iCatCare Advice: Feline Infectious Peritonitis (FIP) (Feline Coronavirus)

This article first appeared on iCatCare here.

Feline infectious peritonitis (FIP) is a viral disease of cats that occurs throughout the world. It is a complex disease to diagnose and in the last few years, significant developments have occurred in the treatment of this previously fatal condition.

Further information for veterinarians can be found here.

Novel treatments

FIP was previously fatal for most cats. However, in the last 3 years, significant developments have occurred in treatment. Studies have shown that antivirals can be effective in curing FIP. This has led to a growth in illegal supplies of these medications, sold to understandably worried caregivers. Since 2021 a legal supply of remdesivir and GS-441524 has been available in the UK and Australia, and via export to other countries. Please click here for more specific information about these drugs and speak to your vet to discuss treatment options if your cat is diagnosed with FIP.


FIP is caused by infection with a virus known as feline coronavirus. Coronaviruses are a common group of viruses that often infect the upper respiratory tract (nose and throat) or gastrointestinal tract (intestines) in different animals. COVID-19 is a coronavirus but the coronavirus causing FIP is not the same and only infects cats. Coronavirus infection is extremely widespread in cats, especially where large numbers of cats are kept together. It is estimated that 25–40% of household pet cats have been infected with FCoV, but the infection rate increases to 80 – 100 per cent for cats kept in multi-cat households or colonies.

Coronaviruses are common and found in the faeces of many cats. In most cats, infection causes no signs or just mild diarrhoea that resolves without treatment. However, occasionally, the virus mutates (changes) inside a cat, and if their immune system reacts in a certain way, they could develop a disease called FIP. This is important, as finding coronavirus in a cat does not mean they have FIP, and it is not possible to consistently tell the difference between a coronavirus causing FIP and one causing no signs at all by just finding the virus itself.

In a cat that develops FIP, the virus spreads throughout the body and can cause a wide range of different signs due to the way it interacts with the cat’s immune system.

Signs of FIP

FIP can cause many different clinical signs, depending on the parts of the body affected and how the immune system reacts. Early signs of FIP are usually very vague with a fluctuating fever (high temperature), lethargy and reduced appetite being common. After a period of several days or weeks (or sometimes even many months) other signs usually develop. Classically, FIP has been divided into ‘wet (effusive)’ and ‘dry’ forms, but actually cats are likely to have both at different times, or a mixture of both presentations. One of the challenges of diagnosing FIP is that the clinical signs are often vague and not specific for FIP and can be seen with other diseases.

Wet or effusive disease

In this form or stage of disease there is accumulation of fluid within the abdominal cavity (resulting in abdominal distension) and/or the chest cavity (resulting in breathing difficulties). The fluid accumulates because infection with FIPV causes damage to and inflammation of blood vessels (called ‘vasculitis’) which results in fluid leaking from the blood into the abdomen or chest. Cases that develop fluid accumulation in the abdomen are responsible for the original name of this disease, ‘peritonitis’ referring to the inflammation that occurs in the lining of the abdominal cavity. The fluid that forms typically is thick and bright yellow in colour and contains a lot of protein. However, other diseases (including some liver diseases and cancers) can also cause a similar fluid accumulation.

FIP can present in different ways but affected cats can have an effusion (fluid) in their chest or abdomen.

Other forms of FIP

FIP can affect any organ of the body and so other forms of FIP (often termed ‘dry’ FIP) involve the formulation of granulomas (masses of virus and inflammatory cells) and the signs will vary according to the organ affected. This inflammation affects the eyes in around 30% of cases and the brain in around 30% of cases, but can also affect almost any tissues in the body including the liver, kidneys, lungs and skin. Thus, a wide range of signs may be observed including neurological disease (e.g., a wobbly and unsteady gait or fits), bleeding in the eyes and other vague signs of disease that may occur with lesions in the liver, kidneys or other internal organs.

What cats are most often affected with FIP?

Although FIP can occur in cats of any age, it is most often seen in young cats. Around 80% of cases diagnosed are in cats less than 2 years old, and many cases are seen in kittens around 4-12 months old. FIP is also more common in cats kept in groups or colonies (especially breeding households) as this is an environment where FCoV infections are spread easily. A crowded environment may also contribute to stress, which can be a factor in disease development as it compromises the cat’s immune response. There is evidence that genetics can also play a role in susceptibility to disease, although this is complex. Many cats that develop FIP are now in single cat homes, despite coming from multi-cat environments.

How is FIP diagnosed

FIP is a very difficult disease to deal with because there are no clinical signs that are specific for the diagnosis of FIP, and no simple blood test to confirm a diagnosis. FIP may be considered more likely when:

  • Cats are showing clinical signs compatible with FIP
  • Cats are in a higher risk category (e.g., younger cats, colony cats, etc.)
  • Typical changes are seen on routine blood tests – these may include
    • Lymphopenia (low numbers of lymphocytes, a type of white blood cell)
    • Neutrophilia (increased numbers of neutrophils, a type of white blood cell)
    • Anaemia (low red blood cells)
    • Elevated globulin concentrations (one of the major groups of proteins in the blood)
    • Elevated liver enzymes (eg, ALT, ALP)
    • Elevated bilirubin (and jaundice or yellowing of the gums and eyes)

None of these blood changes are specific for FIP, occurring with other diseases too, but if multiple changes are seen in combination with appropriate signs, a diagnosis of FIP becomes more likely. Many of these abnormalities may also not be present in the early stages of the disease but may become evident as the disease progresses. Thus, some tests that give normal results may have to be repeated later.

