FIP in Cats: Complete Guide to Symptoms and Treatment

The vet calls back. Your stomach drops. They use three letters you have never heard before: FIP. Within an hour, you are deep in forums, screenshotting lab values, comparing protocols, and trying to understand whether your cat is going to be okay.

FIP in Cats: Complete Guide to Symptoms and Treatment

Don't panic. Take a breath. Feline Infectious Peritonitis (FIP) is no longer the diagnosis it was a decade ago. Thanks to the antiviral GS-441524, more than 100,000 cats have been treated since 2019, and the conversation has shifted from "how long do we have" to "which protocol fits your cat."

This is the pillar guide. It walks you through what FIP actually is, the symptoms to watch for, how veterinarians arrive at a diagnosis, and how modern GS-441524 treatment works in the United States in 2026.

What FIP in Cats Actually Is

FIP is a viral disease caused by a mutated form of feline coronavirus (FCoV). Most cats encounter feline coronavirus at some point, especially in multi-cat households, shelters, and catteries. In the vast majority of cases, the virus stays in the gut, causes mild or no symptoms, and resolves on its own.

In a small percentage of cats, however, the virus mutates inside the body. Once mutated, it stops behaving like a simple gut infection and starts replicating inside immune cells called macrophages. That mutation is what triggers FIP.

This is why FIP is not contagious in the traditional sense. Your cat did not "catch FIP" from another cat. They were exposed to feline coronavirus, and in their body, that virus changed.

Who Gets FIP

FIP can appear in any cat, but it most commonly affects:

1. Kittens and young cats under two years of age

2. Senior cats with weakened immune systems

3. Purebred cats (Ragdolls, Bengals, Persians, British Shorthairs)

4. Cats from high-density environments such as shelters or breeders

5. Cats under recent stress (rehoming, surgery, new household members)

The Two Classic Forms of FIP

Veterinarians traditionally divide FIP into two presentations. In real life, many cats show a mix of both.

Wet FIP (Effusive)

Wet FIP is defined by fluid accumulation. The mutated virus damages blood vessels, and protein-rich fluid leaks into body cavities.

Common signs include:

• A swollen, distended abdomen (often described as "pot-bellied")

• Labored breathing if fluid collects in the chest

• Yellow, viscous fluid drawn on abdominocentesis

• Rapid weight loss in the limbs and spine, even as the belly grows

Wet FIP tends to progress quickly. It is often the more visually dramatic form, and it is what brings many cat parents into the emergency clinic.

Dry FIP (Non-Effusive)

Dry FIP is the quieter, slower form. Instead of fluid, the virus drives the formation of granulomas, small inflammatory lesions, on internal organs.

Signs can include:

• Persistent low-grade fever that does not respond to antibiotics

• Weight loss and reduced appetite

• Eye changes (uveitis, color shifts in the iris, cloudiness)

• Neurological signs (wobbliness, seizures, behavioral changes)

• Enlarged lymph nodes or palpable masses on organs

Neurological and ocular FIP are sub-forms of dry FIP and require longer, more carefully managed treatment protocols because the virus crosses into the eye and central nervous system.

Symptoms of FIP in Cats: What to Watch For

FIP is notorious for mimicking other illnesses. Here is the symptom pattern that most often shows up in early veterinary visits:

1. A fever that comes and goes and does not respond to standard antibiotics

2. Progressive lethargy, your cat is hiding more, playing less

3. Inappetence and weight loss

4. Jaundice (yellowing of the gums, ears, or eyes)

5. Abdominal distension or difficulty breathing

6. Eye changes, especially in young cats

7. Neurological signs in advanced cases

If two or more of these signs appear together, and especially if your cat is under two years old or recently came from a multi-cat environment, ask your veterinarian directly about FIP.

How FIP Is Diagnosed

There is no single test that says "yes, this is FIP" with 100% certainty in a living cat. Diagnosis is built from a combination of clinical signs, bloodwork, imaging, and targeted tests.

Bloodwork Clues

Veterinarians look at the chemistry panel for a familiar pattern:

• Elevated globulins and a low albumin to globulin (A:G) ratio (often under 0.6)

• Mild to moderate non-regenerative anemia

• Elevated bilirubin and liver enzymes

• Lymphopenia (low lymphocyte counts)

• Elevated SAA (serum amyloid A) on inflammatory markers

Here is what most cat owners don't find out until later: no single value is diagnostic. It is the pattern that matters.

Fluid Analysis

If your cat has effusion, the fluid itself is one of the most informative samples available. FIP effusion is typically:

• Yellow and viscous

• High in protein (often over 3.5 g/dL)

• Low in cell count, with a mixed population of macrophages and neutrophils

• Positive on a Rivalta test

PCR and Immunostaining

PCR testing on effusion, tissue, or aspirates can detect feline coronavirus RNA. Immunocytochemistry or immunohistochemistry can confirm coronavirus antigen inside macrophages, which is highly supportive of FIP.

Imaging

Ultrasound and radiographs help identify effusion, enlarged lymph nodes, kidney changes, and organ granulomas. In suspected neurological FIP, MRI is sometimes used.

Dr. Niels Pedersen and the team at UC Davis have published extensively on this diagnostic puzzle, and modern U.S. veterinary practice draws heavily on their work.

GS-441524: The Hero Antiviral

GS-441524 is the small-molecule antiviral nucleoside analog that changed everything for FIP. It works by inserting itself into the viral RNA replication process, stopping the mutated coronavirus from making functional copies of itself.

