FIP in Cats Explained: Causes, Symptoms, and What to Know

Key takeaway: Feline Infectious Peritonitis ("FIP") is a serious illness caused by a mutated form of the common feline coronavirus (FCoV). It takes four forms, wet, dry, ocular, and neurological, and today it is treatable: GS-441524 is the antiviral at the core of modern FIP treatment, given under veterinary supervision over a 12-week (84-day) course. If your cat has just been diagnosed, the most important next steps are confirming the form of FIP with your veterinarian and starting the right protocol quickly.

A cat resting comfortably at home.

What is Feline Infectious Peritonitis (FIP) in cats?

Feline Infectious Peritonitis ("FIP") is a viral disease that develops when the common feline coronavirus (FCoV) mutates inside a cat's body and triggers widespread inflammation. Most cats carry harmless feline coronavirus in their gut at some point, but in a small number the virus changes and causes FIP.

FIP was once considered almost always fatal. That has changed. GS-441524, the antiviral at the core of today's FIP treatment, has given thousands of cats a real path back to eating, playing, and coming home. Across our network, more than 100,000 cats have been treated since 2019.

If you are just starting your research, our plain English guide to what FIP is in cats is a good companion to this article.

What causes Feline Infectious Peritonitis (FIP)?

Feline Infectious Peritonitis ("FIP") is caused by a mutated form of the common feline coronavirus (FCoV). The original coronavirus is widespread and usually harmless, often causing only mild gut signs or none at all. FIP develops when that virus mutates in an individual cat and the immune system reacts with intense inflammation.

Feline coronavirus spreads easily between cats, especially in multi-cat homes, shelters, and catteries, mainly through shared litter boxes. It is important to understand that the harmless coronavirus is contagious, but the FIP mutation itself happens inside one cat and is not passed cat to cat as FIP.

Younger cats, cats in crowded environments, and cats under stress appear to be at higher risk. Still, FIP can affect a cat of any age or background.

What are the four forms of Feline Infectious Peritonitis (FIP)?

Feline Infectious Peritonitis ("FIP") takes four distinct forms, and each one shapes the signs you see and the treatment plan your veterinarian builds. The four forms are wet (effusive), dry (non-effusive), ocular, and neurological. Recognizing them early matters, because the form guides the GS-441524 dose.

Wet (effusive) Feline Infectious Peritonitis (FIP)

Wet, or effusive, FIP is marked by fluid buildup in the abdomen or chest. This is often the fastest-moving form.

Common signs include:

• A swollen, rounded belly from fluid in the abdomen

• Labored or rapid breathing when fluid collects in the chest

• Fever that does not respond to antibiotics

• Lethargy, poor appetite, and weight loss

Dry (non-effusive) Feline Infectious Peritonitis (FIP)

Dry, or non-effusive, FIP produces little or no fluid. Instead, inflammatory lesions form in organs, which can make it harder to spot and slower to diagnose.

Common signs include:

• Persistent fever and gradual weight loss

• Reduced appetite and low energy

• Signs that vary depending on which organs are affected

Ocular Feline Infectious Peritonitis (FIP)

Ocular FIP affects the eyes and is a separate form in its own right. The eye changes are often one of the first visible clues families notice.

Common signs include:

• A change in iris color

• Cloudiness in the eye

• Inflammation inside the eye (uveitis)

• Vision loss from lesions in the eye

Neurological Feline Infectious Peritonitis (FIP)

Neurological FIP affects the brain and nervous system and is also a distinct form. Because the virus reaches the central nervous system, treatment is dosed carefully.

Common signs include:

• Seizures

• Head tilt and loss of balance

• Partial paralysis or weakness

• Disorientation and behavior changes

For a stage-by-stage view of how symptoms progress, see our FIP symptoms checklist from early signs to advanced stages.

How is Feline Infectious Peritonitis (FIP) diagnosed?

