When a playful kitten suddenly becomes lethargic, stops eating, or develops a swollen belly, few words are more alarming to a cat parent than “FIP.” The good news? What was once considered uniformly fatal is now often treatable with modern antivirals when guided by a veterinarian. This comprehensive, fact-rich guide explains what Feline Infectious Peritonitis is, how it presents, how vets diagnose it, and the current treatment options that are giving many cats a second chance.
What is FIP?
Feline Infectious Peritonitis (FIP) is an immune-mediated disease that develops in a small percentage of cats infected with feline coronavirus (FCoV). Most cats exposed to FCoV have either no signs or mild, self-limiting diarrhea; only a minority experience internal mutation of the virus that enables it to spread beyond the gut and trigger FIP. Young cats (especially under two years) and those in multi-cat settings are at the highest risk.
Forms of FIP: Wet (Effusive) vs. Dry (Non-effusive)
FIP typically appears in two overlapping forms:
- Wet (effusive) FIP: Characterized by protein-rich fluid accumulation in the abdomen and/or chest. Cats may show a distended abdomen, labored breathing, fever, and weight loss.
- Dry (non-effusive) FIP: Marked by granulomatous (inflammatory) lesions in organs such as kidneys, liver, intestines, eyes, or brain. Signs can include persistent fever, weight loss, anemia, ocular inflammation (uveitis), or neurological signs (wobbliness, seizures).
Many cats show features of both forms, and ocular or neurologic FIP can occur with either.
How FIP Spreads (and Doesn’t)
Transmission: The common feline coronavirus spreads primarily via the fecal-oral route in litter boxes and shared environments. FIP itself is rarely contagious; it’s the underlying FCoV that passes between cats. Reducing viral load and stress in multi-cat homes and shelters lowers the risk of mutation to FIP in susceptible individuals. Importantly, FIP is not a human health risk.
Key Symptoms to Watch For
Because FIP causes systemic inflammation, signs are variable. Common red flags include:
- Persistent or recurrent fever unresponsive to antibiotics
- Lethargy, inappetence, weight loss, dehydration
- Abdominal distension and/or breathing difficulty (effusion)
- Jaundice, diarrhea, or vomiting
- Eye changes (cloudiness, color change, bleeding)
- Neurologic signs (ataxia, tremors, seizures)
Early veterinary evaluation matters—especially in kittens, purebred cats, or cats from group housing.
How FIP is Diagnosed
There is no single, perfect blood test for FIP. Diagnosis relies on a multimodal approach that interprets clinical signs together with labs, imaging, and (when possible) analysis of effusion or affected tissue.
- Routine labs: Many cats show hyperglobulinemia, hypoalbuminemia, and a low albumin: globulin (A:G) ratio; mild to moderate anemia and elevated liver enzymes may be present. These changes are supportive—not definitive.
- Effusion analysis: If fluid is present, its high protein content with low to moderate cell counts is typical. A Rivalta test on effusion is quick and inexpensive; a positive result increases the likelihood of FIP, but it is not 100% specific. Reported sensitivity is high with moderate specificity.
- PCR testing: RT-PCR on effusion or tissue can detect FCoV RNA; some assays target mutations associated with FIP, but false negatives and positives occur, so results must be interpreted in context.
- Definitive diagnosis: Histopathology with immunohistochemical detection of coronavirus antigen in lesions remains the gold standard, though obtaining biopsies isn’t always feasible in fragile patients.
A veterinarian familiar with the disease will synthesize these data to reach a practical, treatment-guiding diagnosis.
Current Treatment Options
Antiviral therapy (GS-441524 and remdesivir)
Multiple studies and real-world cohorts now show high remission and survival rates for cats treated with nucleoside analog antivirals targeting viral replication:
- GS-441524: A direct antiviral widely used internationally for FIP, with strong evidence of efficacy across effusive, noneffusive, ocular, and neurologic disease when dosed appropriately and for an adequate duration.
- Remdesivir: A prodrug of GS-441524; both injectable and oral protocols have been used under veterinary supervision with positive outcomes, including for effusive FIP.
Access and legality vary by country. In the United States, an important change occurred on June 1, 2024: a compounded oral GS-441524 product became available to veterinarians via a 503A pharmacy collaboration (not an FDA-approved drug, but accessible under compounding guidance). Owners should obtain antivirals legally and through their veterinarian to ensure correct dosing, monitoring, and product quality.
Supportive care
Cats may also need fluid therapy, anti-nausea medication, appetite stimulants, pain control, and nutritional support. Corticosteroids are no longer first-line therapy but may be used selectively for inflammation while antivirals take effect—always under veterinary direction. Antibiotics do not treat FIP itself.
Prognosis
Before antivirals, FIP was typically fatal. Today, many properly treated cats achieve clinical remission and return to normal life, although neurologic or advanced ocular disease may require longer or higher-intensity protocols. Early diagnosis and prompt, vet-guided therapy improve outcomes.
Preventing FIP in Multi-Cat Homes and Shelters
You can’t eliminate risk, but you can reduce it:
- Litter box hygiene: Scoop twice daily, disinfect boxes, and provide one box per cat plus one extra. Place boxes away from food/water to limit fecal-oral transmission of FCoV.
- Reduce crowding and stress: Maintain stable social groups, ensure adequate resources (food, water, resting spots), and use careful introduction practices. Stress management reduces FCoV shedding and immune dysregulation.
- Test wisely: FCoV antibody tests show exposure, not FIP. Routine screening of healthy cats has limited value for predicting FIP risk.
- Vaccination: An intranasal FIP vaccine exists but has questionable efficacy and is not routinely recommended by major feline health groups. Focus on environmental management over vaccination for prevention.
Frequently Asked Questions
Is FIP contagious to other cats?
The underlying feline coronavirus is contagious, but FIP itself is rarely transmitted between cats. Most exposed cats never develop FIP. Good hygiene and reduced crowding are your best defenses.
Can people catch FIP?
No. FIP is a feline disease and not zoonotic.
How long is treatment?
Antiviral courses commonly last several weeks to months, with dosing tailored to disease form (effusive, dry, ocular, neurologic) and guided by veterinary monitoring of weight, bloodwork, and clinical response. Never self-medicate; work closely with your veterinarian.
What about “black-market” drugs?
Product quality, dosing accuracy, and safety can be unreliable. With legitimate compounding channels now available in some regions, partnering with your vet helps ensure safe, effective therapy and proper follow-up. Availability varies by country and current regulations.
Take-Home Message
Feline Infectious Peritonitis is no longer a hopeless diagnosis. Recognizing FIP symptoms early, pursuing a veterinary diagnosis that may include effusion analysis, Rivalta testing, and targeted PCR, and initiating antiviral treatment promptly can significantly improve outcomes for many cats. Because protocols and legal access differ by location—and because each cat’s case is unique—consult your veterinarian immediately if you suspect FIP. They can tailor a plan, monitor progress, and guide you through today’s rapidly evolving, evidence-based treatments.
If you are interested to learn more about other viral diseases, such as rabies, click here.
