What Is FIP in Cats? A Plain English Guide for Cat Parents

Key takeaway: Feline Infectious Peritonitis (FIP) is a serious viral disease caused when a common feline coronavirus mutates inside a cat's body. Once considered fatal, FIP is now treatable with GS-441524, the antiviral at the core of modern FIP treatment, which reported a 92% success rate in the UC Davis study (Pedersen, 2019). Treatment runs as an 84-day protocol under veterinary supervision, and more than 100,000 cats have been treated since 2019.

If your veterinarian has raised the possibility of FIP, it is normal to feel frightened. This guide explains what FIP is, how it shows up, and what the evidence says about treatment, in clear language you can act on.

A cat resting comfortably at home.

What is FIP in cats?

Feline Infectious Peritonitis (FIP) is a viral disease that develops when a common, usually harmless feline enteric coronavirus mutates inside an individual cat and triggers an aggressive immune response. FIP is not the same as the mild intestinal coronavirus that many cats carry without ever getting sick.

The mutation happens inside one cat's own body. This is why FIP is not spread cat to cat in the way a cold is, even though the underlying coronavirus is common in multi-cat homes and shelters.

FIP was long regarded as almost always fatal. That has changed. GS-441524 based antiviral treatment has reshaped outcomes, and you can read more about how effective GS-441524 is for FIP when you are ready for the detail.

What causes FIP in cats?

FIP is caused by a mutated form of feline coronavirus (FCoV). Many cats carry the ordinary intestinal version with few or no symptoms, but in a small percentage the virus changes and spreads through white blood cells, causing the inflammatory disease we call FIP.

Several factors raise the risk. Young cats under two years, cats from crowded or high-stress environments, and cats with weakened immune systems are diagnosed more often.

Because the ordinary coronavirus is widespread, exposure alone does not mean a cat will develop FIP. FIP is the outcome of a specific mutation in one animal, not a routine infection everyone catches.

What are the symptoms of FIP in cats?

Early FIP symptoms are often vague: a fever that does not respond to antibiotics, low appetite, weight loss, lethargy, and poor coat condition. As the disease progresses, more specific signs appear depending on which form of FIP develops.

FIP takes four forms, wet, dry, ocular, and neurological, and each shapes the treatment plan. Below, each form is described in its own section so you can match what you are seeing to the right category. If FIP is suspected, what to do when your vet suspects FIP walks through the next practical steps.

Wet (Effusive) FIP

Wet FIP, also called effusive FIP, is marked by fluid buildup in body cavities. You may notice a swollen, rounded belly, or labored breathing if fluid collects around the lungs.

Wet FIP often progresses faster than the dry form because the fluid accumulation is visible and worsens quickly. A distended abdomen that feels full of liquid is one of the more recognizable signs.

Dry (Non-effusive) FIP

Dry FIP, also called non-effusive FIP, produces little or no fluid. Instead, inflammatory masses called granulomas form in organs such as the liver, kidneys, and lymph nodes.

Dry FIP can be harder to spot because the signs are subtle and slow. Persistent fever, gradual weight loss, and vague illness are common, and jaundice in cats can appear when the liver is involved.

Ocular FIP

Ocular FIP affects the eyes and may occur on its own or alongside other forms. Watch for cloudiness, color changes in the iris, an inflamed appearance, or changes in the pupil.

Ocular signs matter because they can indicate the virus has reached tissues that require a higher antiviral dose. Any sudden change in your cat's eyes should be examined by your veterinarian promptly.

Neurological FIP

Neurological FIP affects the brain and spinal cord. Signs include wobbliness or loss of balance, tremors, seizures, unusual behavior, and weakness in the hind limbs.

Neurological FIP is treated at the highest dosing tier because the antiviral must cross the blood brain barrier to reach the affected tissue. Prompt veterinary assessment is essential when these signs appear.

How is FIP diagnosed in cats?

There is no single test that confirms FIP on its own, so veterinarians combine clinical signs with several results to build a picture. Common tools include bloodwork, fluid analysis when effusion is present, imaging, and specific coronavirus testing.

Bloodwork often shows telling patterns, such as anemia, an altered albumin to globulin ratio, and raised inflammatory markers. Tracking these numbers also matters during treatment, and what your cat's blood work is really telling you explains how the values move as a cat recovers.

Because FIP can mimic other diseases, an accurate diagnosis depends on your veterinarian interpreting the full set of results together. FIP is sometimes caught late, and it is one of the feline viruses vets diagnose too late when early signs are quiet.

Can FIP in cats be treated?

Yes. FIP is now treatable with GS-441524, an antiviral that stops the virus from replicating, and treatment has moved FIP from an almost always fatal diagnosis to a condition many cats survive. The UC Davis study reported a 92% success rate for GS-441524 (Pedersen, 2019).

Treatment is not instant and it is not casual. A standard course runs 84 days under veterinary supervision, with daily dosing throughout.

