Feline Infectious Peritonitis (FIP) was once considered a death sentence for cats, with mortality rates approaching 100% after diagnosis. However, the emergence of antiviral drugs—especially GS‑441524, commercially known as Pronidesivir—has ushered in a new era where FIP is no longer untreatable. This article presents a comprehensive review of FIP’s origin, clinical manifestations, diagnostic challenges, and leading-edge treatment protocols, supported by clinical research from around the world.

I. What Is FIP: From Benign Virus to Lethal Mutation

FIP originates from a mutation of feline coronavirus (FCoV), a virus prevalent in multi-cat environments. While most cats infected with FCoV remain asymptomatic or only develop mild gastrointestinal signs, an estimated 5–10% may experience viral mutation into the more pathogenic feline infectious peritonitis virus (FIPV). This mutated strain can invade macrophages and disseminate throughout the body, triggering systemic inflammation and multi-organ failure.

Notably, this transformation is not due to external infection, but rather a spontaneous mutation within the infected cat, often affecting kittens, purebred cats, or cats under chronic stress or immunosuppression.

Reference: Pedersen NC. (2014). Feline infectious peritonitis. Vet Clin North Am Small Anim Pract.

II. Clinical Types: Wet vs. Dry FIP

FIP is classically categorized into two clinical forms:

• Wet (effusive) FIP: Characterized by accumulation of high-protein yellow effusion in body cavities—typically the abdomen or thorax—causing distended abdomen, respiratory distress, and persistent fever.

• Dry (non-effusive) FIP: Presents with granulomatous lesions in various organs. Symptoms include chronic fever, weight loss, ocular signs (e.g., uveitis), and neurological abnormalities (e.g., seizures, ataxia, paralysis).

Some cats may show mixed forms or transition from one type to another during the course of disease.

III. Diagnostic Challenges and Methods

There is still no single “gold-standard” test for FIP. Diagnosis requires a combination of clinical signs, laboratory data, and molecular or histological confirmation:

• Blood chemistry: A/G ratio < 0.4, mild to moderate non-regenerative anemia

• Effusion testing: Positive Rivalta’s test, high total protein and specific gravity

• qRT-PCR: Quantitative RNA detection of FCoV in effusion, blood, or tissues

• Immunohistochemistry (IHC): Currently the most definitive, requires tissue biopsies

Important note: FCoV antibody titers alone cannot confirm FIP, as healthy FCoV carriers may also test positive.

IV. Therapeutic Breakthrough: GS‑441524 (Pronidesivir)

GS‑441524 is a nucleoside analog that inhibits viral RNA-dependent RNA polymerase (RdRp), effectively halting viral replication. It is the parent nucleoside of remdesivir and demonstrates superior bioavailability and tissue penetration in feline species.

Pronidesivir, the oral formulation of GS‑441524, is now widely used as a first-line treatment for FIP across multiple countries, supported by robust clinical data.

V. Evidence-Based Efficacy of GS‑441524

Multiple peer-reviewed studies have confirmed the remarkable efficacy of GS‑441524:

• 286-cat multicenter study: Overall cure rate of 83.2%, with >90% remission in wet FIP and 70–80% in dry or neurological FIP.

• 99 relapsed cases: Treated with GS‑441524 + Molnupiravir combination; 99.9% achieved remission, with survival >1,043 days.

• Single case neurological FIP: Rapid response to injectable GS‑441524, maintained with oral Pronidesivir over 12 weeks with full recovery.

• Australian retrospective study (650 cats): High safety profile and long-term tolerability with oral GS‑441524.

• Short-course trial (42 days): Mild FIP cats showed full remission without recurrence.

References: Pedersen NC et al., 2019; Krentz D et al., 2021; Front Vet Sci 2024; MDPI Pathogens 2023.

VI. Pronidesivir Treatment Guidelines

Administration Tips:

• Best given on an empty stomach (1 hour before or 2 hours after meals)

• Monitor body weight, temperature, appetite, and liver/kidney function

• Continue for 2 additional weeks after full symptom resolution

VII. Safety Profile and Side Effects

Pronidesivir is a tablet formulation that eliminates injection pain and stress. Clinical data shows a high margin of safety with minimal side effects—rare cases of mild diarrhea or temporary appetite loss do not typically interfere with treatment outcomes.

VIII. Legal and Regulatory Status

• Approved for veterinary use in the UK, Australia, the Netherlands, and several Southeast Asian countries

• In the USA, use remains under compassionate or special-access frameworks

• In China, platforms like Miaite are pioneering compliance channels for regulated availability

IX. Adjunct Therapies and Second-Line Options

• Molnupiravir: Often used in cases of relapse or GS-resistant FIP

• GC376: A protease inhibitor with limited efficacy in trials

• Interferons / Corticosteroids: Immune modulators, but not curative

• Nutritional Support: Milk thistle, taurine, B-complex vitamins for liver and systemic recovery

X. Prevention: Avoiding the Next Tragedy

There is no effective vaccine for FIP. Therefore, prevention relies on management:

• Reduce crowding and chronic stress in multi-cat households

• Sanitize litter boxes regularly to curb fecal-oral transmission

• Avoid breeding FCoV-positive cats

• Screen new cats and isolate suspected carriers

Conclusion: Science Offers Hope, and Pronidesivir Offers a Cure

FIP is no longer a hopeless diagnosis. With GS‑441524-based treatments like Pronidesivir, FIP has evolved from an untreatable disease into a manageable—and often curable—condition.

At Miaite, we work alongside veterinarians, researchers, and cat owners globally to push the boundaries of science and bring the gift of health and hope to every cat affected by FIP.