Feline Infectious Peritonitis (FIP) in Cats — A Complete Guide

Feline Infectious Peritonitis (FIP) was once considered a 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 — thanks to a 92% success rate now achievable with GS-441524-based treatment protocols.

This guide explains what Feline Infectious Peritonitis (FIP) is, how it develops, what symptoms to look for across all four FIP types, how it is diagnosed, and what treatment options are available — including the latest dual-compound approach using GS-441524 and EIDD-1931.

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 that — without treatment — is almost always fatal.

Important: 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. Young cats under two years old are most frequently affected, though FIP can develop at any age — including in senior cats.

Is FIP Contagious?

FIP itself is not directly contagious between cats. The underlying feline coronavirus (FCoV) is contagious and spreads via the faecal-oral route — primarily through shared litter boxes. However, FIP develops only when FCoV mutates within an individual cat. This mutation cannot be transmitted to another cat.

A cat living with an FIP-diagnosed cat is not at significant risk of developing FIP — though they may carry and shed feline coronavirus. Maintaining strict litter box hygiene remains important in multi-cat households.

Types of Feline Infectious Peritonitis (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 — called granulomas — around blood vessels in the organs, most commonly the kidneys, liver, lungs, and brain, without significant fluid buildup. It is harder to diagnose than wet FIP because the signs are more subtle and develop more gradually.

Dry FIP symptoms include:

  • Unexplained weight loss — often the earliest and most consistent sign
  • Persistent fever — temperature that does not resolve with standard treatment
  • Loss of appetite and lethargy — reduced food intake, low energy
  • Vomiting and diarrhoea — gastrointestinal involvement
  • Excessive thirst and urination — if kidneys are affected
  • Jaundice — yellowing of the skin, eyes, or gums if the liver is affected
  • Neurological signs — seizures, uncoordinated movement if the brain is involved

Wet FIP (Effusive)

Wet FIP is the most visually recognisable form. Inflammatory reactions damage blood vessel walls, causing protein-rich fluid to leak into the abdominal cavity and, in some cases, the chest cavity. The hallmark sign is a visibly distended, fluid-filled abdomen — sometimes described as a "balloon belly."

Wet FIP typically progresses faster than dry FIP. Early treatment is critical.

Wet FIP symptoms include:

  • Abdominal swelling / bloated belly — progressive fluid accumulation in the abdomen
  • Laboured or rapid breathing — if fluid accumulates in the chest cavity
  • Lethargy — marked reduction in activity and energy
  • Persistent fever — fluctuating high temperature unresponsive to antibiotics
  • Loss of appetite — reduced or absent interest in food
  • Weight loss — often rapid in wet FIP due to protein loss into the fluid

Neurological FIP

Neurological FIP develops when the feline coronavirus invades the central nervous system. It can occur as a progression of wet or dry FIP, or as a primary presentation. Neurological FIP requires higher antiviral dosing (typically 8–12 mg/kg/day GS-441524) and closer monitoring throughout the 12-week treatment course.

Neurological FIP symptoms include:

  • Seizures — ranging from mild focal episodes to generalised convulsions
  • Loss of balance / unsteady gait — ataxia, stumbling, falling
  • Muscle weakness or partial paralysis — particularly in the hindlimbs
  • Behavioural changes — disorientation, aggression, personality changes
  • Head tilt and circling — signs of vestibular or cerebellar involvement
  • Nystagmus — rapid, involuntary eye movement

With the right protocol — including high-dose GS-441524 and in some cases EIDD-1931 dual-compound therapy — remission is achievable even in advanced neurological FIP cases.

Ocular FIP

Ocular FIP occurs when the virus affects the eyes, causing inflammation and progressive vision impairment. It frequently presents alongside neurological involvement and should be treated as a serious, urgent presentation.

Ocular FIP symptoms include:

  • Uveitis — inflammation inside the eye, visible as redness or cloudiness
  • Colour change or cloudiness in the iris — the iris may appear brown, murky, or discoloured
  • Unequal pupil sizes — one pupil significantly larger than the other (anisocoria)
  • Sensitivity to light — squinting or avoiding bright environments
  • Visible bleeding within the eye — blood pooling in the anterior chamber (hyphaema)
  • Vision loss — reduced responsiveness to visual stimuli, bumping into objects

What Are the Symptoms of FIP?

