Feline Infectious Peritonitis (FIP) in Cats — A Complete Guide
Feline Infectious Peritonitis (FIP) in Cats — A Complete Guide
Feline infectious peritonitis (FIP) was once considered an automatic death sentence for cats. Today, with the right antiviral treatment, the majority of cats diagnosed with FIP can achieve full remission and go on to live normal, healthy lives.
This guide explains what FIP is, how it develops, what symptoms to look for, and what treatment options are available.
What is Feline Infectious Peritonitis (FIP)?
FIP is a progressive, systemic viral disease caused by a mutated strain of the feline coronavirus (FCoV). Feline coronavirus itself is extremely common — it infects a large proportion of the domestic cat population, typically causing only mild gastrointestinal symptoms or no symptoms at all. In a small percentage of infected cats (approximately 5–10%), the virus mutates into a virulent form known as feline infectious peritonitis virus (FIPV), which triggers a severe, body-wide inflammatory response.
FIP is not the same coronavirus that causes COVID-19 in humans. Feline coronavirus is an alphacoronavirus specific to cats and poses no risk to humans.
FIP is more common in multi-cat households, shelters, and catteries where feline coronavirus circulates freely between animals. Young cats under two years old are most frequently affected, though FIP can develop at any age.
Types of FIP in Cats
FIP presents in four main forms. Some cats present with a single form; others develop multiple forms simultaneously or progressively.
Dry FIP (Non-Effusive)
Dry FIP causes inflammatory lesions around blood vessels in the organs — most commonly the kidneys, liver, lungs, and brain — without significant fluid accumulation. It is harder to diagnose than wet FIP because the signs are less visually obvious.
Common signs: weight loss, fever, loss of appetite, vomiting, excessive thirst and urination, seizures, uncoordinated movement.
Wet FIP (Effusive)
The most visually recognisable form. Inflammatory reactions damage blood vessel walls, causing fluid to leak into the chest and abdominal cavities. Cats with wet FIP often develop a visibly distended abdomen — sometimes described as a "balloon belly" — and may experience laboured breathing if fluid accumulates in the chest.
Common signs: abdominal distension, difficulty breathing, lethargy, fever, weight loss.
Neurological FIP
When the feline coronavirus invades the central nervous system, neurological FIP develops. This can occur as a progression of wet or dry FIP, or as a primary presentation. Neurological FIP requires higher antiviral dosing and closer monitoring.
Common signs: seizures, muscle weakness, loss of balance, unsteady gait, behavioural changes, paralysis.
Ocular FIP
Ocular FIP occurs when the virus affects the eyes, causing inflammation and progressive vision impairment. It frequently presents alongside neurological involvement.
Common signs: eye inflammation (uveitis), cloudiness or colour change in the iris, sensitivity to light, visible bleeding within the eye, vision loss.
Ocular and neurological FIP require prompt treatment. With the right antiviral protocol, remission is achievable in both forms — early intervention significantly improves outcomes.
What Are the Symptoms of FIP?
FIP symptoms vary depending on the form and stage of the disease. Early signs are often non-specific and easily mistaken for other conditions, which is why diagnosis can be challenging.
Early warning signs common across all forms:
- Persistent or fluctuating fever that does not respond to antibiotics
- Unexplained weight loss
- Loss of appetite
- Lethargy and reduced activity
- Mild diarrhoea
If your cat is showing any combination of these signs — particularly a fever that does not resolve — consult your veterinarian promptly and ask about FIP testing.
How is FIP Diagnosed?
FIP cannot be diagnosed from a single test. Diagnosis is based on a combination of clinical signs, bloodwork patterns, and where necessary, additional confirmatory testing. Your veterinarian will assess:
Blood and biochemistry markers associated with FIP:
- Low albumin and high globulin levels (low A/G ratio — below 0.4 is strongly suggestive)
- Lymphopenia (reduced white blood cell count)
- Elevated bilirubin and liver enzymes
- Elevated total protein
Additional diagnostic tools:
- Serum protein electrophoresis — assesses globulin fractions
- AGP (alpha-1-acid glycoprotein) — an acute-phase protein elevated in FIP
- FCoV antibody titre — elevated titres support but do not confirm FIP
- Fluid analysis — if effusion is present, the protein content and appearance of the fluid are highly informative
- PCR testing of fluid or tissue samples
- Ultrasound and X-ray — to identify fluid accumulation, organ changes, or enlarged lymph nodes
- Biopsy — the most definitive confirmatory method where other tests are inconclusive
A diagnosis of FIP should be made by a veterinarian. If you have received a suspected or confirmed FIP diagnosis and need guidance on next steps, our treatment team is available via live chat.
