Feline Infectious Peritonitis (FIP) in Cats: 12 Early Warning Symptoms Every Owner Must Know (2026)

Quick answer: The earliest signs of FIP in cats are a persistent fever above 39.5°C that does not respond to antibiotics, gradual weight loss, reduced appetite, and unusual lethargy. As the disease progresses, one of four forms typically emerges: wet FIP (fluid buildup in the abdomen or chest), dry FIP (granulomas on internal organs), neurological FIP (seizures, wobbly gait, behaviour changes), or ocular FIP (eye colour changes, cloudiness, uveitis).

What Is Feline Infectious Peritonitis (FIP)?

Feline Infectious Peritonitis (FIP) is a viral disease caused by a mutation of the feline coronavirus (FCoV). It is not related to the human COVID-19 virus. Most cats exposed to FCoV experience only mild digestive problems, but in a small percentage, the virus mutates and triggers an aggressive immune response. Three facts every cat owner should know:

  • The mutation usually happens in cats under two years old, in older cats with weaker immune systems, or in cats under chronic stress.
  • FIP used to be considered fatal. With modern GS-441524 antiviral therapy, it is now treatable in most cases when caught early.
  • The diagnosis is rarely made from a single test. Vets combine clinical signs, blood work, fluid analysis, and PCR to reach confidence.

The 12 Earliest Symptoms of FIP

General signs (all forms):

  1. Persistent fever above 39.5°C that does not resolve with antibiotics
  2. Gradual weight loss, often visible along the spine and hindquarters
  3. Reduced appetite or refusing food
  4. Lethargy and unusual sleepiness; reluctance to play
  5. Rough or dull coat and reduced self-grooming
  6. Pale or yellow gums (jaundice can indicate liver involvement)
  7. Hiding more than usual or withdrawing from family

Form-specific signs:

  1. Swollen, fluid-filled belly (wet FIP)
  2. Difficulty breathing or panting (wet FIP with chest fluid)
  3. Eye colour changes, cloudiness, or unequal pupil size (ocular FIP)
  4. Wobbly walking, head tilt, seizures, or tremors (neurological FIP)
  5. Sudden behaviour changes: disorientation, aggression, or unusual vocalising (neurological FIP)

If you see any combination of these signs lasting more than a few days, contact a veterinarian familiar with FIP immediately. The earlier GS-441524 treatment begins, the better the outcome.

The 4 Forms of FIP

Form Hallmark Sign What’s Happening Inside How Fast It Progresses
Wet FIP Swollen belly, breathing difficulty Fluid leaking into abdomen or chest Days to weeks
Dry FIP Weight loss, fever, no fluid Granulomas forming on organs Weeks to months
Neurological FIP Wobbly gait, seizures FIP crossing the blood-brain barrier Weeks
Ocular FIP Eye colour change, cloudiness FIP inflammation in the eye Days to weeks

Wet FIP

Fluid builds up in the abdomen or chest. Cats appear pot-bellied even as the rest of the body wastes. The GS-441524 20 mg/ml injectable is the standard starting point at 6 mg/kg once daily for 84 days.

Dry FIP

Affects internal organs without visible fluid. Granulomas form on the liver, kidneys, or lymph nodes. Signs include persistent low-grade fever, weight loss, and fatigue. Standard dose: 8 mg/kg once daily for 84 days.

Ocular FIP

The virus inflames the eye structures. Signs include uveitis, cloudiness, colour changes, and anisocoria. Treatment with GS-441524 30 mg/ml at 10 mg/kg once daily for 84 days.

Neurological FIP

FIP crosses the blood-brain barrier causing tremors, seizures, ataxia, and behaviour changes. Requires higher doses: GS-441524 30 mg/ml at 10 mg/kg once daily for 84 days under close veterinary supervision.

