What is FIP (Feline Infectious Peritonitis)?

Feline Infectious Peritonitis (FIP) is a fatal disease caused by a feline coronavirus (Feline Coronavirus, FCoV). While FCoV is widespread among cats, especially in multi-cat environments, and usually causes only mild gastrointestinal symptoms (e.g., diarrhea), in rare cases, the virus can mutate within the body into a more pathogenic strain called FIPV (FIP virus), triggering a severe systemic inflammatory response—this is known as FIP.

The pathogenesis of FIP primarily involves the mutated FIPV being misidentified by the immune system as an “antigen complex,” leading to an excessive inflammatory response, particularly perivascular inflammation. This results in protein-rich effusions (ascites or pleural effusion) or organ damage.

According to a review published in the Journal of Feline Medicine and Surgery (Pedersen et al., 2009), although the incidence rate of FIP is low (about 5–10% among cats infected with FCoV), once diagnosed, the disease is nearly 100% fatal without effective treatment.

Clinical Signs of FIP

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

• Persistent fever (unresponsive to antibiotics)

• Loss of appetite, weight loss

• Lethargy, decreased activity

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

• Jaundice, anemia

• Ocular signs: uveitis, iris discoloration, vision loss

• Neurological signs: ataxia, seizures, behavioral changes

A study by Addie et al. (2015) emphasized that FIP diagnosis requires a comprehensive assessment of clinical signs, laboratory markers (e.g., low A/G ratio, elevated total protein, α1-acid glycoprotein), and imaging.

Types of FIP

FIP is classified based on clinical signs and fluid accumulation into:

• Wet FIP (effusive type): Most common form, characterized by ascites or pleural effusion—yellow, viscous, protein-rich fluids easily detected by ultrasound or sampling.

• Dry FIP (non-effusive type): No significant effusion, but granulomatous lesions in organs like liver, kidney, or lymph nodes. Often accompanied by ocular or neurological symptoms; more difficult to diagnose.

• Ocular FIP: Affects intraocular structures causing uveitis, retinitis—typically a localized manifestation of dry FIP.

• Neurological FIP: Presents with ataxia, seizures, coma, circling—one of the most challenging forms of FIP.

Many cats have multi-system involvement, and overlapping types are common in clinical cases.

Treatment Advancements (GS-441524 / NeoFipronis)

Historically considered a death sentence, FIP treatment was revolutionized in 2019 with the emergence of antiviral nucleoside analog GS-441524.

• Mechanism: GS-441524 is an adenosine analog that inhibits viral RNA polymerase, blocking FIPV replication. It is the most validated treatment option to date.

• Clinical Efficacy: Multiple studies led by Pedersen (2019, UC Davis) report a cure rate of 80–85%, especially in early or moderate cases.

Approved Drug Example:

NeoFipronis® (Pronidesivir) is one of the first officially approved oral GS-441524 tablets worldwide. It offers precise dosing and high safety for:

• Wet and dry FIP

• Neurological and ocular FIP (higher doses recommended)

Treatment Protocols:

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

• Neurological/ocular dose: 20–30 mg/kg/day

Treatment requires close monitoring of A/G ratio, fsAA, ascites, and weight change.

FIP Prevention Strategies

Although no vaccine can completely prevent FIP, the following measures significantly reduce risk:

• Control FCoV infection rates

• Good hygiene: daily litter box cleaning, regular disinfection

• Reduce stress and overcrowding (limit frequent new cat introductions)

• Zoning: isolate FCoV-positive cats from healthy ones

• Boost immunity

• Proper nutrition, routine deworming, and vaccinations

• Avoid early weaning; ensure sufficient maternal antibodies

• Screen high-risk groups regularly

• In multi-cat homes or breeding facilities, test for FCoV antibodies and fecal PCR

According to ESFM (European Society of Feline Medicine), current FIP vaccines are not recommended for routine use due to limited efficacy and narrow coverage.

FIP has entered an era of treatability, thanks to scientific advances and veterinary medicine progress. The emergence of GS-441524-based drugs like NeoFipronis has saved many feline lives. Early detection, evidence-based treatment, and professional support are key to successful FIP management.

If you suspect your cat has FIP, seek immediate veterinary attention for comprehensive diagnosis and treatment planning.