FIP In Cats: Signs, Diagnosis, Treatment And Prognosis

Written by Dr Leigh Davidson BVSc, BApplSc

Feline infectious peritonitis (FIP) is a serious and often fatal viral disease that affects cats and is caused by certain strains of the feline coronavirus (FCoV) family.

While most cats that become infected with FCoV mount a good immune response and eliminate the virus, others will go on to develop FIP.

FIP is a systemic vasculitis that mainly affects the young, with over 70% of cases being 1yr old or less. This however doesn’t mean that older cats cannot develop FIP.

While FIP is a complex and poorly understood disease and can be difficult to diagnose and treat, not all hope is lost with new treatment options available today.

Read on to learn more about FIP, how it is caught, signs, treatments and prevention.

What Are The Signs Of FIP In A Cat?

Cats with FIP may exhibit a range of signs that will vary depending on the form of the disease and the severity of the infection.

There are two forms of FIP:

1. Effusive ‘wet’ FIP
2. Non-effusive ‘dry’ FIP

Effusive ‘Wet’ FIP

Cats with wet FIP typically have fluid accumulation in the abdomen (ascites) or chest (pericardial or pleural effusion) which can cause abnormal abdominal swelling and breathing difficulties.

Often cats will have a fever, loss of appetite, weight loss and lethargy.

Wet FIP is often more severe and has a poorer prognosis than dry FIP.

Non-effusive ‘Dry’ FIP

Cats with dry FIP typically have chronic pyogranulomatous inflammation in various organs such as the liver, spleen or kidneys.

They too will often have a fever, loss of appetite, weight loss and lethargy.

Dry FIP may also progress to wet FIP over time.

How Is FIP Diagnosed?

FIP can be challenging to diagnose as the signs and symptoms of the disease are often non-specific and may resemble other feline diseases.

There is no single simple blood test that can confirm or rule out FIP in every cat.

Diagnosis is usually made by combining the cat’s age, history, clinical signs, blood results, imaging findings, effusion analysis and, where possible, direct testing for feline coronavirus in macrophages or affected tissue.

Radiographs (xrays) and ultrasonography are often utilised to help rule out other possible causes of the signs your cat is showing.

Blood changes that may increase suspicion for FIP include high globulins, low or low normal albumin, a low albumin to globulin ratio (albumin:globulin >0.8), non regenerative anaemia, lymphopenia, neutrophilia and sometimes raised bilirubin.

Analysis of fluid effusions will show:

High protein content often greater than 35g/L

Cytology will often show macrophages and neutrophils (pyogranulomatous inflammation).

Immunocytochemistry of the effusion fluid can detect FCoV within monocytes and macrophages. If these virus particles are found it means the cat has FIP, however not all samples will contain the viral antigen so a negative result doesn’t necessarily rule out a diagnosis of FIP.

RT-PCR of the effusion fluid is occasionally used to test for FIP however this is not a preferred test in Australia.

Often vets will utilise a quick, cheap in-clinic test called the Rivalta’s test on effusions.

The Rivalta’s test has a good sensitivity for FIP at 91-100% which means that if it is negative, then the likelihood of the cat having another disease causing the effusion is much more likely than it being caused by FIP.

A positive result could indicate another disease process that produces a high protein count such as lymphoma or peritonitis/pleuritis, however, these diseases can be easily diagnosed by cytology and culture.

Ultrasound is often performed to assess for fluid accumulation and the presence of abnormal lesions in organs and lymph nodes.

A fine needle aspiration (FNA) can easily be performed to gather cells for further testing.

What Is The Treatment For FIP?

For many years, FIP was one of the most heartbreaking diagnoses a cat owner could receive.

Before 2019, feline infectious peritonitis was widely considered almost always fatal. Supportive care could help cats feel better for a short time, but it did not stop the disease.

Thankfully, that has changed.

Dr Niels Pedersen of UC Davis has been instrumental in the vast amount of research on FIP in cats over the past 40-plus years and along with colleagues discovered that GS-441514 the active ingredient in Remdesivir was able to cure infected cats.