We are clear about what the data shows and what it does not. The 92% figure comes from GS-441524 monotherapy delivered by injection, and it is the stat matched to that treatment route.

How does GS-441524 treatment work?

GS-441524 works by blocking the FIP virus from copying itself, which gives the cat's body room to recover. It is given daily for the full 84-day protocol, and the dose is set by body weight and by which form of FIP is present.

CureFIP offers GS-441524 in injectable form across several concentrations, and the dose per kilogram rises with the severity and location of the disease. The table below shows the by-type dosing exactly as listed for the injectable products.

FIP form	GS-441524 dose	Schedule	Duration
Wet	6 mg/kg	1 subcutaneous injection per day	84 days (12 weeks)
Dry	8 mg/kg	1 subcutaneous injection per day	84 days (12 weeks)
Ocular	10 mg/kg	1 subcutaneous injection per day	84 days (12 weeks)
Neurological	10 mg/kg	1 subcutaneous injection per day	84 days (12 weeks)

Reference for the injectable dosing and schedule: Pedersen et al., UC Davis (PMC6435921). Your veterinarian will confirm the exact concentration and volume for your cat's weight.

Injectable strengths and prices

CureFIP lists GS-441524 Antiviral Injectables in several concentrations so dosing can match your cat's size:

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

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

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

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

Two versions add vitamin B12: GS-441524 Antiviral with Vitamin B12 20mg/ml at $75.00 and 30mg/ml at $95.00. All follow the same by-type dosing and the same 84-day protocol.

Is there an oral option for FIP treatment?

Yes. For cats who tolerate them, CureFIP offers Cure FIP™ Oral Capsules at $129.00, a dual antiviral route that combines GS-441524 with EIDD-1931 and is positioned for wet and dry FIP. Dosing is by weight band, not by type.

Weight band	GS-441524	EIDD-1931
Under 5.5 lb	25 mg	5 mg
5.5 to 11 lb	35 mg	8 mg
Over 11 lb	50 mg	12 mg

The capsules are given as one capsule per day for a recommended 12 weeks. Some regions note the oral dual route is not recommended once ocular or neurological signs are present, or if a cat cannot eat or defecate, so your veterinarian should guide the choice.

The dual antiviral approach was studied in relapsed cases. A dual antiviral protocol reached 78.3% remission in previously relapsed FIP cats (Li and Cheah, 2025), and you can read why the dual antiviral option is stronger for certain cats to understand where it fits.

Injectable vs oral: which route fits?

The injectable and oral routes suit different situations, and the right choice depends on your cat's form of FIP, size, and tolerance. The table compares them using catalog figures only.

Route	Best matched to	Efficacy stat	Dosing basis
GS-441524 injectables	Wet, dry, ocular, neurological FIP	92% success rate, UC Davis (Pedersen, 2019)	By type, 6 to 10 mg/kg
Cure FIP™ Oral Capsules (dual antiviral)	Wet and dry FIP	78.3% remission, Li and Cheah 2025 (relapsed cases)	By weight band

These two figures are not interchangeable and should never be blended. Each stat belongs to its own product and protocol, and your veterinarian will help you choose the route that fits your cat.

FAQ

Is FIP in cats contagious to other cats or people?

FIP itself is not readily spread from cat to cat, because it develops from a mutation inside one individual cat rather than from passing the disease along. The underlying feline coronavirus is common and can be shared, especially in multi-cat homes, but most exposed cats never develop FIP. FIP does not infect humans.

How long does FIP treatment take?

A standard course runs 84 days (12 weeks) under veterinary supervision, with dosing given every day. Your veterinarian monitors progress with regular checkups and bloodwork and confirms when a cat reaches remission.

What is the success rate of GS-441524 for FIP?

The UC Davis study reported a 92% success rate for GS-441524 monotherapy by injection (Pedersen, 2019). A separate dual antiviral protocol combining GS-441524 and EIDD-1931 reported 78.3% remission in previously relapsed cats (Li and Cheah, 2025). The figures apply to different treatments and are not averaged together.

Which form of FIP is the most serious?

Neurological and ocular FIP reach sensitive tissues and are treated at the highest dosing tier, 10 mg/kg for the injectables, because the antiviral must reach the eyes or nervous system. Wet FIP can progress quickly due to fluid buildup. Your veterinarian will determine the form and set the dose accordingly.

How much does FIP treatment cost with CureFIP?

CureFIP GS-441524 Antiviral Injectables range from $49.00 for the 15mg/ml, 8ml presentation to $119.00 for the 40mg/ml, and Cure FIP™ Oral Capsules are $129.00. The total for a full 84-day protocol depends on your cat's weight and form of FIP, which your veterinarian can help you calculate.

A FIP diagnosis is frightening, but you do not have to face it without clear information or support. When you are ready, explore the FIP treatment options at CureFIP and talk them through with our team and your own veterinarian, who can build the plan that fits your cat best.