FIP Symptom Timeline: Week 1 to Week 4

  • Week 1: Intermittent fever, slightly reduced appetite, mild lethargy — easy to mistake for a passing illness.
  • Week 2: Fever becomes persistent. Cat hides more, grooms less, starts losing weight.
  • Week 3: Form-specific symptoms emerge: swollen belly, jaundice, or eye changes.
  • Week 4+: Without treatment, condition deteriorates rapidly. Neurological symptoms may appear.

Do not wait. A vet visit at week 1 gives your cat the best possible outcome.

Kittens vs Adult Cats

Kittens under two years deteriorate faster — even mild signs warrant immediate investigation. Kittens more often develop wet FIP; adult cats more often develop dry FIP. Both forms respond well to GS-441524 when started early. Read our guide on how long cats can live with FIP.

How Vets Diagnose FIP

  • CBC: anaemia, low lymphocytes, inflammation markers
  • A:G ratio: below 0.6 is highly suggestive; below 0.4 is strongly indicative
  • Rivalta test: ~91% sensitivity on abdominal or chest fluid
  • PCR testing: detects FCoV genetic material
  • Imaging: ultrasound or X-ray for fluid pockets or granulomas

Combining these tools reaches diagnostic confidence above 90% without a tissue biopsy.

Is FIP Contagious?

The mutated FIP virus is not contagious between cats. The underlying FCoV can spread through shared litter trays and grooming, but only a small fraction of FCoV-positive cats ever develop FIP.

Survival Rates With the 84-Day GS-441524 Protocol

GS-441524 injectable monotherapy carries a 92% success rate documented by UC Davis (Pedersen, 2019). The protocol runs 84 days followed by a 12-week observation period. Key factors affecting outcome:

  • How early treatment starts — the single biggest factor
  • Which form of FIP: wet and ocular respond fastest; neurological needs higher doses
  • Correct dosing: under-dosing is the most common cause of relapse
  • Age and prior health

Read our FIP relapse guide on why finishing the full 84 days matters.

What To Do If You Suspect FIP

  1. See a vet. Request the A:G ratio, Rivalta test, and PCR.
  2. Confirm form and weight. Wet: 6 mg/kg. Dry: 8 mg/kg. Ocular/neurological: 10 mg/kg. All once daily for 84 days.
  3. Start treatment. FIP CURE Plus supplies GS-441524 20 mg/ml and GS-441524 30 mg/ml with pricing in AED.
  4. Monitor weekly. Most cats improve within 7–14 days. See our FIP diet guide.
  5. Complete all 84 days. Early stopping is the most common cause of relapse.

FAQ

What are the first signs of FIP?
Persistent fever above 39.5°C, weight loss, reduced appetite, and lethargy. Form-specific signs follow within days to weeks.

Is FIP contagious?
No. The mutated FIP virus does not spread between cats.

Can FIP be cured?
Yes. GS-441524 injectable monotherapy carries a 92% success rate (UC Davis, Pedersen 2019).

How long can a cat with FIP survive?
Without treatment, weeks. With early GS-441524 therapy, most cats achieve full remission. Read our FIP survival guide.

What does a cat with FIP look like?
Wet FIP: swollen belly, weight loss, dull coat. Dry FIP: thin and unwell despite eating. See our complete FIP symptoms guide.

Can FIP return after treatment?
Relapse is uncommon when the full 84-day protocol is completed. Read our FIP relapse guide.

What causes FIP?
A mutation of the feline coronavirus (FCoV) that triggers an aggressive immune response in a small percentage of cats.

Conclusion

FIP was once a devastating diagnosis. Today, with GS-441524 antiviral treatment, it is treatable in the vast majority of cases. Recognising the 12 early warning signs and acting quickly is the single biggest factor in your cat’s recovery.

Explore the FIP CURE Plus GS-441524 injectable range or reach the team in English and Arabic at info@fipcureplus.com.

FIP CURE Plus — transparent, evidence-based GS-441524 treatment for cats across the Gulf.

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