FIP symptoms vary significantly depending on the form and stage of the disease. Early signs are often non-specific — easily mistaken for other conditions — which is why FIP is frequently diagnosed late. Knowing the full symptom picture is critical for early intervention.

Early Warning Signs Common Across All FIP Forms

If your cat shows any combination of the following — particularly a fever that does not resolve with antibiotics — consult your veterinarian promptly and ask specifically about FIP testing:

  • Persistent or fluctuating fever — body temperature above 39.5°C (103.1°F), returning after antibiotic treatment
  • Unexplained weight loss — visible muscle wasting along the spine and hindquarters
  • Loss of appetite — reduced interest in food, skipping meals, or complete refusal to eat
  • Lethargy — reduced activity, sleeping more than usual, reluctance to play or interact
  • Mild diarrhoea — loose or watery stools, potentially with blood

⚠️ When to act urgently: If your cat has a swollen abdomen, difficulty breathing, seizures, loss of balance, or sudden vision changes — seek veterinary attention immediately. These signs indicate advanced FIP requiring urgent diagnosis and treatment.

Fever in Cats — FIP Warning Sign

A persistent fever that does not respond to antibiotics is one of the most reliable early indicators of FIP. Normal cat body temperature is 38–39.2°C (100.4–102.6°F). Sustained fever above 39.5°C in a cat that is also losing weight and appetite should prompt immediate FIP investigation.

Loss of Appetite in Cats — FIP Warning Sign

A cat that has stopped eating or significantly reduced food intake — particularly when combined with lethargy, weight loss, or fever — may be showing early FIP symptoms. Cats with dry FIP often present with a gradual decline in appetite over weeks before other signs become obvious.

Lethargy in Cats — FIP Warning Sign

Unexplained lethargy in a previously active young cat — particularly in a multi-cat household — should trigger FIP on the differential diagnosis list. Cats with FIP often show a progressive decline in energy: sleeping more, withdrawing from interaction, and losing interest in food and play simultaneously.

Swollen Abdomen in Cats — Wet FIP

A visibly distended abdomen filled with fluid is the hallmark sign of wet FIP. The abdomen may feel firm or fluid-filled when gently pressed. In severe cases, swelling can significantly impact breathing as the distended abdomen presses against the diaphragm. This is a medical emergency — seek veterinary care immediately.

Weight Loss in Cats — FIP Warning Sign

Rapid or progressive unexplained weight loss — particularly visible muscle wasting along the spine and hindquarters — is a consistent FIP sign across all forms. In wet FIP, protein lost into the abdominal fluid accelerates weight loss dramatically. Cats with dry FIP may lose weight more slowly but consistently over weeks.

Seizures in Cats — Neurological FIP

Seizures in a cat with no prior history — especially when combined with other FIP signs — strongly suggest neurological FIP. Feline seizures may present as sudden collapse, paddling limbs, loss of consciousness, or more subtle focal episodes such as facial twitching or sudden behavioural change. Any seizure event warrants urgent veterinary assessment.

Jaundice in Cats — Dry FIP with Liver Involvement

Jaundice — yellowing of the skin, whites of the eyes, or gums — indicates significant liver involvement. In FIP, this occurs when inflammatory granulomas affect liver function. Jaundice combined with weight loss, fever, and lethargy in a young cat is a high-suspicion FIP presentation.

Eye Problems in Cats — Ocular FIP

Eye changes are one of the most visually distinctive signs of FIP. Look for: cloudiness or colour change in the eye, unequal pupils, redness within the eye, squinting or light sensitivity, or sudden apparent vision loss. Ocular signs may appear alone or alongside neurological symptoms.

Vomiting and Diarrhoea in Cats — FIP

While vomiting and diarrhoea alone have many causes, their combination with fever, weight loss, lethargy, and appetite loss — especially in a young cat — raises FIP suspicion. Cats with gastrointestinal FIP involvement may show chronic, intermittent vomiting or loose stools that do not resolve with standard treatment.