Treatment Options for FIP
FIP is treatable. This is a relatively recent development that has transformed outcomes for cats diagnosed with the disease.
GS-441524 — The Primary Treatment
GS-441524 is a nucleoside analogue antiviral that works by inhibiting the RNA-dependent RNA polymerase of the feline coronavirus, blocking viral replication at the molecular level. It is available in both injectable and oral capsule form.
Clinical studies and real-world outcomes consistently report remission rates of 80–95% across all forms of FIP, including wet, dry, neurological, and ocular presentations. Since 2019, CureFIP™ has helped save over 87,000 cats globally using GS-441524-based protocols.
Treatment duration is typically 84 days (12 weeks), with dosing adjusted by body weight and FIP form.
EIDD-1931 — An Effective Alternative
EIDD-1931 (the active metabolite of molnupiravir) is an oral antiviral with demonstrated efficacy against FIP. It is an effective alternative where GS-441524 is unavailable or cost-prohibitive, and has shown strong results as a rescue treatment for cats that have relapsed on GS-441524.
Supportive Treatments
In addition to antiviral therapy, some cats benefit from supportive care during treatment — particularly in the early weeks when the disease burden is highest. This may include anti-nausea medication, appetite stimulants, fluid therapy, or corticosteroids in specific presentations such as immune-mediated complications or severe neurological signs.
Can FIP Be Prevented?
There is a commercially available FIP vaccine, though its adoption within the veterinary community remains limited and it is not universally recommended. Discuss vaccination with your veterinarian as part of your cat's routine care plan.
Practical steps to reduce FIP risk in multi-cat environments:
- Maintain strict litter box hygiene — clean daily and use one box per cat plus one additional
- Avoid overcrowding cats in shared spaces
- Quarantine new cats before introducing them to an existing household
- Minimise stress — chronic stress is a known factor in FCoV mutation risk
- Schedule regular veterinary checkups, particularly for cats in high-exposure environments
Frequently Asked Questions about FIP in Cats
1. What is Feline Infectious Peritonitis (FIP)?
FIP is a fatal viral disease in cats caused by a mutated form of the feline coronavirus. Without treatment, it is almost always fatal. With antiviral treatment using GS-441524, remission rates of 80–95% are achievable.
2. What are the first symptoms of FIP in cats?
The earliest signs are often non-specific — persistent fever, weight loss, reduced appetite, and lethargy. Wet FIP may also present with abdominal swelling. If your cat shows these signs, consult your veterinarian and ask about FIP testing.
3. What is GS-441524?
GS-441524 is an antiviral drug that inhibits feline coronavirus replication. It is the most effective treatment currently available for FIP, with clinical remission rates of 80–95% across all FIP forms.
4. Is FIP contagious between cats?
FIP itself is not directly contagious. The underlying feline coronavirus (FCoV) is contagious and spreads between cats via faecal-oral transmission. However, FIP develops only when FCoV mutates within an individual cat — this mutation cannot be transmitted to other cats.
5. How long does the treatment for FIP last?
The standard treatment course is 84 days (12 weeks). Some cats with neurological or ocular involvement may require an extended course. Treatment duration is guided by clinical response and bloodwork at regular checkpoints.
6. Where can I get help treating FIP?
Our treatment specialists are available via live chat seven days a week. We can review your cat's blood results, recommend the appropriate treatment protocol, and support you through every stage of the 84-day course. [Contact our team]
CureFIP™ — Provider of clinically proven antiviral treatments for Feline Infectious Peritonitis. Trusted by cat owners and veterinarians across 50+ countries since 2019.