Since the work in 2019 that discovered this medication, thousands of cats with FIP from around the world have been treated with a cure rate of just over 90% (1. Jones et al.,2021).

The two antiviral drugs most often discussed in FIP treatment are GS-441524 and remdesivir.

GS-441524 is the active metabolite of remdesivir. In simple terms, it is the part of the drug that helps block viral replication. Remdesivir is an antiviral medication that was initially developed for the treatment of Ebola virus disease in humans.

Can Treatment Start Before FIP Is Fully Confirmed?

Sometimes, yes.

FIP can be difficult to diagnose with one single test.

In some cats, especially those that are very unwell, your vet may recommend starting treatment when the history, clinical signs, blood results, imaging and fluid analysis are strongly suggestive of FIP.

This is because waiting too long can reduce the chance of a good outcome.

That does not mean every sick cat should be treated for FIP.

It means your vet will weigh up the likelihood of FIP, the severity of your cat’s illness, and whether there are other possible explanations for the signs.

GS-441524 For FIP In Cats

Oral GS-441524 is now commonly used as a first line antiviral treatment for FIP where it is legally available.

It is usually better tolerated than repeated subcutaneous injections (Remdesivir), which can be painful for cats.

For many cats, oral treatment also reduces stress for both the cat and the owner.

Published studies and clinical reports have shown encouraging results, with many reporting survival or remission rates above 80%, and several reporting outcomes above 90%.

Results vary depending on how sick the cat is when treatment begins, the form of FIP, whether there is eye or neurological involvement, and whether the cat responds as expected.

Current Dose Recommendations For Oral GS-441524

Dose protocols vary between countries, suppliers and treating veterinarians.

The following doses are examples of protocols used in current veterinary treatment discussions.

Effusive or wet FIP

10 mg/kg once daily by mouth

Non effusive or dry FIP

15 to 20 mg/kg once daily by mouth

Ocular FIP

15 mg/kg once daily by mouth

General current protocol

15 mg/kg once daily by mouth is now commonly used in many cats.

Some cats may require higher or twice daily dosing. Doses of 10 to 15 mg/kg twice daily may be considered in cats that are not responding as expected, or in cats with ocular or neurological FIP.

Your cat’s dose must be based on current bodyweight. This matters because many cats gain weight quickly once treatment starts working. If the dose is not adjusted as the cat grows or gains weight, the cat may become underdosed.

This is especially important in kittens.

How Long Is FIP Treatment With GS-441524?

For many years, the standard treatment course for FIP was 84 days, or 12 weeks.

This is still commonly used, especially for cats with dry FIP, ocular FIP, neurological FIP, severe disease, slow clinical response, or blood results that take longer to normalise.

More recent research suggests that some cats with effusive FIP may respond well to a shorter 42 day course of oral GS-441524.

That does not mean every cat should stop treatment at 42 days.

The decision to stop treatment should be based on the individual cat, including:

• return of appetite
• weight gain
• normal temperature
• resolution of fluid
• improved energy
• improvement in eye or neurological signs
• normalising blood results
• albumin and globulin trends
• bilirubin trends
• serum amyloid A, where available

Most treatment failures occur when treatment is stopped too early, the dose is too low, the cat gains weight but the dose is not adjusted, medication is missed, or the diagnosis is not correct.

Remdesivir For FIP In Cats

Injectable remdesivir is still very useful, especially in cats that are severely ill.

Your vet may recommend remdesivir when:

• your cat is too sick to take tablets or liquid medication
• your cat is vomiting
• there is severe diarrhoea
• gut absorption is a concern
• your cat is hospitalised
• rapid injectable treatment is preferred
• oral GS-441524 is not available

Some cats start with injectable remdesivir and later change to oral GS-441524 once they are eating, stable and easier to medicate.

Current Dose Recommendations For Injectable Remdesivir

Dose protocols vary, and your vet will choose a dose based on the form of FIP, severity of disease and current treatment guidance.

