Feline Infectious Peritonitis (FIP)

What is FIP?

FIP is a severe, systemic disease in cats caused by a mutant form of Feline Coronavirus (FCoV). Most cats infected with FCoV develop only a mild or asymptomatic gut infection - but in a minority of cats, the virus mutates and triggers an abnormal immune response that leads to FIP, which was historically nearly always fatal.

The Virus

Feline Coronavirus (FCoV) is common in cats, especially those living in multi-cat households or catteries (prevalence up to 90% in some groups)
The FIP-causing mutation typically happens inside the individual cat - the mutant strain is usually NOT directly transmitted between cats
The mutant virus infects macrophages and spreads via the bloodstream

Two Forms of FIP

Feature	Wet (Effusive)	Dry (Non-Effusive)
Fluid buildup	Yes (chest/abdomen)	No
Progression	Fast (weeks)	Slower (months)
Common signs	Distended abdomen, breathing difficulty	Weight loss, neurological signs, eye issues
Diagnosis	Fluid analysis + PCR	Harder to diagnose

Some cats can have a mixed form with features of both.

Clinical Signs

Fever that doesn't respond to antibiotics
Lethargy and weight loss
Swollen abdomen or fluid around the lungs
Jaundice (liver involvement)
Neurological signs (seizures, wobbling, behavioral changes) - in neurological FIP
Eye changes (uveitis, cloudiness) - in ocular FIP
Young cats (under 2 years) and elderly cats are most susceptible

Diagnosis

No single perfect test exists. Diagnosis is based on a combination of:

Clinical signs and history
Blood work - elevated protein (especially globulins), low albumin:globulin ratio, elevated bilirubin
Fluid analysis - characteristic straw-colored, protein-rich fluid with a low cell count
Rivalta test (a simple bedside test on the fluid)
RT-PCR for FCoV in fluid or tissue
Immunostaining of macrophages in tissue biopsy (the gold standard, but invasive)

Treatment - A Revolution in Recent Years

FIP was once considered a death sentence. That has changed dramatically since ~2019-2020.

GS-441524 (the main treatment)

A nucleoside analogue antiviral that blocks viral RNA replication
Induces remission in the majority of cats (>85% success rates reported)
Typical dose: 4-10 mg/kg once daily for at least 12 weeks (84 days)
Higher doses (up to 15-20 mg/kg) are used for neurological or ocular FIP
Available as oral tablets/suspension or injectable form
A 2025 systematic review in Pathogens (PMID: 40732763) confirmed GS-441524 as highly effective across both wet and dry FIP forms

Remdesivir (GS-5734)

The prodrug of GS-441524; improves cellular penetration
Often used at the start of treatment (injectable), then transitioning to oral GS-441524
Licensed versions are now legally available in many countries including the UK and Australia (since 2020)

Molnupiravir

An alternative antiviral shown to be similarly effective to GS-441524 in some studies
Useful when GS-441524 fails or is unavailable

Treatment duration

Standard cases: 12 weeks (84 days)
Neurological/ocular cases: may need up to 24 weeks or longer
Cats are monitored for 84 days after treatment ends before being declared in remission

2026 Guidelines Update

The European Advisory Board on Cat Diseases (ABCD) published updated FIP treatment guidelines in April 2026 (PMID: 42043241), and International Cat Care released a July 2025 update - reflecting growing clinical experience and a move toward fully oral treatment protocols.

Prognosis

With treatment: >85% remission rate for uncomplicated FIP
Neurological FIP: lower success rates but still treatable - requires higher doses
Without treatment: nearly always fatal within days to weeks (wet form) or months (dry form)
A small number of cats relapse and need retreatment

Key Takeaway

FIP went from one of the most feared feline diseases to a treatable and often curable condition within just a few years, thanks to the development of antiviral therapy. If you have a cat suspected of having FIP, early diagnosis and starting antivirals quickly makes a significant difference in outcomes.

Do you want more detail on any particular aspect - diagnosis, treatment protocols, or something else?

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