If fluid is present in the abdomen or chest cavity, obtaining a sample of the fluid and analysing the cell and protein content can be extremely helpful. With FIP the fluid invariably has a high protein content and further analysis of cell types and types of protein present can be useful. Further tests on fluid can be performed to look for the virus (PCR for example). Looking for fluid is an important part of investigating a cat for FIP so xrays, and particularly ultrasound, can be very helpful and again may need to repeated later as the disease progresses.

Some cats with FIP develop a build-up of fluid in their abdomen or chest, it is typically yellow and should be sampled for further testing.

Other tests that can also sometimes be helpful include:

  • Further analysis of proteins in the blood (e.g. measuring the protein acid-1-alpha glycoprotein [AGP])
  • MRI scans of the brain
  • Evaluation of a cerebrospinal fluid sample (the fluid that surround the brain and spinal cord) in cases where there are neurological signs
  • Evaluation of fluid from the eye (aqueous humour) in ocular cases
  • Needle samples of lesions in organs and lymph nodes
  • Biopsies (tissue samples) taken at surgery

Looking for the presence of antibodies against the virus in a blood sample (coronavirus serology) is of very limited value – antibodies against FCoV develop when a cat meets coronavirus – this does not mean they have or will get FIP. A very large number of perfectly healthy cats are therefore positive on this test, and it should not be used for diagnosis.

Confirming a diagnosis of FIP

To confirm a diagnosis is very difficult, but in general finding the typical type of effusion or inflammation and finding the virus in the same areas can be useful. Ways to look for the presence of the virus include immunocytochemistry and immunohistochemistry (staining the virus so it can be detected) and PCR (testing for the viruses genetic material). These tests can be performed on fluids, needle samples, biopsies and other samples. Remember, it is not enough to find just the virus as cats without FIP may have coronavirus, we must put together the puzzle of clinical signs, imaging findings fluid and needle samples and virus detection tests.

Treatment for FIP

There have been significant recent developments in the management of this once fatal condition. Recent research by Professor Niels Pedersen of University of California, Davis, has shown that some newer anti-viral drugs such as GS-5734 and GS-441524, used in humans against some emerging viruses, may be effective. Additionally, remdesivir, a prodrug of GS-441524 has become available legally in the UK and Australia. The drug has been used to treat human viral infections including SARS-CoV-2 (COVID-19). Initial experience has been positive with response rates of around 80% and therefore we have reason to be optimistic. The treatment remains expensive and requires a long course of treatment (84 days) so veterinarians should be mindful of commitment and cost involved plus potential for relapse. We look forward to learning more about the treatment of FIP over the next few years. Black market products remain available, but of unknown content and safety so legal drugs should be selected for the treatment of FIP in cats.

Prevention of FIP

A commercial vaccine is available in some countries to help protect against FIP. However, this vaccine (while showing some efficacy) can only be given to kittens over 16 weeks of age. The major indication for using such a vaccine would be in breeding households, especially with a history of FIP, but by the time a kitten can be vaccinated (at 16 weeks) they would almost invariably have already been exposed to FCoV infection and so the vaccine probably has little or no value.

Reducing the risk in breeding households

FIP is least common in household pets. The risk can be minimised by obtaining cats from a source with relatively few cats and by keeping cats in small stable groups (less than five cats in a household).

In breeding catteries, eradicating coronavirus infections is extremely difficult, as the virus is so ubiquitous, and it is unsuitable in most situations to attempt this. A more practical approach is to use measures to reduce the risk of FIP occurring, but recognising that on occasions, this may happen even in the best run catteries. Good practice to minimise the risk of FIP would include:

  • Avoid keeping large groups of cats and having multiple litters of kittens at any one time
  • Keep cats in small isolated groups (ideally no more than four cats in each group – this reduces the risk of endemic FCoV infection)
  • Have at least one litter box for every two cats, located in easy to clean and disinfect areas
  • Keep litterboxes away from food and water bowls, and clean/disinfectant them regularly (at least daily)
  • Avoid stress and maintain good hygiene and preventive healthcare for all cats

Wherever FIP occurs is a problem in a group of breeding cats:

  • Consider preferentially breeding from older cats, as these will less likely be shedding FCoV
  • Consider isolating queens just before they give birth and keeping the queen and kittens isolated from all other cats until the kittens are homed, as a means of reducing the risk of FCoV spread to kittens
  • Stop breeding from any queens or tom cats that repeatedly produce litters of kittens that develop FIP as they may be passing on FCoV infection or may be passing on genetic susceptibility to disease
  • Carefully review management and hygiene policies
  • If faced with an outbreak of FIP, stop all breeding for several months

Reducing the risk in rescue and rehoming facilities

Good hygiene and avoiding overcrowding are essential strategies for minimising the risk of FIP. Cats should ideally be housed individually, or if this is not possible, they should be kept in small stable groups. Litter boxes and cleaning/disinfection should be managed as in breeding households.

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