Feline infectious peritonitis (FIP) is caused by a mutated form of the feline coronavirus and was once considered universally fatal. A new antiviral drug, GS-441524, has transformed FIP from a death sentence to a treatable disease — with remission rates exceeding 80% in treated cats. Early recognition of signs is critical because treatment works best when started promptly.
Last reviewed: June 2026
What Is FIP in Cats?
Feline infectious peritonitis is caused when the ubiquitous feline enteric coronavirus (FCoV) mutates within an individual cat into a form capable of infecting macrophages (a type of white blood cell) and causing a systemic, often fatal vasculitis and inflammatory disease. FCoV itself is harmless — most cats shed it without any illness. The dangerous mutation occurs sporadically and unpredictably; it cannot be transmitted to other cats in its FIP form.
FIP primarily affects young cats (under 2 years) and older immunocompromised cats, though any age can be affected. Purebred cats and cats in multi-cat households are at higher risk due to greater exposure to FCoV. As described in Greene's Infectious Diseases of the Dog and Cat, the mutation that transforms FCoV into FIP virus involves genes encoding for the viral spike protein and involves loss of the cat's ability to contain infection within the gut.
Types of FIP
FIP occurs in two classic forms, though "non-effusive" and "effusive" often overlap:
Effusive (wet) FIP accounts for approximately 60–70% of cases. Fluid accumulates in the abdominal cavity (ascites), the chest cavity (pleural effusion), or around the heart (pericardial effusion). Affected cats have a progressively distended, fluid-filled abdomen or show difficulty breathing if fluid is in the chest. The fluid is characteristically yellow, viscous, and high in protein.
Non-effusive (dry) FIP involves granulomatous inflammation of organs without significant fluid accumulation. Signs depend on which organ is affected: eyes (uveitis, cloudiness, changing iris color), brain and spinal cord (ataxia, seizures, personality changes), kidneys, or liver.
Signs of FIP in Cats
- Persistent fever that doesn't respond to antibiotics
- Progressive weight loss and muscle wasting
- Lethargy and reduced activity
- Decreased appetite or complete food refusal
- Distended, fluid-filled abdomen (wet form)
- Difficulty breathing, labored breathing (chest fluid)
- Yellow tinge to eyes, skin, or gums (jaundice — liver involvement)
- Cloudy eyes, uveitis, or change in iris appearance
- Neurological signs: ataxia, head tilt, seizures, behavior changes
According to the AAFP-AAHA Feline Life Stage Guidelines, 2021, persistent fever and weight loss in a young cat warrant prompt evaluation including testing for FIP, particularly in cats from multi-cat environments or catteries.
Diagnosis
FIP diagnosis has historically been challenging because no single test is definitive. A combination of findings is used:
- Fluid analysis: yellow, viscous, high-protein (>35 g/L) fluid with a specific ratio of albumin to globulin supports FIP
- Feline coronavirus antibody titer: high titers support FCoV exposure but are not specific for FIP
- Alpha-1 acid glycoprotein (AGP): elevated in most FIP cats; available as a point-of-care test
- Immunostaining of macrophages in fluid: specific for FIP when positive
- PCR for FIP mutant virus on fluid, tissue, or CSF: increasingly available; highly specific
- Reverse transcriptase PCR (RT-PCR): in CSF for neurological FIP
Treatment: The GS-441524 Revolution
GS-441524, a nucleoside analogue antiviral, and its prodrug remdesivir-related compounds have transformed FIP treatment. Clinical remission is achieved in over 80% of cats with effusive FIP and 70–80% with neurological FIP when treatment is completed appropriately. Treatment protocol is typically 12 weeks of daily GS-441524 injections or oral tablets, followed by monitoring for 12 weeks after stopping.
Treatment is not inexpensive, but the costs are falling as licensed veterinary products become available in more countries. In the US, GS-441524 is available via veterinary compounding pharmacies and recently through licensed veterinary products.
What Does FIP Treatment Cost?
Diagnostics including bloodwork, fluid analysis, PCR testing, and imaging typically run $400–1,200. GS-441524 treatment for 12 weeks costs approximately $1,500–4,000 depending on the cat's weight and the source of medication. Licensed products tend to be at the higher end; compounded versions vary. Total treatment including diagnostics and monitoring often ranges $2,500–6,000. This was previously a disease with no treatment cost because the outcome was euthanasia — the investment now leads to survival in most cats.
When to See a Vet
Call your vet today if:
- Your cat has had a fever for more than 3–4 days that isn't responding to antibiotics
- Your cat's abdomen appears distended or pot-bellied
- Your cat is losing weight rapidly despite eating
- You notice eye changes — cloudiness, color change in the iris
Go to the ER immediately if:
- Your cat is having difficulty breathing or is breathing with an open mouth
- Your cat is having seizures or sudden neurological signs
- Your cat is jaundiced (yellow gums or eyes) and not eating
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Frequently Asked Questions
Is FIP contagious to other cats? FIP itself is not contagious — the mutated virus that causes FIP arises spontaneously within an individual cat and does not spread in the FIP form. The parent virus (feline enteric coronavirus, FCoV) is highly contagious between cats, but most cats exposed to FCoV never develop FIP. Cats from the same household as a FIP cat are not at directly elevated risk of FIP from that cat.
Can cats survive FIP? Yes — with GS-441524 antiviral treatment, over 80% of cats with effusive FIP achieve clinical remission. Neurological FIP has a slightly lower success rate (70–80%) but many cats recover fully. Treatment must be completed for the full 12-week course. Monitoring after treatment completion is essential to detect relapse.
What is the life expectancy of a cat with untreated FIP? Without antiviral treatment, wet FIP is typically fatal within days to weeks after the onset of clinical signs. Dry FIP progresses more slowly but is also uniformly fatal without treatment. This is why early recognition and rapid initiation of antiviral therapy are critical — the earlier treatment starts, the better the outcome.
How much does treating FIP in cats cost? Diagnostics run $400–1,200. GS-441524 treatment for 12 weeks costs $1,500–4,000 depending on the cat's weight and medication source. Total treatment including monitoring often runs $2,500–6,000. Costs have been falling as more licensed veterinary products become available.
How do I know if my cat has FIP vs. other causes of ascites? Fluid characteristics are the key: FIP fluid is yellow, viscous, foamy when shaken, and high in protein. Other causes of ascites (heart disease, liver disease, low albumin) produce different fluid types. Your vet will analyze the fluid and assess the albumin-to-globulin ratio and AGP level. PCR testing on the fluid can confirm FIP specifically.
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