FIP in Cats


Feline Infectious Peritonitis is a relatively uncommon disease (affecting approximately 0.02% of the domestic cat population) caused by the feline Coronavirus. A high proportion of cats (up to 40% of the general population, but up to 100% of cats in multicat colonies) will have been exposed to Coronavirus at some point in their life. In the vast majority of cases, it will cause no symptoms at all, or just mild self limiting diarrhoea or flu like symptoms (sneezing, watery eyes etc).

The virus infects the cells of the intestinal tract and replicates. It is then shed in the faeces, potentially surviving in the environment for up to several weeks (though it can be destroyed with the application of basic disinfectants). It is spread via the faeco-oral route.

Once the virus has been ingested, most cats are able to fight off the infection, though some may become persistent carriers and continue to shed virus into the environment long term. Queens carrying the virus through pregnancy and lactation will usually have passed it onto their kittens by the age of 5-8 weeks.

However, a small proportion of cats will develop a mutated strain of virus as it replicates inside their gut. This more aggressive virus has a much increased potential to cause disease by spreading to other parts of the body inside the cat's own white blood cells. If the cat cannot destroy this strain of virus, or there is an inadequate or inappropriate immune response, then it will develop FIP. Although dangerous to the host cat, this mutated virus does not get shed to the same extent as normal Coronavirus, so would not be considered highly infectious to others.

Clinical signs are variable between cats, and not specifically diagnostic of FIP. Initially, fever (which does not improve with antibiotics), lethargy, weight loss and poor appetite may be seen. This will progress over days or weeks to develop into the ‘ wet ‘ or ‘ dry ‘ forms of FIP, or a combination of both. The latter is thought to arise in cats who are able to mount a partial immune response.

The unequivocal diagnosis of FIP is difficult as there are no pathognomic symptoms (ie specific to FIP only), and no simple diagnostic test that can give a definite positive or negative answer.

The vet has to take into account a number of factors in combination – signalment (the disease is most common in young cats and kittens, in cats from multi-cat households, shelters or breeding colonies, and appears to be more prevalent in exotic or pure bred cats, so there may be a complex genetic predisposition)

    – history
  • – symptoms
  • – test results

Routine blood tests may demonstrate changes in the proportions of different populations of white blood cells, anaemia, increased levels of globulin ( a blood protein ) and elevated liver parameters ( indicative of liver damage ). Unfortunately these results may be seen in other disease processes, though the high globulin level can be considered more suspicious for FIP.

In wet FIP, imaging with ultrasound or x-ray can confirm the presence of fluid in the abdomen and chest. Samples of this fluid can be taken and analysed. Typically the fluid is clear yellow in colour, with a high protein concentration. There are tests which can demonstrate the presence of Coronavirus in the fluid ( but cannot confirm that it is the FIP strain ). However, a number of other conditions can result in the accumulation of fluid in the abdomen or chest, including liver disease, certain cancers and heart failure.

It is possible to check Coronavirus antibody in the blood, but this is generally not helpful as so many normal healthy cats have had previous exposure and will test positive.

In dry FIP, biopsies of damaged tissues can confirm the presence of the virus, but realistically most cats at this stage would be considered to be too ill to undergo an anaesthetic and surgical procedure for diagnostic purposes.

Wet ( effusive ) FIP

Typically the patient will develop a bloated swollen abdomen and/ or breathing difficulties due to the development and accumulation of fluid in the abdomen or chest. This happens because the virus triggers a vasculitis – inflammation of the blood vessels, which allows fluid to leak out of the blood stream into the body cavities.

Dry ( non-effusive ) FIP

Intense inflammation caused by the interaction between the virus and the cat’s immune system result in lesions associated with the blood vessels developing in various organs including the eyes, brain, liver, kidneys, lungs and skin. This can cause a variety of symptoms associated with the affected systems including excessive thirst, jaundice, weakness and in-coordination or poor vision.

This immune mediated disease aspect of FIP makes it unique in animals and humans – no other viral infection causes such significant organ damage through the response of the body’s own defense system.


Unfortunately, once symptoms of FIP have developed, it is considered to be incurable, rapidly progressive ( with an average life expectancy of 2 months or less from time of diagnosis ) and ultimately fatal disease.

Supportive care with nursing, nutrition, steroid and antibiotic treatment may provide some palliation. More interventional treatments such as intravenous fluid therapy, blood transfusions and drainage of abdominal or chest fluid to alleviate respiratory distress or discomfort may be considered on an individual cat basis, taking the associated risks and potential benefits into account.

But sadly for most cats with FIP, euthanasia may be the kindest and most humane decision, taking into account the patient’s quality of life and likely rapid rate of deterioration. Always an extremely difficult and heartbreaking decision, but especially so for FIP cats who are generally still juveniles.


Although a licenced vaccine has been produced, studies suggest that it is of limited benefit as it is given from 4 months of age onwards, thus too late to prevent maternal spread of the disease to young feeding kittens.

Obtaining cats from individually housed or small group only settings, and keeping them in small stable groups can help to minimise the spread of Coronavirus. This also helps to reduce stress, which itself can impact on the immune system.

In any colonies of cats, hygiene, particularly with regards to litter trays and food bowls, is of major importance.

FIP is a unique and particularly challenging disease in cats, but it is worth remembering that it is thankfully relatively rare, particularly in our domestic moggies.