Feline Infectious Peritonitis, commonly known as FIP, is a serious and complex viral disease that affects cats worldwide. For decades it was considered uniformly fatal, but recent advances in antiviral treatment have transformed outcomes dramatically. Understanding FIP — what causes it, how it presents, and how it is now treated — is essential knowledge for any cat owner.
FIP is caused by a mutation of Feline Coronavirus (FCoV), a highly common virus that most cats encounter during their lifetime. The disease can affect cats of any age, but young cats under two years and older cats over ten are at highest risk. Multi-cat households and catteries see higher rates due to widespread coronavirus exposure.
How FIP Develops
Feline Coronavirus itself is widespread and usually harmless, causing mild gastrointestinal symptoms or no symptoms at all. Most cats that become infected clear the virus naturally. The problem arises in a small percentage of infected cats — estimates range from 5% to 12% — in which the virus mutates within the cat’s own cells.
This mutated virus, called FIPV (Feline Infectious Peritonitis Virus), infects and replicates within macrophages, a type of immune cell. Instead of being destroyed by the immune system, the virus exploits these cells to spread throughout the body, triggering an extreme inflammatory response that attacks multiple organ systems.
Why the mutation occurs in some cats and not others is not fully understood. Genetic susceptibility, immune system response, viral load at the time of infection, and stress all appear to play roles. The disease is not directly contagious from cat to cat — FIP itself cannot be transmitted, though the underlying coronavirus can be shed and spread.
Types and Symptoms of FIP
FIP presents in two main forms, though many cats show features of both:
Wet (Effusive) FIP: The most recognizable form. The inflammatory process causes fluid to accumulate in body cavities — the abdomen (causing a characteristic pot-bellied appearance), chest (causing difficulty breathing), or around the heart. This form progresses more rapidly, sometimes over days to weeks.
Dry (Non-Effusive) FIP: Characterized by inflammatory lesions (granulomas) forming in organs without significant fluid accumulation. Symptoms depend on which organs are affected and may include neurological signs (seizures, ataxia, personality changes), eye problems (uveitis, cloudy eyes), kidney or liver disease, or intestinal masses. This form tends to progress more slowly but can be harder to diagnose.
General symptoms common to both forms include persistent fever unresponsive to antibiotics, lethargy, loss of appetite, weight loss, and jaundice in some cases. The disease can closely mimic other conditions, making diagnosis challenging.
Diagnosing FIP
FIP is notoriously difficult to diagnose definitively without advanced testing. No single test confirms FIP with certainty. Veterinarians use a combination of clinical signs, bloodwork, fluid analysis, imaging, and specific tests to build a diagnosis.
Blood tests often reveal low lymphocyte counts, elevated total protein, low albumin-to-globulin ratio, and elevated liver enzymes. The FCoV antibody titer measures exposure to coronavirus, but a positive result only confirms exposure, not FIP. High titers combined with clinical signs increase suspicion.
The most reliable diagnostic tests include PCR testing of fluid or tissue samples to detect FIPV RNA, and immunohistochemistry of biopsy tissue to identify the virus within macrophages. The AGP (alpha-1-acid glycoprotein) test can support a diagnosis when elevated alongside other findings. Rivalta’s test on effusion fluid is a useful screening tool.
Treatment: The Antiviral Revolution
Until recently, FIP carried a near-100% fatality rate. The development of nucleoside analog antiviral drugs — particularly GS-441524 (the active metabolite of remdesivir) and molnupiravir — has revolutionized treatment outcomes. Clinical trials and real-world reports show remission rates exceeding 85% in cats treated with these antivirals.
Treatment typically involves daily injections or oral administration of the antiviral drug for a minimum of 84 days. Cats are monitored closely throughout treatment with repeat bloodwork and physical examinations. After completing the treatment course, cats enter an observation period of at least 84 additional days. If clinical signs do not return during this time, the cat is considered to be in sustained remission — effectively cured.
Access to these drugs varies by country. In some regions they are available through veterinary prescription; in others, cat owners have obtained them through compounding pharmacies or international sources. Treatment is expensive, often running several thousand dollars, but outcomes for treated cats are increasingly positive. Working with a veterinarian experienced in FIP management is strongly recommended.
Prognosis and Prevention
With antiviral treatment, the prognosis for FIP has changed dramatically from universally fatal to largely treatable. Cats that complete treatment and achieve sustained remission can go on to live normal, healthy lives. Young cats generally respond particularly well.
Prevention focuses on minimizing coronavirus exposure and stress. In multi-cat environments, keeping group sizes manageable, maintaining strict litter box hygiene (coronavirus spreads via feces), and quarantining newly arrived cats for at least two weeks reduces transmission risk. A vaccine against FIP exists in some countries (Primucell FIP), though its efficacy is debated and it is not widely used.
If you have a cat diagnosed with FIP, seek out a veterinarian knowledgeable about antiviral treatment options as soon as possible. The earlier treatment begins, the better the outcome tends to be.