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. A suspected FIP diagnosis should always be made by a veterinarian.

Blood and biochemistry markers associated with FIP:

  • Low albumin : globulin ratio (A/G ratio) — below 0.4 is strongly suggestive of FIP; below 0.8 warrants further investigation
  • Lymphopenia — reduced lymphocyte count, common in active FIP
  • Elevated bilirubin — indicates liver involvement; may cause visible jaundice
  • Elevated total protein — driven by high globulin levels
  • Elevated liver enzymes — ALT, AST, ALP elevated in hepatic involvement

Additional diagnostic tools:

  • FCoV antibody titre — elevated titres support but do not confirm FIP
  • AGP (alpha-1-acid glycoprotein) — consistently elevated in active FIP
  • Rivalta test — simple, inexpensive test for effusion fluid; positive result strongly supports wet FIP
  • Fluid analysis — protein content, appearance, and PCR of effusion fluid are highly informative
  • PCR testing — of fluid, tissue, or CSF; detects viral RNA directly
  • Ultrasound and X-ray — identifies fluid accumulation, organ changes, or enlarged lymph nodes
  • Biopsy — the most definitive confirmatory method where other tests are inconclusive

If you have received a suspected or confirmed FIP diagnosis and need guidance on next steps, contact our treatment team— available seven days a week.

Treatment Options for FIP

FIP is treatable. Since the widespread availability of GS-441524-based treatments from 2019, cats that would previously have been given days to live have achieved full remission and returned to normal, healthy lives.

GS-441524 — The Gold-Standard FIP 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. Available in both injectable and oral capsule form.

Clinical studies and real-world outcomes consistently report remission rates of 92% across all forms of FIP — wet, dry, neurological, and ocular — when treatment is started early and the full 84-day protocol is followed. Since 2019, CureFIP has helped save over 87,000 cats globally.

EIDD-1931 — A Path to Dual- Therapy

EIDD-1931 (β-D-N4-hydroxycytidine) is an advanced antiviral that works alongside GS-441524 through a complementary mechanism — lethal mutagenesis. While GS-441524 halts viral chain elongation, EIDD-1931 floods the viral RNA copying process with errors until the virus can no longer replicate.

This dual-compound approach is particularly recommended for:

  • Neurological FIP — maximum antiviral suppression across the blood-brain barrier
  • Incomplete remission — cats that have not fully responded to GS-441524 alone
  • Relapse prevention — cats at high risk of FIP recurrence
  • Calicivirus co-infection — EIDD-1931 also has demonstrated activity against feline calicivirus

Supportive Treatments

In addition to antiviral therapy, some cats benefit from supportive care during the early treatment weeks. This may include anti-nausea medication, appetite stimulants, fluid therapy, or corticosteroids for immune-mediated complications or severe neurological signs.

FIP Prognosis, Life Expectancy & Final Stages

Without treatment, FIP carries a very poor prognosis. Wet FIP can progress from diagnosis to death within days to weeks. Dry FIP may allow weeks or a few months. Neurological and ocular FIP progress rapidly without antiviral intervention.

With treatment: with early diagnosis and the right antiviral protocol, 92% of cats achieve remission. Cats that complete the full 84-day course and maintain remission through the observation period are considered cured and go on to live normal, healthy lives with normal life expectancy.

Final Stages of FIP in Cats

In untreated cats, or cats in whom treatment is started very late, the final stages of FIP include:

  • Severe, uncontrollable fluid accumulation
  • Extreme weight loss and muscle wasting
  • Complete loss of appetite and inability to eat
  • Severe breathing difficulty
  • Neurological deterioration — seizures, paralysis, coma
  • Organ failure

⚠️ Time is critical. Treatment started earlier in the disease course gives significantly better outcomes. If you suspect your cat has FIP, contact our team immediately.

FIP in Kittens

Kittens are the highest-risk age group for FIP. The majority of diagnoses occur in cats under 2 years old, with peak incidence at 4–18 months. Kittens in shelters, catteries, or multi-cat households are at greatest risk.

