FIP is one of the most complex and frequently tested feline infectious diseases on the NAVLE. For decades it was a death sentence. That changed with GS-441524 antiviral therapy. You need to understand the classic diagnostic approach AND the current treatment landscape.
Pathogenesis
Feline Enteric Coronavirus (FECV) infects 80–90% of cats in multicat households, causing mild enteritis or nothing at all. In a small percentage of cats, FECV mutates within macrophages to become FIPV. The mutation occurs in the Spike protein or ORF3c gene region. FIP is NOT spread cat-to-cat—the mutant virus is poorly infectious and FIP arises de novo within the individual cat. Young cats (<2 years) and old cats (>10 years) are overrepresented. The pathological hallmark is pyogranulomatous vasculitis—immune complex deposition in vessel walls driving systemic disease.
Clinical Forms
Wet (Effusive) FIP
- Yellow-amber viscous effusion—ascites, pleural, pericardial
- Rivalta test positive
- Total protein >3.5 g/dL
- A:G ratio <0.4 (strongly supportive)
- Rapid accumulation; young cats
Dry (Non-Effusive) FIP
- Pyogranulomatous organ lesions
- Neurological signs (meningoencephalitis)
- Ocular: uveitis, hypopyon, chorioretinitis
- Renal granulomas, hepatic lesions
- More difficult to diagnose; CSF pleocytosis
Diagnosis
Rivalta Test
Add 1 drop of effusion to water + acetic acid. Positive = the drop maintains shape (sinks slowly like a jellyfish) = high protein effusion. Sensitivity ~91% for FIP, specificity ~66%. Good screening tool but not definitive alone.
Effusion Analysis
Serum Markers
Hyperglobulinemia (elevated gamma globulins) is the hallmark. A:G ratio <0.4 on serum is highly specific. Alpha-1 acid glycoprotein (AGP) >1,500 μg/mL is strongly supportive. Lymphopenia is typical.
FCoV Serology
FCoV antibody titers are NOT diagnostic for FIP. Because FECV infects most multicat-household cats, a high titer only confirms coronavirus exposure. Many healthy cats have high titers. A very low/negative titer makes FIP less likely, but high titers are meaningless diagnostically.
Definitive Diagnosis
Immunohistochemistry (IHC) of biopsy tissue showing FCoV antigen within macrophages is the gold standard. In practice, most diagnoses are made clinically using the combination of effusion analysis, A:G ratio, and supportive labs.
Treatment: GS-441524
GS-441524 is a nucleoside analog (active metabolite of remdesivir) that inhibits viral RNA polymerase. It has transformed FIP from a death sentence to a treatable disease.
- Dose: 6–12 mg/kg SQ or PO SID (higher for neuro/ocular FIP: 12–15+ mg/kg)
- Duration: 84 days minimum
- Remission criteria: clinical recovery + normalized labs + NO recurrence for 12 weeks after completing treatment
- Remission rate: >85% for wet form; ~60–70% for neurological FIP
- Relapse: if recurrence within 12-week observation period, second longer course required