Feline Infectious Peritonitis ("FIP") is diagnosed by your veterinarian using a combination of clinical signs, bloodwork, fluid analysis when fluid is present, and imaging. There is no single at-home test, so a professional workup is essential to confirm FIP and identify its form.

Because the four forms look different, your veterinarian will match the diagnosis to the form. That form then determines the correct GS-441524 dose, which is why getting an accurate diagnosis first is so important.

How is Feline Infectious Peritonitis (FIP) treated?

Feline Infectious Peritonitis ("FIP") is treated with GS-441524, the antiviral at the core of modern FIP care, given as a daily subcutaneous injection over a 12-week (84-day) course under veterinary supervision. The daily dose is matched to the form of FIP your cat has.

The schedule is one subcutaneous injection per day, 7 days a week, for the full 84 days. Here is how the dose maps to each form:

FIP form	GS-441524 dose	Schedule	Course
Wet (effusive)	6 mg/kg	1 injection/day, 7 days/week	12 weeks (84 days)
Dry (non-effusive)	8 mg/kg	1 injection/day, 7 days/week	12 weeks (84 days)
Ocular	10 mg/kg	1 injection/day, 7 days/week	12 weeks (84 days)
Neurological	10 mg/kg	1 injection/day, 7 days/week	12 weeks (84 days)

These dose levels are consistent with current clinical practice. Gokalsing et al. 2025 found the best outcomes in the 5 to 10 mg/kg once-daily range, adjusted for FIP type and for neurological or ocular signs, and Ansari Mood et al. 2025 reported mean starting doses in a similar range that were adjusted upward over treatment.

During treatment, your veterinarian will also advise avoiding certain products that can interfere with care, including fluoroquinolone antibiotics, spot-on flea medications, lysine, and immune boosters.

What does the evidence show for GS-441524?

The evidence behind GS-441524 is strong and specific. In Pedersen et al. 2019 (J Feline Med Surg 21(4):271-281), 24 of the 26 cats that completed injectable GS-441524 treatment remained healthy, a 92% success rate, meaning more than 9 out of 10 cats that completed injectable GS-441524 treatment stayed healthy. That study used 2.0 to 4.0 mg/kg and excluded severe neurological and ocular cases.

More recent work has looked at larger groups. Ansari Mood et al. 2025 (Sci Rep 16:1037) followed 629 cats on GS-441524 for at least 12 weeks and reported survival of 94.12%, meaning more than 9 out of 10 cats survived. In a systematic review of 11 studies and 650 cats, Gokalsing et al. 2025 (Pathogens 14(7):717) reported overall success of 84.6%, meaning more than 8 out of 10 cats were treated successfully.

These are statistics from real cats, not promises. Every cat is different, and your veterinarian will guide what to expect for yours. You can read more in our overview of the GS-441524 success rate and what the UC Davis FIP trial shows.

What GS-441524 products are available for Feline Infectious Peritonitis (FIP)?

For Feline Infectious Peritonitis ("FIP"), the injectable GS-441524 options differ by concentration, so your veterinarian can match the volume to your cat's weight and dose. Here are the current listings:

Product	Strength	Size	Price
GS-441524 Antiviral Injectables	15mg/ml, 8ml	15 mg/ml	8ml	$49.00
GS-441524 Antiviral Injectables	20mg/ml	20 mg/ml	8ml, 10ml	$69.00
GS-441524 Antiviral Injectables	30mg/ml	30 mg/ml	8ml, 10ml	$89.00
GS-441524 Antiviral Injectables, 40mg/ml	40 mg/ml	8ml, 10ml	$119.00

For cats that may benefit from added support, GS-441524 Antiviral with Vitamin B12 | 20mg/ml (20 mg/ml + B12 50mcg/ml) is $75.00 and GS-441524 Antiviral with Vitamin B12 | 30mg/ml (30 mg/ml + B12 50mcg/ml) is $95.00. The B12 is an addition, not a separate antiviral, and the dosing and indications are the same as the standard injectable.

