Feline Infectious Peritonitis (FIP): Causes, Symptoms, & Treatment

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Feline Infectious Peritonitis (FIP) is a complex, serious, and until very recently, it was always ultimately a fatal disease of cats. FIP is caused by a biotype of Feline Coronavirus (FCoV), called Feline Infectious Peritonitis virus or FIP virus (FIPV).

The disease has a global distribution, and only affects cats: dogs, humans and other species cannot be infected. FIP has been one of the most challenging infectious diseases of cats known to veterinary medicine.

The fact that this is a feline coronavirus infection brought the disease into particular focus in 2020, with the emergence of COVID-19 in humans: it should be stressed that there is no connection between these diseases, other than the fact that the two different viruses belong to the same viral family.

How Cats Get Feline Infectious Peritonitis?

One of the complicated aspects of FIP is that while Feline Coronavirus (FCoV) is very common, Feline Infectious Peritonitis is rare.

FCoV, also known as Feline Enteric Coronavirus (FeCV) is found in 80-90% of cats living in multicat houses, but nearly all of these infections are harmless, with no signs of illness, or just mild signs of diarrhea. The virus is shed in the faeces, spreading easily between cats, commonly via litter trays or boxes, and surviving in the environment for up to seven weeks.

A new cat in a household will be infected orally, mostly via contact with litter trays or boxes. Direct spread from cat to cat rarely happens.

Following infection, the virus can be found in the gastrointestinal tract and the blood for a short period, and then maybe shed in the faeces, for weeks, months, or in some cases, for life. However, in a very small proportion of FCoV-infected cats, the relatively benign virus mutates to become the Feline Infectious Peritonitis Virus virulent biotype, which goes on to cause Feline Infectious Peritonitis.

This mutation may involve point deletions or mutations at the 3c gene but the full details have not yet been established. The mutation happens in an individual cat, with the mutant virus multiplying rapidly, infecting the macrophages and monocytes of the host cat and spreading throughout the body, causing disease.

The mutated FIPV virus is not shed in the same way as FCoV in the faeces, so is not thought to be directly infectious in the same way.

There are still questions about how this mutation happens and how the development of FIP follows:

There are thought to be links between how much FCoV a cat is exposed to (a higher virus burden is more likely to lead to FIP), and how an individual cat’s immune system responds to the virus. Cats seem more likely to develop FIP if they experience stress (such as visits to a cattery, post-surgery, or being rehomed), a phenomenon linked to the fact that stress affects the functioning of the immune system.

Most cases of FIP happen in young cats, aged less than one year, although cats can be affected at any age, and it’s more common in multicat households, and in pedigree cats (e.g. Birman, Bengal, and Orientals). Male cats are more likely to develop FIP than females, and cats that are FeLV positive are also more likely to develop the condition.

How Feline Infectious Peritonitis Affects Cats?

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Feline infectious peritonitis in cats affects the body in a variety of ways, with the illness manifesting in either a wet or dry form.

FIP is described as a pyogranulomatous vasculitis, with effects throughout the body. The signs of illness are variable, depending on which part of the body is affected. Classically, there are two forms of FIP, depending on the cat’s immune response: wet (effusive) and dry (non-effusive). However, these are not necessarily absolutes: some cases have elements of both the wet and dry disease

• Wet FIP (the effusive form) is a more acute, rapidly developing, disease, developing weeks or months after a period of stress such as rehoming or surgery. The clinical signs are more directly related to vasculitis (inflammation of blood vessels) with protein and fluid leaking out of blood vessels into body cavities, leading to the accumulation of fluid. The specific signs of the disease in an individual cat depend on which part of the body is affected by this fluid accumulation.
• The dry (non-effusive) form of FIP is a more chronic, slower onset and gradually developing disease, following a partial immune response by the cat’s body. Pyogranulomatous lesions (plaques that represent a combination of virus plus the body’s immune reaction to the virus) are found in organs around the body, including the liver, kidneys, spleen, lymph nodes, and brain, as well as in the abdominal cavity. Again, the clinical signs depend on which part of the body is affected. The dry form can develop into the wet form at a later stage of the disease.

Clinical Signs of Feline Infectious Peritonitis

The clinical signs of Feline Infectious Peritonitis are variable, depending on which type of disease is present, and which part of the body is affected.

