FIP Symptoms in Cats: 2026 U.S. Checklist for Owners

You notice your cat is hiding more than usual. The food bowl is barely touched. Maybe the belly looks rounder than last week, or one eye has a strange cloudiness that wasn't there yesterday. You start typing symptoms into your phone at midnight, and the search results take you somewhere terrifying.

Don't panic. Feline Infectious Peritonitis (FIP) is now a treatable disease, and recognizing the signs early is the single most important thing you can do for your cat. This 2026 checklist is written for U.S. cat parents who need calm, evidence-based guidance, not scare tactics.

FIP Symptoms in Cats: 2026 U.S. Checklist for Owners

Below you will find the full symptom map: early warning signs, the four clinical forms of FIP, and the red-flag combinations that should send you to a veterinarian today. CureFIP has supported more than 100,000+ cats treated since 2019, and almost every successful case started with an owner who trusted their gut and acted fast.

What FIP Actually Is (In Plain English)

FIP develops when a common, mostly harmless feline coronavirus mutates inside a specific cat and triggers a severe inflammatory immune response. It is not contagious in the way a cold is. Your other cats are very unlikely to develop FIP just because one cat in the household has it.

Historically, FIP was considered uniformly fatal. That changed when Dr. Niels Pedersen and the UC Davis team published the foundational work on GS-441524, the antiviral nucleoside analog that is now the hero ingredient behind CureFIP's protocols. Today, an 84-day antiviral course gives most cats a realistic path to remission.

But treatment depends on diagnosis, and diagnosis starts with you spotting the signs.

Early FIP Symptoms: What to Watch for First

Early FIP symptoms are notoriously vague. That is what makes them dangerous. Here is what most cat owners don't find out until later: by the time the classic textbook signs appear, the disease has often been brewing for weeks.

Watch for this early cluster:

1. Fluctuating fever that does not respond to standard antibiotics (often 103 to 105 F).

2. Lethargy that is out of character. Your cat sleeps in odd spots, stops greeting you, or skips the windowsill.

3. Progressive weight loss even when food intake looks okay.

4. Reduced appetite, picky eating, or sudden food rejection.

5. Poor coat quality, with a dull, greasy, or unkempt look.

6. Stunted growth in kittens and young cats under 2 years.

7. Mild jaundice, with yellowing of the gums, the whites of the eyes, or the inside of the ears.

If two or more of these persist for more than five to seven days, especially in a cat under 2 years old or in a recently adopted shelter or rescue cat, FIP should be on the differential list your veterinarian considers.

The Four Types of FIP and Their Signs

FIP presents in four clinical forms. The form matters because it changes the dosing protocol, the prognosis, and the urgency. Use this section as a symptom map.

1. Wet (Effusive) FIP

The wet form is defined by fluid buildup inside body cavities. It is often the fastest to diagnose because the changes are visible.

Signs to look for:

• A visibly distended, pear-shaped abdomen that feels soft and full, not firm.

• Labored or rapid breathing, open-mouth breathing, or reluctance to lie flat (this suggests chest fluid).

• Pale or yellow gums.

• Muffled heart sounds noted by your veterinarian on exam.

• Persistent fever and rapid weight loss despite the swollen belly.

Wet FIP can progress within days. If your cat's belly looks bigger week over week, do not wait.

2. Dry (Non-Effusive) FIP

The dry form produces inflammatory lesions called granulomas in organs like the kidneys, liver, lymph nodes, and intestines, without obvious fluid.

Signs to look for:

• Chronic, low-grade fever that comes and goes.

• Weight loss and muscle wasting along the spine and hips.

• Enlarged lymph nodes that your veterinarian may feel under the jaw or behind the knees.

• Mesenteric masses, sometimes felt as firm lumps in the abdomen.

• Jaundice and elevated liver values on bloodwork.

• Vomiting or diarrhea that does not respond to standard treatment.

Dry FIP is the great mimic. It can look like inflammatory bowel disease, lymphoma, or chronic kidney issues. Targeted testing is essential.

3. Ocular FIP

Ocular FIP affects the eyes, often before other systemic signs become obvious. Many owners catch FIP early because they noticed something off about one eye.

Signs to look for:

• Color change in the iris, often a green or brown tint appearing where it wasn't before.

• Cloudiness in the front chamber of the eye, sometimes described as a haze.

• Uveitis, with redness, squinting, or sensitivity to light.

• Visible flecks or keratic precipitates on the inside of the cornea.

• Anisocoria, where the two pupils are different sizes.

• Sudden vision loss or bumping into furniture.

Any new eye change in a young cat with vague systemic signs deserves an FIP workup, not just an eye drop prescription.

4. Neurological FIP

Neurological FIP involves the brain, spinal cord, or both. It is the most challenging form and requires higher antiviral dosing, so accurate recognition is critical.

Signs to look for:

• Wobbly gait, ataxia, or stumbling, especially in the back legs.

• Head tilt or circling.

• Seizures, twitches, or tremors.

• Behavior changes, including hiding, aggression, or unresponsiveness.

• Incontinence or sudden loss of litter box habits.

• Nystagmus, where the eyes flick back and forth involuntarily.

• Hyperesthesia, with the skin rippling or the cat overreacting to gentle touch.

Neurological signs can develop in a cat that previously had wet or dry FIP, or they can be the first presentation. Either way, they require immediate veterinary evaluation and a protocol designed to cross the blood-brain barrier.

Red-Flag Symptom Combinations

What the numbers are really telling you matters more than any single symptom in isolation. These combinations carry the highest predictive value for FIP in U.S. clinical practice in 2026:

• Young cat (under 2 years) + fluctuating fever + weight loss + recent shelter or multi-cat exposure.

• Pot-bellied appearance + jaundice + low appetite.

