What Is Feline Infectious Peritonitis (FIP)?

Feline Infectious Peritonitis (FIP) is a fatal systemic inflammatory disease caused by a mutation of feline coronavirus (FCoV). FCoV is widespread among cats, especially in multi-cat households, shelters, and catteries. While most FCoV infections only cause mild gastrointestinal symptoms such as diarrhea, approximately 5–10% of infected cats may experience a viral mutation that transforms FCoV into a more pathogenic form known as FIP virus (FIPV). This mutated strain can trigger a severe immune-mediated response, resulting in systemic vasculitis and organ damage.

The core mechanism of FIP involves the formation of immune complexes that deposit around blood vessels, leading to pyogranulomatous inflammation and protein-rich fluid accumulation (effusion) in the abdomen or chest, or the development of granulomatous lesions in organs.

Reference: Pedersen NC. A review of feline infectious peritonitis virus infection: 1963–2008. J Feline Med Surg. 2009;11(4):225–258.

Clinical Signs of FIP

The clinical signs of FIP vary depending on the organs affected. Common symptoms include:

• Persistent fever unresponsive to antibiotics

• Loss of appetite and weight loss

• Lethargy and reduced activity

• Abdominal distension (ascites) or labored breathing (pleural effusion)

• Jaundice, anemia

• Ocular symptoms: uveitis, iris color changes, impaired vision

• Neurological symptoms: ataxia, seizures, behavioral changes

Diagnosis is based on a combination of clinical signs, lab results (low albumin/globulin ratio, elevated total protein, α1-acid glycoprotein), and imaging findings.

Reference: Addie DD et al. Journal of Feline Medicine and Surgery, 2015.

Types of FIP

FIP is classified into the following forms depending on fluid accumulation and affected systems:

1. Wet (Effusive) FIP

The most common form, characterized by the accumulation of yellow, viscous, protein-rich fluid in the abdomen or chest. This type is often easier to detect via ultrasound or fluid analysis.

2. Dry (Non-effusive) FIP

No obvious fluid accumulation. It presents with granulomatous lesions in organs such as the liver, kidneys, or lymph nodes. Often involves the eyes or central nervous system. Diagnosis is more challenging.

3. Neurological FIP

Affects the brain and spinal cord, leading to ataxia, seizures, circling, or coma. It is one of the most difficult forms to treat.

4. Ocular FIP

Targets the eyes, leading to uveitis, retinal changes, and visual impairment. It is often a localized manifestation of dry FIP.

Cats may show overlapping signs from multiple types simultaneously.

Treatment Advances (GS-441524 / NeoFipronis®)

Historically, FIP was considered a death sentence. However, since 2019, the emergence of antiviral nucleoside analogs such as GS-441524 has revolutionized FIP treatment.

Mechanism of Action

GS-441524 is an adenosine nucleoside analog that inhibits viral RNA-dependent RNA polymerase, thereby halting FIPV replication at the molecular level.

Clinical Evidence

Research conducted by Pedersen et al. at the University of California, Davis (2019), demonstrated 80–85% cure rates, particularly in early or moderate cases.

Commercial Treatment Example: NeoFipronis®

NeoFipronis® (generic name: Pronidesivir) is among the world’s first officially approved oral GS-441524 formulations, offering:

• Accurate dosing and high safety profile

• Effective for both wet and dry FIP

• Applicable for neurological and ocular FIP at higher dosages

Recommended dosage:

• Standard: 15 mg/kg/day for 84 consecutive days

• Neurological/Ocular: 20–30 mg/kg/day

Monitoring of A/G ratio, fsAA, effusion volume, and body weight is essential during treatment.

Preventive Measures

Although there is no fully protective vaccine for FIP, the following steps can significantly reduce the risk:

Reduce FCoV Spread

• Daily cleaning of litter boxes

• Regular disinfection of shared areas

• Limit crowding and reduce new-cat introductions

Strengthen Immune Defense

• Provide balanced nutrition

• Maintain routine deworming and vaccinations

• Avoid early weaning to ensure maternal antibody transfer

Screen High-Risk Populations

• Perform periodic PCR or antibody testing in multi-cat households or catteries

• Isolate infected from uninfected cats

Note: According to the European Society of Feline Medicine (ESFM), the current FIP vaccine is not widely recommended due to limited efficacy and narrow applicability.

Conclusion

FIP has shifted from an incurable fatal disease to a treatable condition thanks to scientific breakthroughs in antiviral therapy. Medications like NeoFipronis® offer hope and recovery for cats worldwide.

Early detection, proper diagnosis, and science-based treatment are the keys to saving lives.

If you suspect your cat may have FIP, consult a qualified veterinarian immediately for thorough evaluation and treatment planning.