Examples of previously used dose recommendations include:

Effusive ‘wet’ FIP

10mg/kg once daily by subcutaneous injection

Non-effusive ‘dry’ FIP

15-20mg/kg once daily by subcutaneous injection

Ocular FIP

15mg/kg once daily by subcutaneous injection

Cats with neurological FIP may require higher doses than cats with wet or dry FIP without eye or brain involvement.

Subcutaneous remdesivir injections can be painful. This is one reason oral GS-441524 is now preferred for many cats when it is appropriate and available

Is FIP Treatment Approved?

This depends on where you live.

In many countries, FIP treatment involves compounded or specially accessed antiviral medication.

Availability, legal pathways and prescribing rules vary between Australia, New Zealand, the UK, the US and other countries.

In Australia, veterinarians have access to compounded antiviral options (these do not have APVMA drug approval).

In the US, compounded GS-441524 became more accessible through veterinary prescription from 2024, although it is not the same as having a fully FDA approved animal drug.

The important point is this:

Do not buy FIP medication from an unknown online source without veterinary guidance.

Black market products may have variable strength, poor quality control, incorrect dosing or unsafe ingredients.

FIP treatment is too important to guess.

Current Cost Of Remdesivir Treatment Of FIP In Australia (2026)

FIP treatment is still expensive.

The cost depends on:

• your cat’s weight
• the type of FIP
• whether the cat needs hospital care
• whether injectable or oral medication is used
• the dose required
• the length of treatment
• whether complications are present
• the country you live in
• the pharmacy or supplier used
• how often blood tests and rechecks are needed

In the earlier days of legal treatment, owners in Australia could spend several thousand dollars on antiviral medication alone. Costs have changed as more options have become available, but treatment is still a serious financial commitment.

You also need to allow for diagnosis, blood tests, imaging, fluid testing, hospital care, supportive medication and follow up monitoring.

The cost to treat a 2.5kg cat with effusive FIP (remember most cats are young and light) for 84 days with remdesivir (25mg per day or 2.5ml) will require approximately 21 vials @ $110/vial. The full medication will cost approximately $2,350 plus tax.

This does not include initial diagnostics or any supportive care such as fluids that might be required in the initial stages.

Current Cost Of GS-441524 Tablets For FIP Treatment (2026)

In Australia, the tablets come with a 50mg concentration (now also has a liquid suspension)

The cost is currently $600 for 10 x 50mg tablets.

The cost to treat a 2.5kg cat with effusive FIP for 12 weeks with GS-441524 (25mg per day or HALF a 50mg tablet) requires approximately 42 tablets at a cost of approximately $2500.

The optimum treatment period has been found to be 84 days, that said, some cats will be cured earlier, likewise – some later.

The decision to stop treatment is based on a full return to clinical health including normal blood test values.

Most treatment failures occur when treatment is stopped prematurely.

There does not appear to be any difference between success rates in those cats with wet or dry FIP that don’t have ocular or neurological involvement.

In those cats with ocular or neurological forms, the success rate was lower at 80% and 76% respectively. (FIP Warriors data).

It is pleasing to note that the long-term success of treatment is positive with few relapses or recurrences of FIP. (2. FIP Warriors data).

What About Molnupiravir And Other Antivirals?

Molnupiravir is another antiviral that has been used in some cats with FIP. It is converted to EIDD-1931 in the body.

Some studies and reports describe good outcomes, including use as a rescue treatment in cats that relapse or do not respond as expected to other antivirals.

However, molnupiravir and EIDD-1931 are not usually considered first choice where GS-441524 or remdesivir are available.

This is because there are still questions around ideal dosing, long term safety, side effects and mutagenic potential.

Other treatments that have been investigated or discussed include:

At this stage, these should not be viewed as replacements for antiviral treatment with GS-441524 or remdesivir.

Do Steroids Help Cats With FIP?

Steroids are not a cure for FIP.

Glucocorticoids, such as prednisolone, are not recommended as routine sole treatment because they do not stop the virus.

However, they may be used in selected situations, including:

In these cases, steroids are used as part of a wider treatment plan, not instead of antiviral therapy.

Supportive Care During FIP Treatment

Antiviral medication is the main treatment, but supportive care can make a big difference, especially in the first few days.

