FIP in Cats

The Disease That Used to Be a Death Sentence

Feline Infectious Peritonitis, or FIP, is one of those diagnoses that historically struck fear into the hearts of veterinarians and cat owners alike. For many years, a diagnosis of FIP was essentially considered a death sentence.

Thankfully, that has changed dramatically.

FIP is caused by feline coronavirus (FCoV). Feline coronavirus itself is extremely common, particularly in multi-cat households, shelters and catteries. Most cats exposed to feline coronavirus do not develop FIP.

So how does FIP happen?

In some cats, the coronavirus undergoes changes within the individual cat that allow it to behave very differently. The virus can then infect and replicate within certain immune cells, resulting in the severe inflammatory disease we recognize as FIP.

Importantly, having feline coronavirus does not automatically mean a cat has FIP, and FIP itself is not considered to spread between cats in the same way ordinary feline coronavirus does.

What does FIP look like?

FIP can be incredibly challenging because it can look like many other diseases.

Cats may develop:

* Persistent or recurrent fever

* Lethargy and depression

* Poor appetite or weight loss

* Anemia

* Jaundice

* Fluid accumulation in the abdomen or chest

* Eye inflammation or other ocular changes

* Neurologic abnormalities

* Enlarged lymph nodes

* Changes in kidney or liver function

Traditionally, we have talked about “wet” and “dry” FIP, although the disease is actually much more complicated than those two categories.

Effusive FIP occurs when inflammatory fluid accumulates in the abdomen or chest.

Non-effusive FIP may produce granulomatous lesions in organs without large amounts of fluid.

And some cats have a combination of both.

Neurologic and ocular FIP can be particularly challenging to diagnose and treat especially with the cost and scarcity of advanced imagine such as CT and MRI imaging in veterinary medicine.

How do we diagnose FIP?

This is one of the most important things I want cat owners to understand:

There is not one simple blood test that can definitively diagnose every case of FIP.

Diagnosis is generally based on putting together the entire picture—history, physical examination, bloodwork, imaging, fluid analysis when available, and sometimes cytology, PCR, or tissue testing.

Bloodwork may show findings such as elevated globulins, low albumin, anemia, changes in white blood cells, or abnormalities in liver and kidney parameters. These findings can raise our suspicion, but they are not specific for FIP.

If fluid is present, analyzing that fluid can provide extremely valuable information.

And when we can safely obtain affected tissue, histopathology and additional testing may provide stronger evidence.

The really important news…

For years, veterinarians were often left with very few options once FIP became highly suspected.

That is no longer the case.

Antiviral therapy has completely changed the FIP conversation.

In Part 2, we’ll talk about the medications that have changed FIP treatment—including GS-441524, remdesivir and molnupiravir—and what these treatments mean for cats and their families.

A diagnosis of FIP should no longer automatically mean that we have to give up.

FIP Treatment Has Changed Everything

If you have been following veterinary medicine for a while, you probably remember when FIP was one of the most heartbreaking diagnoses we could make.

Today, the conversation is very different.

Antiviral medications have transformed FIP from a disease that was historically considered almost uniformly fatal into a disease that is now treatable and frequently curable.

GS-441524

The biggest breakthrough has been GS-441524, an antiviral medication that interferes with coronavirus replication.

It is currently the most extensively studied antiviral for FIP, and published treatment experience has demonstrated very high rates of clinical remission and survival.

In the United States, compounded oral GS-441524 became available through veterinary prescribing pathways in 2024. The FDA has stated that, because there is no FDA-approved drug available to treat FIP, it does not intend to enforce certain new-animal-drug approval requirements for qualifying patient-specific compounded GS-441524 prescribed by a veterinarian under its compounding guidance. It is important to distinguish this from saying that the compounded product itself is FDA-approved—it is not.

This is a huge improvement over the earlier situation, when desperate owners frequently had to turn to unregulated sources for medication of uncertain concentration and purity.

What about remdesivir?

Remdesivir is another antiviral option. It is a prodrug that is converted in the body to GS-441524.

It can be particularly useful when a cat is extremely ill and cannot tolerate oral medication. Injectable remdesivir can be used initially, with oral antiviral therapy often substituted once the cat is stable enough to take medication by mouth.

There is also emerging evidence supporting oral remdesivir as a treatment option. A recent randomized clinical trial in cats with non-effusive FIP found oral remdesivir met the study’s criteria for non-inferiority compared with oral GS-441524 for survival and disease-free remission at 16 weeks.

What about molnupiravir?

Another antiviral being investigated and used in some treatment protocols is molnupiravir, also known as EIDD-2801.

It works differently from GS-441524 and remdesivir and may have an important role in certain situations, including some cases where cats relapse or do not respond adequately to an initial antiviral.

However, availability, legal status, formulation and treatment protocols vary considerably by country, and I am not comfortable with this potential option yet.

How long does treatment take?

Historically, 84 days (12 weeks) of antiviral treatment became the standard protocol.

But this is another area that is evolving.

Newer evidence suggests that some cats may successfully complete treatment with shorter courses, including 42-day protocols, although treatment duration still needs to be individualized based on the type and severity of FIP and the cat’s response to therapy.

This is why FIP treatment should not simply be approached as:

“Give this drug for this many days and you’re done.”

Cats need monitoring throughout treatment, including their clinical response, weight, appetite and laboratory parameters. Treatment may need to be adjusted depending on response.

Supportive care still matters

Antivirals are the cornerstone of treatment, but severely ill cats may also need supportive care, both during treatment, but also while waiting on this medication. Remember that. Compounded medication is not just sitting on our pharmacy shelves. It has to be ordered for a specific patient, compounded by the pharmacy and the n shipped to our hospital. But there is still a lot we can do to get started.

This can include:

* Fluids

* Nutritional support

* Management of nausea

* Treatment of secondary complications

* Drainage of significant chest or abdominal effusions when appropriate

* Blood transfusions in selected cases

* Treatment of neurologic or ocular complications

The antiviral treats the virus, but we still have to support the patient while the body recovers from the damage caused by the disease.

One important caution

Because these medications have revolutionized FIP treatment, there is understandably a tremendous amount of information—and misinformation—circulating online.

Please don’t try to navigate FIP treatment entirely on your own.

Drug concentration, dosing, formulation, route of administration, treatment duration and monitoring matter. Antiviral resistance is also an area of active research, particularly with widespread antiviral use.

The good news is that we now have legitimate veterinary treatment pathways that didn’t exist just a few years ago.

So if your cat is diagnosed with—or strongly suspected of having—FIP, please don’t assume that there is nothing we can do.

There is hope. And as veterinarians, we now have something incredibly important that we didn’t have before.

Dr. Cassie Floral City Animal Clinic, Inverness, Fl ©Copyrights 2026