In the landmark UC Davis field trial led by Dr. Niels Pedersen in 2019, GS-441524 injectable monotherapy achieved a 92% success rate in cats with FIP. That number reframed FIP from a terminal diagnosis into a treatable viral disease.

GS-441524 is the hero ingredient of CureFIP. It is the foundation of every protocol we build around, and the reason more than 100,000 cats have been treated since 2019.

How GS-441524 Treatment Works

The modern protocol is typically built around an 84-day treatment course. The general framework looks like this:

1. Daily antiviral dosing for 84 consecutive days

2. Weekly or biweekly weight checks, because dose is weight-based

3. Periodic bloodwork to track recovery (A:G ratio, SAA, hematocrit, globulins)

4. An 84-day observation period after treatment ends, during which relapse is monitored

Dosing and product selection should always be coordinated with a veterinarian familiar with FIP. Because individual cats vary by weight, form of FIP (wet, dry, ocular, neurological), and underlying labs, there is no single dose that fits every cat. Your veterinarian will tailor the protocol to your cat's specifics.

Dual Antiviral Protocols

For some cases, especially neurological FIP, relapses, or cats that need oral-only treatment, a dual antiviral approach combining GS-441524 with EIDD-1931 has shown strong results. The Li and Cheah 2025 study reported 78.3% remission across 46 cats on dual antiviral therapy.

Which protocol is right for your cat depends on form, severity, and prior treatment history. This is a decision to make with your vet, not from a forum thread at 2 a.m.

What Recovery Looks Like

Recovery on GS-441524 is often faster than cat parents expect, and that speed can be misleading.

The First Two Weeks

Most cats show visible improvement within 3 to 7 days:

• Fever resolves

• Appetite returns

• Energy levels climb

• Effusion begins to reabsorb in wet cases

This early turnaround is encouraging, but it is not the finish line. The virus is still present in the body, and stopping treatment early is the single biggest cause of relapse.

Weeks 3 Through 12

This is the long middle. Your cat may look completely normal. Their bloodwork is improving. They are gaining weight. The temptation to ease off is real.

Don't. The full 84-day protocol exists for a reason. Cutting it short, even by a week, can mean restarting the entire course at a higher dose.

Post-Treatment Observation

After day 84, your cat enters an 84-day observation window. No antivirals, just monitoring. Most relapses, if they happen, occur in this window. A clean bloodwork panel at day 84 plus day 168 is considered functional remission.

Supporting Your Cat Through Treatment

Antiviral therapy is the engine of FIP treatment, but supportive care is the chassis.

1. Feed a high-protein, high-calorie diet to rebuild lost muscle

2. Keep stress low (limit travel, new pets, household upheaval)

3. Monitor for jaundice or new neurological signs and report them immediately

4. Track weight weekly, dose adjustments depend on it

5. Run bloodwork at the intervals your vet recommends

6. Liver support may be discussed with your veterinarian for cats with elevated enzymes

What the Numbers Are Really Telling You

When you read "92%" or "78.3%" in FIP literature, these are not interchangeable figures.

• 92% reflects GS-441524 injectable monotherapy in the original UC Davis cohort (Pedersen 2019).

• 78.3% reflects dual antiviral therapy combining GS-441524 with EIDD-1931 across 46 cats (Li and Cheah 2025).

Different protocols, different cohorts, different clinical scenarios. The right number for your cat is the one tied to the protocol your veterinarian recommends.

Where CureFIP Fits

CureFIP is a U.S. focused, GS-441524 based antiviral platform built around the realities American cat parents face: fast access, veterinary coordination, transparent science, and protocol support that lasts the full 84 days and beyond.

More than 100,000 cats have been treated through the broader GS-441524 network since 2019. The science is no longer new. The protocols are no longer experimental in the colloquial sense. What remains essential is calm, evidence-based guidance and a veterinarian in your corner.

If you are at the start of this journey, visit [curefipusa.com](https://curefipusa.com) to learn more about treatment options and to coordinate with your veterinary team.

FAQ

Is FIP in cats always fatal?

No, not anymore. Before 2019, FIP was considered almost universally fatal. With GS-441524 antiviral therapy, the outlook has shifted dramatically. The original UC Davis field trial reported a 92% success rate with GS-441524 monotherapy, and more than 100,000 cats have been treated since.

How long does FIP treatment take?

The standard protocol is 84 consecutive days of daily antiviral dosing, followed by an 84-day observation period. Stopping early is the most common cause of relapse, so completing the full course is essential.

Can my other cats catch FIP from a sick cat?

FIP itself is not directly contagious. The underlying feline coronavirus can spread between cats, but only a small percentage develop the mutation that causes FIP. Your other cats are not at high risk simply because one cat has been diagnosed.

What is the difference between GS-441524 and EIDD-1931?

GS-441524 is a nucleoside analog antiviral and the hero ingredient of CureFIP. EIDD-1931 is a complementary antiviral used in dual antiviral protocols, particularly for neurological FIP and relapse cases. The two work through related but distinct mechanisms.

When should I ask my vet about FIP specifically?

If your cat has a fever that does not resolve with antibiotics, especially combined with weight loss, lethargy, abdominal swelling, eye changes, or neurological signs, ask your veterinarian directly about FIP. Early diagnosis improves outcomes, and modern testing makes confirmation faster than it used to be.