The oral capsule option for Feline Infectious Peritonitis (FIP)

For families who want to move away from daily injections, the GS-441524 + EIDD-1931 oral capsule (Cure FIP™ Oral Capsules, $129.00) has one standout advantage: one capsule a day, chosen simply by your cat's weight class. There is no mg math and no counting multiple pills. One capsule for one weight class.

Weight class	Daily dose
0 to 2.5 kg (0 to 5.5 lb)	1 capsule per day
2.5 to 5 kg (5.5 to 11 lb)	1 capsule per day
over 5 kg (over 11 lb)	1 capsule per day

The dual capsule is usually started after 2 to 4 weeks of GS-441524 injections, or once the cat is stable, which is common practice including in ocular and neurological cases. The recommended course is 12 weeks. In our own data from about 46 treated cats, the dual antiviral GS-441524 + EIDD-1931 reached 78.3% remission, meaning about 8 out of 10 cats on the dual antiviral reached remission.

The dual capsule is not for cats with vomiting or diarrhea, cats under 1 year old with wet (effusive) FIP, cats that cannot eat or defecate normally, or pregnant, nursing, or breeding cats, because it contains EIDD-1931. Your veterinarian will confirm whether it is a good fit.

How much does Feline Infectious Peritonitis (FIP) treatment cost?

The cost of Feline Infectious Peritonitis ("FIP") treatment depends on your cat's weight, the form of FIP, and the product chosen, because dosing and volume vary. Injectable GS-441524 ranges from $49.00 for the 15 mg/ml, 8ml vial to $119.00 for the 40 mg/ml option, and the Cure FIP™ Oral Capsules are $129.00.

We are transparent about one thing up front: there are no guarantees in medicine, and the figures we quote are statistical success rates, not promises. The only guarantee we offer is that you can cancel any time before your order ships for a full refund.

FAQ about Feline Infectious Peritonitis (FIP)

Is Feline Infectious Peritonitis (FIP) contagious to other cats?

The common feline coronavirus that can lead to Feline Infectious Peritonitis ("FIP") is contagious between cats, mainly through shared litter boxes, but the FIP mutation itself happens inside one cat and is not spread cat to cat as FIP. Good litter hygiene and reducing stress and crowding help lower coronavirus spread in multi-cat homes.

Can Feline Infectious Peritonitis (FIP) be cured?

Feline Infectious Peritonitis ("FIP") is treatable today with GS-441524, and many cats go on to live healthy lives after a full course. These outcomes are statistical success rates rather than guarantees, and every cat is different, so your veterinarian is the best guide to what to expect for yours.

How long is the treatment protocol for Feline Infectious Peritonitis (FIP)?

The standard GS-441524 injectable protocol for Feline Infectious Peritonitis ("FIP") is 12 weeks, which is 84 days, with one subcutaneous injection per day, every day. Your veterinarian may adjust the plan based on the form of FIP and your cat's response.

What are the first signs of Feline Infectious Peritonitis (FIP) to watch for?

Early signs of Feline Infectious Peritonitis ("FIP") often include a fever that does not respond to antibiotics, reduced appetite, weight loss, and low energy, with form-specific signs such as a swollen belly, eye color change, or balance problems appearing as the disease progresses. Any of these warrants a prompt veterinary visit.

Does the form of FIP change the GS-441524 dose?

Yes. For Feline Infectious Peritonitis ("FIP"), the GS-441524 dose is matched to the form: 6 mg/kg for wet, 8 mg/kg for dry, and 10 mg/kg for both ocular and neurological FIP, each once daily for 12 weeks under veterinary supervision.

If your cat has just been diagnosed, you are not out of options, and you are not alone. Learn more about the treatment choices and reach out to our team or your own veterinarian by visiting CureFIP's FIP treatment resources, where you can review the protocols and ask your questions. The sooner the right plan begins, the sooner your cat can get back to eating, playing, and curling up at home.