full 84-day course depending on the cat’s weight, disease type, country, and whether licensed or compounded products are used. Online FIP support communities such as FIP Warriors can help owners navigate diagnosis and treatment options.