Feline Infectious Peritonitis (FIP) in Cats: 12 Early Warning Symptoms Every Owner Must Know (2026)
- CUREFIP.COM
- May 15
- 10 min read
Quick answer: The earliest signs of Feline Infectious Peritonitis (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, owners typically notice one of four forms emerging: 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).
Since 2019, CureFIP has supported more than 100,000 cat owners worldwide through Feline Infectious Peritonitis (FIP) treatment. The disease that was once considered fatal is now treatable in most cases when caught early. This guide walks you through every early warning sign you should recognise, how vets confirm a Feline Infectious Peritonitis (FIP) diagnosis, and what to do next.
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 such as soft stool or diarrhoea. However, in a small percentage of cats, the virus mutates inside the body and triggers an aggressive immune response. This is the moment the cat develops Feline Infectious Peritonitis (FIP).
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.
Feline Infectious Peritonitis (FIP) used to be considered fatal. With modern dual-antiviral therapy (GS-441524 combined with EIDD-1931), it is now treatable in most cases.
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 Feline Infectious Peritonitis (FIP)
These are the symptoms cat owners most often report in the days and weeks before a Feline Infectious Peritonitis (FIP) diagnosis. Not every cat shows all twelve, but the appearance of three or more together is a strong reason to ask your vet about FIP testing.
General signs (all forms of FIP):
Persistent fever above 39.5°C (103.1°F) that does not resolve with antibiotics
Gradual weight loss, often visible along the spine and hindquarters, even when the cat is still eating
Reduced appetite or refusing food
Lethargy and unusual sleepiness; reluctance to play
Rough or dull coat and reduced self-grooming
Pale or yellow gums (jaundice can indicate liver involvement)
Hiding more than usual or withdrawing from family
Form-specific signs:
Swollen, fluid-filled belly (wet FIP)
Difficulty breathing or panting (wet FIP with chest fluid)
Eye colour changes, cloudiness, or unequal pupil size (ocular FIP)
Wobbly walking, head tilt, seizures, or tremors (neurological FIP)
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 Feline Infectious Peritonitis (FIP). Time matters: the earlier antiviral treatment begins, the better the outcome.
The 4 Forms of Feline Infectious Peritonitis (FIP)
FIP appears in two main forms, wet and dry, that can each progress to involve the eyes or central nervous system. Here is how to tell them apart.
Form | Hallmark sign | What's happening inside | How fast it progresses |
Wet FIP (effusive) | Swollen belly, breathing difficulty | Fluid leaking into abdomen or chest | Days to weeks |
Dry FIP (non-effusive) | Weight loss, fever, no fluid | Granulomas forming on organs | Weeks to months |
Neurological FIP | Wobbly gait, seizures, behaviour change | FIP crossing the blood-brain barrier | Weeks |
Ocular FIP | Eye colour change, cloudiness | FIP inflammation in the eye | Days to weeks |
Wet FIP (Effusive Form)
Wet FIP develops when fluid builds up in the abdomen or chest cavity. Cats with wet FIP often appear pot-bellied even though the rest of the body is wasting. The fluid is typically straw-yellow, viscous, and high in protein. Your vet can extract a small sample for the Rivalta test, a simple bedside test that takes five minutes and is a strong indicator of Feline Infectious Peritonitis (FIP) when positive. Learn how the Rivalta test works.
Common symptoms of wet FIP:
Swollen or distended belly that may feel tight
Breathing difficulties, panting, or open-mouth breathing
Reduced appetite and energy
Visible weight loss along the spine despite the swollen belly
Occasional coughing or weakness
Dry FIP (Non-Effusive Form)
Dry FIP affects internal organs, most often the liver, kidneys, lymph nodes, or lungs, without producing visible fluid. Affected organs may become enlarged or develop firm nodules called granulomas, which a vet can sometimes feel on physical examination or detect on ultrasound.
Common symptoms of dry FIP:
Gradual weight loss despite normal or near-normal appetite
Persistent low-grade fever
General fatigue and reduced activity
Pale gums or jaundice if the liver is involved
Diarrhoea or vomiting in some cases
Because dry FIP progresses slowly, it is often diagnosed later than wet FIP. This is one reason owners should not wait to investigate persistent low-grade fever.
Neurological FIP
When Feline Infectious Peritonitis (FIP) affects the central nervous system, the symptoms can be dramatic and frightening for owners. Neurological FIP requires higher doses of antiviral treatment because the drugs must cross the blood-brain barrier. EIDD-1931 is particularly important for neurological cases, which is why CureFIP's protocol combines it with GS-441524 rather than using GS-441524 alone.