Typically, infected cats have vague signs at first, such as lethargy, loss of appetite and weight loss. There may be episodes of pyrexia (fever).

In the wet form, the signs depend on where the fluid gathers.

• The abdomen is most commonly affected, resulting in ascites, with the abdomen visibly swollen and full of fluid. Sometimes, hard objects can be felt in the fluid when the abdomen is palpated, representing enlarged lymph nodes or diseased internal organs such as the spleen, liver or kidneys.
• If the fluid gathers in the chest cavity (thorax), there is difficulty breathing (dyspnoea), caused by the fluid preventing normal expansion and functioning of the lungs.
• If the fluid gathers in the sac surrounding the heart (a pericardial effusion), signs of heart disease may be seen, caused by the inability of the heart to contract normally due to the pressure of the fluid.

In the dry form, again the signs depend on which part of the body is affected, and they tend to be slower onset, over a longer period of time. This can range from general signs of illness when the abdominal organs are involved, difficulty breathing when the lungs or chest are involved, neurological signs when the brain or spinal cord is involved (such as ataxia or even seizures), or ocular signs when the eyes are affected (such as visual difficulties, with abnormal appearance of the eyes).

Diagnosis of Feline Infectious Peritonitis

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Your veterinarian can determine whether or not your cat has feline infectious peritonitis.

The only way that a definitive diagnosis of FIP can be made is via histopathology of affected tissues when the pyogranulomatous inflammation can be seen under the microscope, along with identification of the presence of FCOV antigen in macrophages via immunohistochemistry. However, in most cases, it can be difficult to obtain such samples from a living cat, and so this diagnosis may only be made post-mortem, at an autopsy.

More commonly, your DVM veterinarian will carry out a series of diagnostic tests, and the patchwork of results will lead to a likely diagnosis of FIP if this is the underlying cause.

Commonly undertaken investigations include:

• Physical examination, with signs as listed above, along with specific changes e.g. on detailed examination of the eyes with an ophthalmoscope.
• X-rays and ultrasound may show a gathering of fluid in the abdomen and chest and enlargement of affected organs e.g. liver, spleen, lymph nodes.
• Laboratory tests including analysis of fluid sampled from the abdomen or chest are the most common way that a strong suspicion of the diagnosis is formed. This fluid has particular distinctive characteristics: technically, it is referred to as either a modified transudate or an aseptic exudate, depending on its colour, viscosity, cell content and other aspects. A specific Polymerase Chain Reaction (PCR) assay for viral RNA can be carried out on these fluid samples but this does not distinguish between cats with the benign Feline Coronavirus (FCoV) and the pathological FIPV biotype.
• General blood tests, including haematology (lymphopenia, or reduced lymphocytes in the blood cell count, and anemia, are commonly seen) and biochemistry (a high protein level, or hyperglobulinemia, is common, with low albumin: globulin ratio, hyperbilirubinemia is common, and raised liver and kidney parameters are common).
• Specific blood tests, including Alpha-1 acid glycoprotein: which tends to be elevated in cats with FIP (but it can be elevated in cats with other diseases too). Coronavirus antibodies can be measured using antibody titers, but antibodies to the FIP biotype of FCoV cannot be distinguished from the less pathogenic version of FCoV. Antibody levels are elevated in cats that have the benign form of FCoV, so they are often positive in healthy cats, and some cats with FIP do not have elevated levels. Nonetheless, in cats that have signs of FIP, very high elevations of FCoV would be seen as supportive evidence of the diagnosis.
  Around 80-90% of cats in multi-cat environments and up to 50% of cats in single-cat households have FCoV antibodies, but only 5-10% of exposed cats develop FIP.
• Other investigations include a detailed neurological assessment by a specialist, with the possibility of cerebrospinal fluid (CSF) analysis, along with advanced imaging such as MRI, looking for the distinctive changes that are commonly found in FIP.
• Referral to a feline medicine specialist (e.g. a member of the American Association of Feline Practitioners) may be recommended by your local veterinarian.

Feline Infectious Peritonitis Treatment

Up until very recently, treatment of cats with FIP was very limited, and only palliative supportive care was possible, keeping affected cats as comfortable as possible for as long as possible. This generally involved the use of prednisolone or cyclophosphamide to reduce inflammation, and appetite stimulants (such as mirtazapine and cyproheptadine). Other immune-influencing medications such as interferon were sometimes used.