Signs of FIP in kittens often progress faster than in adult cats. A kitten that is suddenly lethargic, not eating, running a persistent fever, or developing a swollen belly should be evaluated for FIP urgently.

GS-441524 is effective and safe for use in kittens. Dosing is weight-based — use our FIP Dosage Calculator for an accurate recommendation, or contact our team for personalised guidance.

Can FIP Be Prevented?

There is a commercially available FIP vaccine, though its adoption remains limited and it is not universally recommended. Discuss vaccination with your veterinarian as part of your cat's routine care.

Practical steps to reduce FIP risk in multi-cat environments:

  • Strict litter box hygiene — clean daily, one box per cat plus one additional
  • Avoid overcrowding — high cat density accelerates feline coronavirus circulation
  • Quarantine new cats — isolate new arrivals for 2–3 weeks before introducing to existing cats
  • Minimise stress — chronic stress is a known factor in FCoV mutation risk
  • Regular veterinary checkups — particularly for cats in high-exposure environments

Frequently Asked Questions about FIP ​​in Cats

What is Feline Infectious Peritonitis (FIP)?

FIP is a progressive viral disease in cats caused by a mutated form of the feline coronavirus. Without treatment it is almost always fatal. With antiviral treatment usingGS-441524, remission rates of 92% are now achievable across all FIP forms.
What are the first symptoms of FIP in cats?

The earliest signs are often non-specific:persistent fever, unexplained weight loss, loss of appetite, and lethargy. Wet FIP may also present with progressive abdominal swelling. If your cat shows these signs — particularly a fever that does not resolve with antibiotics — consult your veterinarian and ask specifically about FIP testing.
What are the symptoms of wet FIP vs dry FIP?

Wet FIP:visibly distended abdomen, fluid in the chest, laboured breathing, fever, lethargy, weight loss.Dry FIP:weight loss, persistent fever, loss of appetite, vomiting, diarrhoea, excessive thirst — without significant fluid accumulation. Some cats develop both forms simultaneously.
Is FIP contagious to other cats?

FIP itself is not contagious.The underlying feline coronavirus spreads via shared litter boxes, but the FIP mutation cannot be passed between cats. Maintaining litter box hygiene is still important to limit FCoV circulation.
How is FIP diagnosed?

FIP diagnosis combines clinical signs, bloodwork (low A/G ratio, lymphopenia, elevated globulins), and additional tests includingAGP levels, Rivalta test, fluid analysis, PCR testing,and ultrasound. No single test confirms FIP — diagnosis is based on the overall clinical picture.
What is the FIP blood test?

The most informative blood test is the albumin-to-globulin (A/G) ratio— below 0.4 is strongly suggestive of FIP. Other useful markers include lymphopenia, elevated bilirubin, elevated total protein, and AGP levels.
What is GS-441524 and how does it treat FIP?

GS-441524is a nucleoside analogue antiviral that blocks feline coronavirus replication at the molecular level. It is the most effective FIP treatment available, with a 92% remission rate. Treatment runs for 84 days with once-daily dosing adjusted by body weight and FIP form.
How long does FIP treatment last?

The standard course is 84 days (12 weeks). Neurological and ocular cases may require an extended course. After completing treatment, cats enter an observation period before being considered in full remission. Use our FIP Dosage Calculatorto plan the full protocol.
Can kittens be treated for FIP?

Yes.GS-441524 is safe and effective in kittens. Dosing is weight-based. Early diagnosis and prompt treatment give the best outcomes.
What are the final stages of FIP in cats?

Without treatment, end-stage FIP involves severe fluid accumulation, extreme weight loss, complete appetite loss, breathing difficulty, neurological deterioration, and organ failure. Treatment started before this stage gives significantly better outcomes. Contact our team immediately if you suspect advanced FIP.

CureFIP™ — Provider of clinically proven antiviral treatments for Feline Infectious Peritonitis. Trusted by cat owners and veterinarians across 50+ countries since 2019.

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