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Feline Infectious Peritonitis (FIP) is a disease that historically was always surrounded with much sadness.   It often occurs in very young cats and kittens and, as it is ultimately fatal without treatment, it quite often leaves us feeling that they have been taken far too soon from us. Fortunately there have been recent developments for treatment protocols over the past 5 years that have allowed us to be more optimistic about these cases.

What is FIP?

FIP is a disease caused by a virus known as a Feline Coronavirus.  Feline Coronavirus is actually quite a common infection in cats and most of the time does not cause any outward signs of disease, except for mild diarrhoea in some cases (Feline Enteric Coronavirus).

In a very small number of cats this coronavirus mutates inside the body, and if the virus is not contained by a good immune response, a cat will develop the disease referred to as Feline Infectious Peritonitis Virus.

The FIP virus then spreads throughout the body in infected  white blood cells and can cause a wide range of different signs including; accumulation of fluid in the abdomen and/or chest cavity, or in others the virus may cause severe inflammation affecting the brain, eyes, liver, kidneys or elsewhere.  The disease is almost always fatal.

Is my cat at risk? 

Generally, cats do not catch the mutated form of the virus directly, however any cat that carries coronavirus is potentially at risk for developing FIP.  Coronaviral infection occurs worldwide (estimated that 25 to 40 per cent of household pet cats are infected) and is extremely common where large numbers of cats are kept together i.e. multi-cat households or colonies (infection rate increases up to 80-100%).

Feline coronavirus can be found in the saliva and faeces of cats during acute infection, and sometimes in recovered or carrier cats.  Coronavirus can be transmitted through cat-to-cat contact and exposure to infected faeces, with virus being able to live in the environment for several weeks.  Most commonly, transmission of Feline Coronavirus occurs when infected female cats pass the virus to their kittens during nursing.

FIP is relatively uncommon in the general cat population. However, the disease rate is much higher in multiple-cat populations due to the often higher prevalence of Coronavirus in these situations.  FIP is most likely to develop in cats with weak immune systems including; kittens, cats infected with feline leukemia virus, and older cats. Most cats that develop FIP are under two years of age, but cats of any age may develop the disease.

What are the symptoms? 

Often, cats that have been initially exposed to Feline Coronavirus will show no obvious symptoms.  Some cats may show mild upper respiratory symptoms (sneezing, watery eyes, and nasal discharge) or mild gastrointestinal symptoms (diarrhoea).  A very small percentage of cats that are exposed to Feline Coronavirus may develop FIP.  This can occur over weeks, months, or even years after initial exposure.

FIP can be difficult to diagnose as early signs are usually very vague and each cat can display different symptoms that are similar to those of many other diseases.  In the early stages of FIP symptoms may include fluctuating fever, lethargy, inappetence and weight loss.

After a period of several days or weeks (or sometimes even many months) other signs usually develop. There are two main clinical forms of the disease: an effusive or “wet” form and a non-effusive or “dry” form.  Many cats may in fact have a mix of these two forms.

“Wet” FIP is characterised by fluid accumulation in the abdomen and/or chest.  This typically causes a swelling of the belly and/or breathing difficulties as these body cavities fill with fluid.

“Dry” FIP  is typically more insidious, with weight loss, anorexia, fever and lethargy commonly present. Depending on which organ systems are involved, other signs may include jaundice, vomiting and diarrhoea, dehydration, excessive urination and drinking, neurological signs or inflammation within the eyes. The occurrence of these secondary signs is highly variable and may mimic many other diseases.

Sadly, without treatment both of these forms are terminal.

Diagnosis

Diagnosing FIP can sometimes be tricky as there is no simple blood test to confirm a diagnosis.  Often, the combination of clinical signs (mentioned above) and changes evident on in-house blood tests are highly suggestive of the disease but do not give a definitive diagnosis.  Increased globulin concentrations (one of the major groups of proteins in the blood), changes to white and red cell counts, and elevated liver enzymes are often (but not always) seen.

A blood test to detect coronavirus antibodies in the blood of a cat is only useful to ascertain whether the cat has been in contact with coronavirus.  This blood test does not differentiate between the different types of Feline Coronavirus (FIP-producing strains from enteritis-producing strains), therefore this test is not definitive and we believe it of minimal diagnostic significance.

With the “wet” form of the disease, a sample can be taken of the fluid effusion in the abdomen or chest cavity and the cell and protein content analysed.  This test can be extremely helpful because few other diseases cause this type of fluid accumulation.  When FIP is suspected a vet will often take X-rays or do an ultrasound examination to see if fluid is present so that a sample can be collected for analysis.

Specific FIP tests that can be performed on effusion fluid include FIP PCR testing and effusion immunofluoresnce for FeCoV in macrophages. These tests are more definitive for diagnosing FIP, however unfortunately can show false negative results (a negative test result when the cat has the disease).

There are other non-specific tests that can sometimes be helpful, including; further analysis of proteins in the blood and evaluation of a cerebrospinal fluid sample (the fluid that surround the brain and spinal cord) in cases where there are neurological signs.

The gold standard test to confirm a diagnosis of FIP is to collect a biopsy from affected tissues (done through a surgical procedure).  Typically, inflammation of abdominal organs is usually seen, which is highly suggestive, but the diagnosis can be confirmed using a technique called ‘immunohistochemistry’ which will demonstrate the presence of the virus itself within the damaged tissues. Unfortunately with many cases of FIP, a cat may be too sick for surgery to be performed, and so in many cases a definitive diagnosis may only made on post-mortem examination (using immunohistochemistry, as above).

Treatment 

Since 2016, promising research had shown there are anti-viral compounds called nucleoside analogues which can treat FIP and put cats into remission. These drugs were also used in trials for treating COVID-19, which has lead to readily available and legal sources of these drugs for veterinary use. Over the course of the past few years, our team has attained significant experience with use of both injectable remdesivir, and now more recently oral GS441526 for treatment of our FIP cases. Our vets have been in collaboration with the team at the University of Sydney to conduct ethical trials to investigate the use of these drugs and how to optimise treatment protocols. We have also been involved in their clinical trials for assessment of mefloquine as a cheaper treatment alternative.

Initial treatment protocols often require intensive care as many patients are very unwell and require supportive care such as feeding tube placement, hospitalisation, and blood transfusions. Each treatment protocol is tailored to the individual cat’s needs and clinical presentation. These drugs come at a high cost which can be limiting for some to afford. We recommend discussing your cat’s needs directly with one of our vets.

Prevention 

As Feline Coronavirus is very common worldwide, it is difficult to prevent kittens becoming exposed especially in a cattery environment.  It is important to try to select healthy kittens that come from a hygienic and non-crowded environment.  Ideally selecting a cattery that has no history of FIP in its breeding line is best, however it is impossible to determine which individuals may develop FIP.

Keeping cats individually or in small, stable groups to reduce stress, along with stringent litter box management (cleaning/disinfection) can reduce the risk of transmission of coronavirus and subsequent potential development of FIP.

Currently there is no effective vaccine available that will prevent FIP.

Updated by Dr Martine van Boeijen Nov 2021

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