What Is FIP in Cats?
Feline Infectious Peritonitis (FIP) is a serious viral disease caused by a mutated form of the Feline Coronavirus (FCoV). For decades, a diagnosis of FIP was considered a death sentence. That changed in 2019, when antiviral treatments — specifically GS-441524 — demonstrated cure rates above 80%.
How FIP Develops
Most cats are exposed to the benign Feline Coronavirus (FCoV) at some point in their lives — especially in multi-cat households. In the majority of cats, the virus causes only mild gastrointestinal symptoms or no symptoms at all.
In approximately 5–10% of infected cats, FCoV mutates into FIPV (Feline Infectious Peritonitis Virus). This mutated virus infects white blood cells called macrophages, allowing it to spread throughout the body and trigger a severe inflammatory response.
Why some cats develop FIP while others don't is still not fully understood, but factors include: - Genetic predisposition - Immune system response - Age (young cats under 2 and older cats over 10 are more susceptible) - Stress - High viral load from multi-cat environments
The Four Types of FIP
Wet FIP (Effusive)
The most common form. Fluid accumulates in the abdomen (ascites) or chest cavity (pleural effusion). Signs include a visibly distended belly, difficulty breathing, and lethargy. Wet FIP typically progresses fastest.
Dry FIP (Non-Effusive)
Less fluid accumulation, but granulomas (inflammatory lesions) form in organs including the kidneys, liver, intestines, and lymph nodes. Signs are more subtle and harder to diagnose.
Ocular FIP
Affects the eyes — uveitis, cloudiness, color changes in the iris, or hemorrhage inside the eye. Can occur alone or alongside other forms.
Neurological FIP
The most complex form. The virus crosses the blood-brain barrier and causes neurological signs: seizures, loss of coordination, behavioral changes, paralysis. Requires higher drug doses and longer treatment.
Who Is At Risk?
- Age: Kittens and cats under 2 years account for ~70% of FIP cases
- Breed: Certain breeds appear more susceptible (Ragdoll, Bengal, Abyssinian, Birman, Himalayan)
- Environment: Catteries, shelters, multi-cat households with higher FCoV circulation
- Sex: Intact males may be slightly more at risk
The Good News
Since 2019, the antiviral drug GS-441524 has transformed FIP from a death sentence into a treatable disease. Clinical studies and real-world reports show cure rates of 80–90% when treatment is started appropriately.
"My cat Mochi was diagnosed with wet FIP at 9 months. After 12 weeks of GS-441524, she's been in remission for over a year." — Member of the SOCK FIP community
If your cat has been diagnosed with FIP, read our complete treatment guide to understand your options.