However the big news about FIP treatment is new therapies (developed by experts such as Professor Niels Pedersen from UC Davis School of Veterinary Medicine) with antiviral medications such as antiprotease inhibitors and nucleoside analogs to reduce replication of FCoV, and Polyprenyl Immunostimulant (PI) to support the immune system.

New treatments have been nothing short of revolutionary, offering cats with FIP a full and complete cure in 85% of affected cats. Even in cases with resistance to first-line medications, or cases of relapse, cats are still responding well to combination therapies and resistance treatment protocols.

For full details on the current status of FIP treatment approaches, including dosing protocols, product availability, and treatment support, see the FIP Treatment Guide for Veterinary Professionals, which is publicly available. Several pages from the guide are further references in the following sections for convenient reference.

There are now a total of 5 treatments available for FIP. Keep in mind that their role in therapy and their availability may vary. For full details on treatment protocols for FIP, see the FIP Treatment Protocols from the FIP Vet Guide.

GS-441524

GS is the active form of the antiviral remdesivir and is the most documented treatment for FIP. It can be given either orally or through a subcutaneous (under the skin) injection.

Molnupiravir

Molnupiravir is a newer treatment for FIP, which was also developed as a COVID-19 treatment for people. A study published in 2022 showed it to be an effective rescue therapy for cats failing GS-441524 therapy. More recent studies have also shown that it is an effective first-line treatment for FIP, with similar efficacy to GS-441524. Molnupiravir offers a more affordable treatment option.

Paxlovid

Many may be familiar with Paxlovid, as it is a medication FDA-approved for treating COVID-19 in people. It contains two antivirals nirmatrelvir and ritonavir. Given that FIP is caused by a mutated coronavirus and COVID-19 is also a coronavirus, some began to look into its use as an FIP treatment.

Currently, Paxlovid is not considered a first-line treatment for FIP due to lack of studies. But it is being used as a rescue therapy for cats failing treatment with GS or molnupiravir alone by being combined with either first-line medications.

Remdesivir

GS-441524 is the active form of the antiviral remdesivir. However, unlike GS which has an oral form, remdesivir only comes as an injectable. Remdesivir itself has also been looked at as an FIP treatment given that it is also FDA-approved for use to treat COVID-19 in people.

Its use for COVID-19 also makes it available globally, however in the US at least it can only be found through human hospitals using it to treat COVID patients and is not available through outpatient pharmacies or veterinary distributors. It is most commonly used in the UK and Australia where it is more available through compounding.

Its place in treating FIP is reportedly more as an initial therapy for a couple of weeks before therapy is changed over to GS-441524.

GC-376

GC-376 was actually the first antiviral that demonstrated a cure for FIP. However, because it has less ability to cross the blood/brain barrier and is therefore not as effective for neurological FIP, it is not used as a first-line treatment. It also only comes in an injectable form.

GC-376 is used the most paired with GS-441524 as a combination therapy to combat viral resistance to GS. A commercial veterinary product is going through trials and as of this writing update, is about 2 years from FDA approval.

Treatment Process and Expectations

So with these new treatments, how does a course of treatment for FIP now look?

Fortunately, most cats will be cured of FIP with a treatment course of 12 weeks, which is considered standard in most cases. Some cats with more mild disease may see treatment resolution in only 6-8 weeks.

The first 2-4 weeks are focused on clinical response with the improvement of body cavity fluid, weight loss, and neurologic or ocular signs (if neurologic or ocular FIP is present).

Any cats with neurologic or ocular FIP will typically have their dosage of medication increased to overcome the blood-brain and blood-ocular barrier.

The amazing thing is that most cats will show noticeable improvement in their disease signs within the first 3-5 days. Fevers break in just 1-2 days. By 1-2 weeks, fluid in the chest and ocular disease is resolved. While noticeably improved, full resolution of neurologic signs and abdominal fluid may take up to 30 days. For more details on what treatment progression looks like, see the Treatment Timeline page of the FIP Vet Guide.

The decision to discontinue treatment for FIP and declare a cat cured of the disease requires a combination of evaluating a cat’s clinical condition with lab work testing. Veterinary exams are recommended every 2 weeks with lab work at least midway through treatment at 6 weeks and again at 12 weeks.

As with the treatment of many diseases, we can see poor clinical response, viral resistance to drug therapies, and relapses. Fortunately, there are therapy protocols described to address these situations.