Common symptoms of neurological FIP:
Tremors or seizures
Loss of balance, unsteady walking, or ataxia
Muscle weakness or partial paralysis in the hind legs
Behavioural changes, disorientation, or reduced alertness
Difficulty jumping or walking in a straight line
Nystagmus (involuntary back-and-forth eye movement)
Head pressing or unusual head tilt
Ocular FIP
Ocular FIP occurs when the virus inflames the structures of the eye. Eyes are sometimes the first place Feline Infectious Peritonitis (FIP) shows itself, which means an ocular sign can be an early warning rather than a late-stage symptom.
Common symptoms of ocular FIP:
Uveitis (inflammation of the iris and surrounding tissue)
Cloudiness or visible change in eye colour
Reduced or lost vision
Cataracts or inflammation around the iris and lens
Anisocoria (one pupil larger than the other)
Aqueous flare visible under vet examination
Ocular symptoms may occur alone or alongside other Feline Infectious Peritonitis (FIP) signs. Supportive eye medication can ease discomfort, but antiviral therapy is the only way to stop the underlying disease.
Feline Infectious Peritonitis (FIP) Symptom Timeline: Week 1 to Week 4
Owners often ask how quickly FIP progresses. Here is a typical pattern, though every cat is different:
Week 1: Intermittent fever, slightly reduced appetite, mild lethargy. Easy to mistake for a passing illness.
Week 2: Fever becomes more persistent. Cat hides more, grooms less, may start losing weight.
Week 3: Form-specific symptoms emerge: swollen belly (wet), jaundice (dry), or eye changes (ocular).
Week 4 and beyond: Without treatment, condition deteriorates rapidly. Neurological symptoms may appear. Cats reach advanced disease.
This timeline is why the message in every FIP support group is the same: do not wait. A vet visit at week 1 with a request to rule out Feline Infectious Peritonitis (FIP) gives your cat the best possible outcome.
Feline Infectious Peritonitis (FIP) Symptoms in Kittens vs Adult Cats
Kittens and young cats under two years old are more vulnerable to Feline Infectious Peritonitis (FIP) because their immune systems are still developing. They also tend to deteriorate faster, so even mild signs warrant immediate investigation.
Signs in kittens:
Sudden tiredness or reluctance to play
Failure to gain weight or rapid weight loss
Swollen belly that feels soft and fluid-filled
Quick progression from mild to severe illness within days
Signs in adult cats:
Slower, more insidious onset
Gradual reduction in playfulness
Mild weight loss over weeks
Increased hiding or withdrawing from family
Adult cats more often develop dry FIP, kittens more often develop wet FIP. Both forms respond well to the modern dual-antiviral protocol when treatment starts early.
How Vets Diagnose Feline Infectious Peritonitis (FIP)
There is no single definitive test for Feline Infectious Peritonitis (FIP). Vets combine multiple findings to reach a confident diagnosis. Here is the typical diagnostic workflow:
Complete Blood Count (CBC): checks for anaemia, low lymphocytes, and inflammation patterns typical of FIP
Albumin-to-Globulin (A:G) ratio: a ratio below 0.6 is highly suggestive; below 0.4 is strongly indicative
Rivalta test: performed on any abdominal or chest fluid; a positive Rivalta has about 91% sensitivity for FIP
PCR testing: detects FCoV genetic material in fluid, blood, or tissue
Immunohistochemistry: confirms FCoV in tissue samples, the closest thing to a definitive test
Imaging: ultrasound or X-ray can identify fluid pockets or organ granulomas
A vet who combines these tools with the clinical picture can reach diagnostic confidence well above 90% even without a tissue biopsy.
Read more about the Rivalta test and how to interpret it.
Is Feline Infectious Peritonitis (FIP) Contagious to Other Cats?
This is one of the most common questions from multi-cat households, and the answer matters for how you manage your home during treatment.
The mutated FIP virus itself is not contagious between cats. Once the feline coronavirus mutates inside one cat's body and becomes FIP, it loses the ability to transmit. A cat diagnosed with Feline Infectious Peritonitis (FIP) does not endanger the other cats in the home with FIP.
The underlying feline coronavirus (FCoV), however, is contagious through shared litter trays, grooming, and close contact. Most cats living together will have been exposed to FCoV already. The good news is that only a small fraction of FCoV-positive cats ever develop the mutation that causes FIP.
Survival Rates With the 84-Day Antiviral Protocol
Before 2019, Feline Infectious Peritonitis (FIP) carried an almost universal fatal prognosis. Today, the picture is completely different.
The Li and Cheah (2025) field study of 46 cats treated with combined oral GS-441524 and EIDD-1931 reported a 78.3% remission rate with a 6.5% relapse rate. The standard protocol runs for 84 days, followed by a 12-week observation period. Cats that complete both phases without symptom recurrence are considered in long-term remission.
