1. Overview of FIP: From Mild Coronavirus to Fatal Inflammatory Storm

Feline Infectious Peritonitis (FIP) is a fatal disease caused by a mutation of the feline coronavirus (FCoV). While FCoV is widespread in cat populations and generally causes no symptoms, a small percentage of infected cats may experience viral mutation, transforming the virus into FIPV (FIP virus), which leads to systemic inflammation, organ damage, and death.

There is currently no effective vaccine for FIP, but antiviral treatments are now available. Among these, Pronidesivir — a GS-441524-based oral nucleoside analog — has shown significant efficacy in various global clinical settings.

2. Pathogenesis: When a Harmless Virus Turns Lethal

• Feline Coronavirus (FCoV): A common enteric virus primarily transmitted via the fecal-oral route. Highly transmissible in multi-cat environments.

• Mutation Mechanism: In certain cats, FCoV mutates into FIPV, gaining the ability to infect macrophages, which then disseminate the virus throughout the body, causing vascular inflammation and organ failure.

This mutation is not an external infection but occurs internally, especially in young cats, purebred cats, or those under prolonged stress.

Reference:

Pedersen NC. Feline infectious peritonitis. Vet Clin North Am Small Anim Pract. 2014. PMID: 25155687

3. Clinical Forms: FIP Is Not Just One Disease

FIP presents in two primary clinical forms:

Wet (Effusive) FIP

• Accumulation of yellow, viscous fluid in the abdomen or chest

• Abdominal swelling, labored breathing

• Fever, lethargy, anorexia

Dry (Non-effusive) FIP

• No visible fluid accumulation

• Neurological or ocular symptoms:

  • Uveitis, retinal detachment, visual impairment

  • Ataxia, seizures, paralysis

  
  

• Gradual weight loss, behavioral changes

Some cats may exhibit mixed forms or progress from dry to wet type.

4. Diagnostic Challenges and Common Tests

There is no single definitive test for FIP. Diagnosis is based on a combination of:

• Blood Tests: Low albumin/globulin ratio (A:G < 0.4), anemia

• Rivalta Test: On abdominal or thoracic effusion

• Effusion Protein Analysis: Often >3.5 g/dL

• RT-PCR Testing: FCoV RNA in blood or effusion

• Immunohistochemistry (IHC): Gold standard but requires tissue samples

Note:

FCoV antibody titers alone are insufficient for diagnosis.

5. Treatment Progress: From Incurable to Treatable

Core Drug: 

Pronidesivir (GS‑441524)

Pronidesivir is an oral antiviral developed for the treatment of FIP. Its active ingredient, GS‑441524, is a nucleoside analog that blocks viral replication by inhibiting RNA-dependent RNA polymerase.

As the parent nucleoside of remdesivir, GS‑441524 has excellent bioavailability and tissue penetration in cats, making it especially effective for systemic infections like FIP.

✅ Clinical Advantages:

• Oral formulation: No injections required

• Rapid absorption: Suitable for all FIP forms

• Excellent safety profile: Low incidence of side effects

• Broad-spectrum efficacy: Effective against wet, dry, ocular, and neurological FIP

Reference:

Pedersen NC et al. Efficacy of a 12-week GS-441524 regimen for treating FIP in cats. J Feline Med Surg. 2019.

6. Recommended Use of Pronidesivir

• Administer on an empty stomach (1 hour before or 2 hours after feeding)

• Extend treatment for 2 weeks after full symptom resolution

• Monitor weight, appetite, temperature, and liver/kidney function during treatment

7. Real-World Effectiveness

A multi-center study of 286 cats with confirmed FIP treated with GS-441524 showed:

• 83.2% overall cure rate

• >90% success in wet FIP

• 70–80% success in dry and neurological cases

Reference:

Krentz D, Zenger K, et al. Outcomes in cats with FIP treated with GS-441524. Vet Sci. 2021.

8. Prevention and Management

Effective FIP control focuses on minimizing FCoV spread:

• Maintain low cat density and minimize stress

• Clean litter boxes frequently

• Isolate FCoV-positive cats, avoid breeding them

• Routine FCoV screening in shelters or catteries

⚠️ Important:

The intranasal FIP vaccine (FIPV-IN) has shown limited effectiveness and is not recommended for routine use.

9. Conclusion: Science Meets Hope

FIP was once considered a death sentence, but with the development of GS‑441524-based treatments like Pronidesivir, it is now a manageable and often curable condition.

Miaite is proud to be at the forefront of FIP research and treatment, collaborating with veterinarians, researchers, and cat owners worldwide to bring hope and healing to cats suffering from this devastating disease.