Cat FIP (Feline Infectious Peritonitis): Signs & Treatment

Feline infectious peritonitis (FIP) is caused by a mutated form of feline coronavirus and was once uniformly fatal, but antiviral drugs approved in recent years have changed that prognosis dramatically — with cure rates above 85% in cats treated early. Recognizing FIP early matters more than ever because treatment is available. Key signs include a fluid-filled, distended abdomen ("wet" FIP) or neurological and eye changes without abdominal fluid ("dry" FIP), combined with fever, weight loss, and failure to thrive.

Last reviewed: June 2026

Understanding FIP: Wet vs. Dry Forms

FIP results when feline enteric coronavirus (FECV) mutates within an individual cat to a systemic, macrophage-tropic form (FIPV). The mutation is not transmitted between cats — so FIP itself is not contagious even though the parent coronavirus is. Young cats (under 2 years) and older immunosuppressed cats are at highest risk.

FIP presents in two main clinical patterns:

Effusive (wet) FIP — the most common form, accounting for roughly 60–70% of cases:

• Progressive, painless accumulation of straw-yellow, sticky fluid in the abdomen and/or chest
• The fluid is characteristically high in protein (Rivalta test positive) and contains inflammatory cells
• Abdomen becomes visibly distended; breathing may become labored if chest fluid accumulates
• Rapid deterioration over days to weeks

Non-effusive (dry) FIP — slower and more insidious:

• Neurological signs: stumbling, head tilt, seizures, behavioral changes
• Ocular signs: uveitis, cloudiness of one or both eyes, irregular pupils, retinal hemorrhage
• Weight loss, fever that waxes and wanes, poor appetite
• Organ granulomas (liver, kidney, intestinal lymph nodes) detectable on ultrasound

A mixed form with both effusion and organ involvement also occurs. Per Ettinger's Textbook of Veterinary Internal Medicine, FIP should be suspected in any young cat with persistent fever unresponsive to antibiotics, combined with any of the above organ-specific signs.

Diagnosis and the New Treatment Landscape

FIP is historically difficult to confirm without biopsy or post-mortem examination, but clinical scoring systems and a combination of tests now allow confident presumptive diagnosis in most cases. Key diagnostics include:

• Complete blood count: elevated total protein, low albumin, lymphopenia
• A:G ratio (albumin-to-globulin): a ratio below 0.4 is highly suspicious for FIP
• Rivalta test on effusion fluid: a positive result (fluid forms a jellyfish shape when dripped into dilute acetic acid) has sensitivity above 90% for FIP
• PCR for FIPV on effusion fluid or CSF
• Abdominal ultrasound: peritoneal effusion, lymph node enlargement, hyperechoic mesenteric fat

Treatment: GS-441524, the antiviral nucleoside analog that targets feline coronavirus replication, has transformed FIP from a death sentence to a treatable disease. In the landmark study by Pedersen et al., 2019, JFMS, 25 of 26 cats with effusive FIP treated with GS-441524 achieved sustained remission. Standard treatment courses run 12 weeks minimum for wet FIP, 12–16 weeks for dry, and longer for neurological cases. Treatment costs for GS-441524 typically range from $2,000–6,000 for a full course depending on the cat's weight and the formulation used.

The AAFP-AAHA Feline Life Stage Guidelines, 2021 underscore the importance of early diagnosis and referral for FIP given the availability of effective antivirals.

When to See a Vet

Call your vet today if:

• Your cat's abdomen appears swollen or distended and they are lethargic or not eating
• You notice one or both eyes appear cloudy, red, or have irregular pupils
• A young cat (under 2 years) has persistent fever, weight loss, and malaise lasting more than a week
• Your cat is stumbling, tilting their head, or showing seizure-like episodes

Go to the ER immediately if:

• Breathing is labored or rapid — chest effusion in FIP can cause respiratory distress that is a medical emergency
• Your cat is unable to stand, is unresponsive, or is having active seizures
• The abdomen is so distended it appears painful to the touch and the cat cannot walk normally

Frequently Asked Questions

How much does FIP treatment cost? Diagnostics (bloodwork, ultrasound, effusion analysis) typically run $300–700. GS-441524 antiviral treatment for a standard 12-week wet FIP course costs approximately $2,000–4,000 for most cats. Neurological FIP requiring longer courses or higher doses may reach $4,000–8,000. Some compounded formulations are available at lower cost through licensed veterinary pharmacies.

Is FIP contagious to my other cats? FIP itself is not contagious — the macrophage-tropic mutation that causes disease does not spread. The parent feline enteric coronavirus (FECV) can spread between cats through fecal-oral contact, but most cats who acquire FECV never develop FIP. Keeping multi-cat households small (under 5 cats), maintaining separate litter boxes, and regular cleaning reduce FECV prevalence.

What is the cure rate for FIP with antivirals? In clinical studies and real-world case series, GS-441524 achieves sustained remission in 85–90% of cats who complete a full treatment course. Neurological FIP has a slightly lower but still meaningful success rate. Relapse is possible — cats should be monitored for 12 months after completing treatment.

Can older cats get FIP? Yes, though FIP is most common in cats under 2 years and in cats over 10 years (where immune senescence increases susceptibility). Middle-aged cats (2–10 years) have the lowest incidence.

What breeds are at higher risk for FIP? Purebred cats — particularly Bengals, Ragdolls, Birmans, and Abyssinians — appear to have higher susceptibility, likely due to reduced genetic diversity and some breed-specific immune factors. Cats from multi-cat environments (catteries, shelters) also carry elevated risk due to higher FECV prevalence.

Still Not Sure if Your Cat Needs a Vet?

When you're not sure if this is wait-and-see or call-tonight, Voyage AI Vet triages in under 2 minutes. Describe what you're seeing in chat, share photos of your cat's abdomen, eyes, or posture, or hop on a live video call if you want a second pair of eyes. Every answer comes with citations to the actual veterinary literature it's pulling from — so you see exactly where the guidance comes from, not just a chatbot's word.

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