What affects survival:
How early treatment starts: the single biggest factor
Which form of FIP: wet and ocular FIP generally respond fastest; neurological FIP needs higher doses
Dual-antiviral therapy vs monotherapy: combining GS-441524 with EIDD-1931 addresses the resistance pathways that cause GS-only treatment to fail in some cats
Age and prior health: kittens and young cats often recover faster than older immunocompromised cats
What To Do If You Suspect Feline Infectious Peritonitis (FIP)
If your cat is showing three or more of the early symptoms above, take action today. Feline Infectious Peritonitis (FIP) is no longer a hopeless diagnosis, but every day matters.
Step 1: Visit a veterinarian familiar with modern FIP treatment. Some traditional vets still believe Feline Infectious Peritonitis (FIP) cannot be cured. Ask specifically about FIP testing and request the A:G ratio, Rivalta test if there is fluid, and PCR if available.
Step 2: Calculate the correct treatment dose for your cat's weight and FIP form. Use the CureFIP Dosage Calculator. It accounts for FIP form (neurological cases need higher doses) and converts to tablet count automatically.
Step 3: Start the 84-day dual-antiviral protocol. CureFIP supplies GS-441524 combined with EIDD-1931 in oral capsule form, manufactured under GMP standards. See the treatment options here.
Step 4: Monitor weekly. Track weight, temperature, appetite, and any new symptoms. Most cats show clear improvement within the first 7 to 14 days of treatment.
Step 5: Complete the full 84 days. Stopping early is the most common cause of relapse. The 12-week observation period after day 84 is just as important as treatment itself.
Frequently Asked Questions
What are the first signs of Feline Infectious Peritonitis (FIP) in cats?
The earliest signs are a persistent fever above 39.5°C that does not respond to antibiotics, gradual weight loss, reduced appetite, and unusual lethargy. Within days to weeks, form-specific signs appear: a swollen belly for wet FIP, jaundice for dry FIP, eye changes for ocular FIP, or wobbly gait for neurological FIP.
Is Feline Infectious Peritonitis (FIP) contagious to other cats?
No. The mutated FIP virus is not contagious between cats. The underlying feline coronavirus (FCoV) can spread, but only a small fraction of FCoV-positive cats develop the mutation that causes Feline Infectious Peritonitis (FIP).
Can Feline Infectious Peritonitis (FIP) be cured?
Yes. With the modern 84-day antiviral protocol using GS-441524 combined with EIDD-1931, FIP is now treatable in most cases. The Li and Cheah (2025) field study reported a 78.3% remission rate with a 6.5% relapse rate in 46 cats.
How long can a cat with Feline Infectious Peritonitis (FIP) survive?
Without treatment, cats typically survive only a few weeks once symptoms appear. With early antiviral therapy, most cats achieve full remission and go on to live normal lifespans.
What does a cat with Feline Infectious Peritonitis (FIP) look like?
A cat with wet FIP typically has a swollen, pot-bellied abdomen filled with fluid, visible weight loss along the spine, a dull coat, and reduced energy. A cat with dry FIP may look thin and unwell despite eating normally. Yellowing of the gums or eyes is common in both forms.
How is Feline Infectious Peritonitis (FIP) diagnosed?
Vets combine multiple findings: a complete blood count, the albumin-to-globulin ratio, the Rivalta test on any fluid, PCR testing for FCoV, and the clinical picture. No single test is definitive on its own, but the combination reaches diagnostic confidence above 90%.
Can Feline Infectious Peritonitis (FIP) return after treatment?
Relapse is rare. The 2025 Li and Cheah field study reported a relapse rate of 6.5%. The standard 84-day treatment is followed by a 12-week observation period before a cat is considered fully recovered.
What causes Feline Infectious Peritonitis (FIP) in cats?
FIP is caused by a mutation of the feline coronavirus (FCoV). Most cats exposed to FCoV experience only mild digestive symptoms, but in a small percentage of cases, the virus mutates and triggers an aggressive immune response that becomes Feline Infectious Peritonitis (FIP).
Conclusion
Feline Infectious Peritonitis (FIP) was once a devastating diagnosis. Today, with the dual-antiviral protocol of GS-441524 and EIDD-1931, it is a treatable disease in the vast majority of cases. Recognising the 12 early warning signs (persistent fever, weight loss, appetite changes, swollen belly, eye changes, wobbly gait) and acting within days rather than weeks is the single biggest factor in your cat's recovery.
If your cat is showing any combination of these symptoms, do not wait. Speak to a veterinarian familiar with modern FIP treatment, use the CureFIP Dosage Calculator to plan the protocol, and explore the dual-antiviral treatment options here.
Since 2019, CureFIP has supported the treatment of 100,000+ cats globally. Early action saves lives.
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