Side Effects of Treatment

Fortunately, serious side effects to FIP treatments are not common. Compared to the terminal nature of FIP that goes untreated, less serious side effects are considered manageable and a good trade-off for overall survival.

Kidney blood work value elevations that are mild or transient appear to be common with GS-441524. Fortunately, these elevations have not led to serious kidney disease in cats that are still a few years into remission and usually resolve with the end of treatment. Cats with pre-existing kidney disease may see more severe elevations and may require fluids to be given under the skin several times a week while being treated.

Injectable forms of GS-441524 and GC-376 may lead to skin irritation, lumps, bumps, and even sterile (non-infected) abscesses. Owners of cats undergoing treatment will often have their cat wear a shirt or surgical recovery suit/onesie to keep them from scratching or opening up any lumps or sores.

Rare, allergic reactions have been reported. Mild cases can be addressed symptomatically while more severe cases may require a treatment protocol change.

Therapy Availability

In the US, the FDA has recently made it legal for veterinarians to prescribe GS-441524 and molnupiravir through compounding pharmacies. The compounding pharmacies Stokes and MixLab offer GS through a veterinary prescription in all 50 US States. Wedgewood Pharmacy similarly offers molnupiravir through a veterinary prescription in all 50 US States.

See the full list of US pharmacies, what FIP treatment products they carry, what US states they service, and approximate costs for a full 12 weeks of treatment.

In the UK, legally available medication has been accessible through a supplier of extemporaneously prepared veterinary medicinal products in the United Kingdom (Bova.co.uk) since 2021. They have also recently become available in European Union member states such as Cyprus, Czechia, Finland, Germany, and Ireland, and are also available in Australia, Canada, and Switzerland.

It may still be difficult to access some of these medical therapies in some countries, though accessibility is improving all the time.

But as a result of this lack of availability, informal groups with names like FIP Warriors have sprung up in many different places (such as Facebook Groups), offering support and even access to medications via the black market. With legal and affordable options now increasingly available, we caution anyone against resorting to black-market suppliers.

Black market suppliers of GS-441524 often will not divulge the ingredients of their products. Dosing protocols recommended by these suppliers have also been found by researchers to be too high, leading to unnecessary side effects.

In countries where the alternative is simply euthanasia, or allowing a fatal illness to take the life of a cat, it is easy to understand why cat carers may consider this route. As the legal forms of the new treatments become increasingly widely available around the world, there hopefully will soon be no need for cat carers to consider the black market route.

Veterinary professionals will also no longer have to face the ethical dilemma of consulting on the use of unregulated, unlicensed drugs to treat a historically deadly disease.

For individual cat owners, the best approach is to discuss the local situation in your country with your own veterinarian. You may wish to show them this article to assist your discussion.

For more information on what options may be available in your country or region, see the FIP Vet Guide’s page on Accessing Antiviral Treatment for FIP. You may also go to the FIP Global Community Advocacy Treatment Support (C.A.T.S.) Facebook group for help looking into FIP treatment options available where you live.

Visit or Join the FIP Global C.A.T.S. Facebook Group

Feline Infectious Peritonitis Prognosis

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Unvaccinated cats catch feline infectious peritonitis (FIP) through close contact with other felines.

Until very recently, FIP was considered a fatal disease, with most cats deteriorating rapidly, requiring euthanasia to prevent terminal suffering. Some cats would have milder signs, with a longer course of illness, but the outcome was ultimately the same.

As a general rule, cats with wet (effusive) FIP used to live just for days or weeks, while cats with dry or non-effusive FIP survived for weeks or months.

However, with the new treatments, cats can be fully cured of all forms of the condition. While the most common first-line treatment, GS-441524, can be expensive, one of the newer therapies, molnupiravir, has been shown in recent studies to be as effective as GS and is much more affordable. The cost of GS has also come down over the last two years and hopefully, that trend will continue.

Even in cases where a cat’s FIP shows resistance to GS alone, there are additional therapy options that can be combined with GS for treatment for successful outcomes.

With the advent of FDA approval and multiple compounding pharmacies now carrying these therapies legally, the availability of treatment is now much better and continues to improve. For more information on the global availability of FIP treatments and what options may be available in your country or region, see the FIP Vet Guide’s page on Accessing Antiviral Treatment for FIP.