Feline Infectious Peritonitis (FIP): What Owners Must Know About This Disease: At a Glance
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FIP: Once Fatal, Now Treatable
Feline Infectious Peritonitis (FIP) was until recently a near-universally fatal diagnosis. In 2019, antiviral drugs -- GS-441524 and remdesivir -- demonstrated 85-96% survival rates in treated cats. This is one of veterinary medicine's most dramatic recent reversals. FIP is still serious, still requires prompt treatment, and still kills cats whose owners don't know options exist. Early diagnosis and treatment initiation matter enormously for outcome.
What FIP Actually Is
FIP is not caused by a single deadly virus -- it's caused by a mutation of feline coronavirus (FCoV). Feline coronavirus infects a majority of cats, especially those in multi-cat households, and usually causes mild or no diarrhea. In roughly 5-10% of infected cats, the virus mutates within the cat's own body into the FIP-causing form (FIPV). This mutated virus infects macrophages (immune cells) and spreads through the body, triggering a severe, misdirected immune response.
FIP is not contagious from cat to cat in its FIP form -- it develops within individual cats. The original feline coronavirus is contagious. Cats in environments with high FCoV exposure (shelters, catteries, multi-cat households) have higher FIP risk.
Two Forms: Wet and Dry
Wet (effusive) FIP: fluid accumulates in the abdomen (causing a "pot-bellied" appearance), chest (causing breathing difficulty), or both. The fluid is characteristic -- yellow, sticky, high-protein. Wet FIP progresses faster than dry FIP, often over weeks.
Dry (non-effusive) FIP: inflammatory granulomas form in organs -- eyes (uveitis, color change, cloudiness), brain and spinal cord (neurological signs: wobbling, seizures, behavior changes), kidneys, intestines, and liver. Dry FIP progresses more slowly and can be harder to diagnose. Some cats develop both forms.
Signs to Watch For
Wet FIP: progressive abdominal distension, difficulty breathing (if chest effusion), weight loss, lethargy, persistent fever that doesn't respond to antibiotics (103-106 degrees F), poor appetite. Dry FIP: eye cloudiness or color change, neurological signs (ataxia, head tilt, seizures), jaundice, persistent fever and illness that doesn't resolve. Young cats (under 2 years) and older cats (over 10 years) are most commonly affected, along with immunocompromised cats and intact males.
Diagnosis
No single test definitively diagnoses FIP. A positive coronavirus antibody titer confirms FCoV exposure but not FIP specifically. Diagnosis relies on a combination of: clinical signs, fluid analysis (Rivalta test, protein levels, cytology), blood work (elevated globulins, low albumin:globulin ratio, anemia, elevated bilirubin), and immunostaining of fluid or tissue samples (most definitive). PCR testing of affected tissue or fluid has high specificity. Rapid FIP RealPCR tests are now commercially available with reasonable accuracy.
Treatment: The Antiviral Revolution
GS-441524 (nucleoside analog antiviral) is the primary treatment. It is not FDA-approved but available through veterinary compounding pharmacies and legitimate overseas sources. Treatment protocol: 12 weeks minimum for wet FIP, 84 days for dry FIP, longer for neurological cases (often 12-20 weeks). Remdesivir (the COVID-19 antiviral) contains GS-441524 as a metabolite and has been used as an alternative. A licensed veterinary version (Xraphconn/GS-441524) gained European approval in 2023. Treatment costs range from $1,000-$5,000+ depending on protocol, cat size, and duration.
Response to treatment typically appears within 24-72 hours -- fever breaks, appetite returns, effusion begins resolving. Cats that respond to treatment need to complete the full course; stopping early leads to relapse. After completing treatment, cats are monitored for 12 weeks for signs of relapse. Survival rate in treated cats: 85-96% depending on form and neurological involvement.
Sources
- Pedersen et al. (2019) "Efficacy of GS-441524 against FIP" JFMS
- Dickinson et al. (2020) "Treatment of FIP with Remdesivir" Viruses
- Cornell Feline Health Center -- FIP Overview
- ISFM (International Society of Feline Medicine) -- FIP Consensus Guidelines 2023