• Eye color change + lethargy + fever.

• Neurological signs + history of vague illness over weeks.

• High total protein, low albumin-to-globulin ratio + persistent fever.

If you see any of these, ask your veterinarian directly about FIP testing, including bloodwork, fluid analysis if applicable, and PCR or imaging as needed.

Which Cats Are Most at Risk?

FIP can affect any cat, but the risk is concentrated in specific groups:

1. Cats under 2 years old, with kittens 4 to 16 months at highest risk.

2. Purebred cats, particularly Ragdolls, Bengals, Birmans, British Shorthairs, and Abyssinians.

3. Cats from shelters, rescues, catteries, or multi-cat households where feline coronavirus circulates widely.

4. Cats recently exposed to stress, including rehoming, surgery, or boarding.

5. Immunocompromised cats, including those with concurrent FIV or FeLV.

If your cat falls into one of these categories and shows early signs, do not let a veterinarian dismiss FIP as too rare to consider.

What to Do the Moment You Suspect FIP

Here is a calm, evidence-based action plan:

1. Document the symptoms. Note start date, frequency, and any changes. Photograph eye changes and belly shape.

2. Book a veterinary exam this week, not next month.

3. Request a full bloodwork panel, including CBC, chemistry, total protein, albumin, and globulin. Ask for the A:G ratio.

4. If there is abdominal or chest fluid, ask for a tap and analysis. Yellow, sticky, protein-rich fluid is a strong FIP indicator.

5. Ask about FCoV antibody titers, AGP, and PCR testing where appropriate.

6. Confirm the form of FIP, wet, dry, ocular, or neurological, because this determines the antiviral dose.

7. Start treatment promptly once FIP is confirmed. Days matter.

How CureFIP Treats Each Form of FIP

CureFIP's protocols are built around GS-441524, the antiviral validated in the original UC Davis work, and our dual antiviral capsules that pair GS-441524 with EIDD-1931 for cats who do better on oral therapy.

Injectable GS-441524, used as monotherapy, has shown 92% success in the foundational UC Davis / Pedersen 2019 study. Our injectable line includes:

• GS-441524 Antiviral Injectables | 15mg/ml, 8ml | $49.00

• GS-441524 Antiviral Injectables | 20mg/ml | $69.00

• GS-441524 Antiviral Injectables | 30mg/ml | $89.00

• GS-441524 Antiviral Injectables, 40mg/ml | $119.00

• GS-441524 Antiviral with Vitamin B12 | 20mg/ml | $75.00

• GS-441524 Antiviral with Vitamin B12 | 30mg/ml | $95.00

Dosing is matched to the form of FIP: wet 6 mg/kg, dry 8 mg/kg, ocular 10 mg/kg, neurological 10 mg/kg, given as one subcutaneous injection per day, seven days a week, for 12 weeks (84 days), per Pedersen et al., UC Davis (PMC6435921).

For cats who tolerate oral therapy, Cure FIP™ Oral Capsules at $129.00 deliver the dual antiviral approach. Dosing is by weight: under 5.5 lb gets GS-441524 25 mg plus EIDD-1931 5 mg; 5.5 to 11 lb gets GS-441524 35 mg plus EIDD-1931 8 mg; over 11 lb gets GS-441524 50 mg plus EIDD-1931 12 mg. One capsule per day, every day, for 12 weeks (recommended). The dual antiviral approach (GS-441524 plus EIDD-1931) showed 78.3% remission in Li and Cheah 2025. Oral dual therapy is generally not recommended once ocular or neurological signs are present, or when a cat cannot eat or defecate normally.

Your veterinarian remains essential throughout. Antiviral therapy for FIP requires monitoring, weight checks, and bloodwork at intervals.

Why Acting Early Changes the Outcome

The number on the label isn't the number that matters. What matters is how early you intervene, how accurate the form classification is, and how consistently the protocol is followed for the full 84 days.

Cats treated within days of clear FIP signs tend to stabilize fast. Fever often breaks within 24 to 72 hours of starting an appropriate antiviral protocol. Appetite returns. Energy comes back. The 84-day finish line is reachable for the great majority of cats whose owners caught the signs and committed to the full course.

That is why this checklist exists. The earlier you recognize FIP symptoms in cats, the better your cat's odds.

FAQ

What are the very first signs of FIP in cats?

The earliest signs of FIP are usually a fluctuating fever that does not respond to antibiotics, lethargy, reduced appetite, gradual weight loss, and a dull coat. These signs are subtle and easy to miss. In kittens and young cats under 2 years, even a few days of these symptoms together justifies an FIP workup.

How fast does FIP progress without treatment?

Wet FIP can progress within one to three weeks, with rapid fluid buildup and breathing difficulty. Dry, ocular, and neurological forms can develop over weeks to months. Once neurological signs appear, deterioration can be very fast. Starting an evidence-based antiviral protocol promptly is the single biggest factor in changing the trajectory.

Can a cat have more than one form of FIP at the same time?

Yes. Many cats present with overlap, for example wet FIP that develops ocular signs, or dry FIP that progresses to neurological involvement. When forms overlap, the protocol uses the higher of the applicable doses, because ocular and neurological FIP require more antiviral to reach the affected tissues.

Is FIP contagious to my other cats or my dog?

FIP itself is not transmitted cat to cat in the way a respiratory virus is. The underlying feline coronavirus is common and spreads through shared litter boxes, but only a small percentage of infected cats develop the mutation that causes FIP. Dogs and humans are not at risk.

Where can I learn more about CureFIP treatment options?

You can review the full antiviral catalog, dosing references, and clinical guidance at curefipusa.com. Always work with a veterinarian to confirm the FIP form, set the correct dose for your cat's weight, and monitor progress through the 84-day protocol.