Effusive FIP: A Classical and Fatal Form of Feline Infectious Peritonitis

Feline Infectious Peritonitis (FIP) is a deadly, systemic inflammatory disease caused by a mutated feline coronavirus (FCoV). According to Professor Pedersen’s research, FIP is classically divided into two types: effusive (wet) and non-effusive (dry) forms【Pedersen, 2009】.

Among them, effusive FIP is the most common, accounting for approximately 60%–70% of all FIP cases. It is characterized by severe vasculitis and the accumulation of protein-rich fluid in body cavities.

1. Pathogenesis

The core mechanism of effusive FIP includes:

• After FCoV mutates into FIPV inside the cat’s body, it triggers a strong immune-mediated inflammatory response.

• Inflammation affects vascular-rich tissues such as the peritoneum and pleura, increasing vascular permeability.

• Plasma proteins and fluids leak massively, resulting in high-protein ascites or pleural effusion【Kipar & Meli, 2014】.

2. Clinical Signs

Cats with effusive FIP exhibit classic fluid-accumulation-related symptoms, including:

• Ascites (abdominal effusion): causing abdominal distension, with a positive fluid wave on palpation.

• Pleural effusion: resulting in dyspnea, labored breathing, and exercise intolerance.

• Decreased appetite and weight loss

• Persistent high fever (unresponsive to antibiotics)

• Jaundice or anemia

• Dull coat and lethargy

The effusion is typically yellow, viscous, and rich in proteins, and the Rivalta test is often positive — a key diagnostic indicator【Felten & Hartmann, 2019】.

3. Diagnostic Features

Effusive FIP can be diagnosed through the following criteria:

1. Clinical signs: notably abdominal or pleural effusion;

2. Laboratory findings:

   • Decreased A:G ratio (<0.4)

   • Elevated α1-acid glycoprotein (AGP)

   • Increased FSαA (feline serum amyloid A)

   
   

3. Fluid analysis:

   • Yellow, sticky, high-protein fluid (>35 g/L)

   • Rivalta test positive

   
   

4. Molecular testing:

   • FCoV RNA detected in the effusion via PCR (high specificity)

4. Treatment Protocol

Effusive FIP was once considered incurable. However, recent studies show that GS-441524 (e.g., NeoFipronis®) has significantly improved survival outcomes.

• Recommended dosage: 15 mg/kg/day for 84 consecutive days;

• Treatment goal: resolution of effusion, weight gain, and normalization of inflammatory markers;

• Monitoring: A:G ratio, body temperature, fsAA, and body weight should be tracked throughout treatment.

Pedersen et al. reported an 80–85% cure rate for effusive FIP using GS-441524 therapy【Pedersen, 2019】.

5. Prognosis

When diagnosed and treated early, effusive FIP has a better prognosis than dry or neurological types. Successfully treated cats often show fluid absorption, increased activity, and regained appetite within 2–4 weeks, though full treatment duration is recommended.

References

• Pedersen NC. (2009). A review of feline infectious peritonitis virus infection: 1963–2008. Journal of Feline Medicine and Surgery, 11(4):225–258.

• Kipar A, Meli ML. (2014). Feline infectious peritonitis: still an enigma? Veterinary Pathology, 51(2):505–526.

• Felten S, Hartmann K. (2019). Diagnosis of feline infectious peritonitis: a review of the current literature. Viruses, 11(11):1068.