Supportive care may include:

Common supportive medications may include maropitant, ondansetron, metoclopramide, mirtazapine, gabapentin, pregabalin or opioids, depending on the cat.

NSAIDs may be used in selected cats, but only when your vet is confident there are no concerns with hydration, kidney function, gut health or blood pressure.

Monitoring During FIP Treatment

Monitoring is not optional. It is one of the most important parts of successful FIP treatment.

Your vet may monitor:

Weight gain is often a good sign. If a cat is not gaining weight, is losing weight, still has fever, or the fluid is not reducing, the treatment plan needs to be reassessed.

If effusion has not reduced significantly after around two weeks of antiviral treatment, your vet may review the dose, current bodyweight, drug formulation, medication administration, absorption and diagnosis.

Can FIP Come Back After Treatment?

Yes, relapse can happen, although many cats do very well long term.

Relapse is more likely if treatment is stopped too early, the cat was underdosed, doses were missed, the cat gained weight and the dose was not adjusted, or the cat had ocular or neurological disease.

Signs of relapse may include:

If your cat has been treated for FIP and any of these signs return, contact your vet quickly.

How Is FIP Transmitted Between Cats?

It appears that cats that live in high-density situations such as shelters, catteries and breeding colonies are the most susceptible to infection with FCoV as are purebred cats, males and geriatric cats.

The most common mode of transmission occurs between an infected Queen and her kittens when they are about 5-8 weeks of age.

The virus is quite contagious and is easily passed from an infected cat to other cats via faecal oral spread – especially from shared litter trays, crowding and multi-cat environments.

Remember though, that whilst most cats may become infected easily, only a small percentage will go on to develop FIP when the FCoV mutates.

FIP virus (the mutated form) is not passed in faeces.

Can Feline Coronavirus Be Prevented?

While there is a vaccination available for feline coronavirus it is not recommended due to lack of efficacy for a couple of reasons:

1. It is licensed for use for kittens from 16 weeks of age – as mentioned earlier most cats are infected between the ages of 5 and 8 weeks.
2.  It also contains a serotype II strain of the FIP virus whereas Serotype I FIP virus is the predominate strain of the virus found in the environment.

However, there are several steps you can take to help prevent the spread of feline coronavirus in cats:

1. Keeping your cats up to date on their vaccinations for other diseases can ensure a robust immune system.
2. Practice good hygiene: clean and disinfect any surfaces or objects that may have come into contact with cats.
3. Keep litter boxes clean and locate them away from food and water dishes.
4. Limit your cat’s exposure to other cats – particularly wild cats. Keep inside.
5. Create a low-stress environment – FIP is more common in cats that live in crowded or stressful environments.
6. Use caution when introducing new cats to your household. Make sure that they are healthy before introducing them to your cats.

Can Other Animals Or Humans Become Infected With Feline Coronavirus?

Infection with FCoV is limited to cats, other animals and humans cannot get sick from feline coronavirus.

Feline coronavirus is not implicated in SARS or Covid-19, human diseases also caused by a coronavirus.

General Outlook For Cats With FIP

FIP is still a serious disease, but it is no longer the hopeless diagnosis it once was.

Oral GS-441524 is now a major treatment option and is commonly used as first line therapy where available. Injectable remdesivir remains valuable for very sick cats, hospitalised cats, cats that cannot take oral medication, or cats with gut absorption concerns.

The best outcomes come from early recognition, prompt antiviral treatment, correct dosing, weight based dose adjustments, good supportive care and careful monitoring.

References

1. Jones S, Novicoff W, Nadeau J, Evans S, 2021. Unlicensed GS-441524-like antiviral therapy can be effective for at-home treatment of feline infectious peritonitis. Animals 11, 2257.
2. FIP Treatment – Czechia /Slovakia. Basic data, 2022. https://docs.google.com/spreadsheets/d/e/2PACX-1vRAnj_FV_fteWIW1HXsROLuJ7YY1i_Sf81BCmM9JT9LbCT2mcnwD1rL9IBsLCTB1U59CcnalOGjFqq/pubhtml?gid=1340189982&single=true (Accessed 4 April2022).