Feline Infectious Peritonitis


Feline Infectious Peritonitis (FIP) is a fatal condition in cats where the causative virus (a coronavirus) which is harmlessly present in most cats, mutates and becomes harmful. This mutation is uncommon, but if it is present in one cat in a group, it will almost certainly spread, as the affected animal sheds the virus in faeces very freely and it will then spread as animals groom and socialise.

How Common is FIP?

Coronavirus is very common in cats and the incidence increases with groups of cats of five or more. It is only possible to estimate how widespread it is and ‘guestimates’ vary between 25% and 40% in domestic cats in one or two cat households. In larger groups of household cats or in catteries and rescue centres, this number is more likely to be 80%. Of these affected cats, some will shed the virus for a short time only and then stop never to do so again. Others will shed it continuously. This sounds a very worrying total, but this is the estimate for the harmless version of the virus – the mutation rate is actually very low. Where FIP becomes a threat is after the mutation has happened; a cat in which this has happened can then infect the whole population and then it is very serious.

Why does the Virus Mutate?

The reasons for the mutation of the virus are not known precisely, but it has been suggested that stress plays a large part in it. Most mutations happen in younger cats and it is at this time of their lives that most cats are most stressed; they are taken from their mothers and home, they are vaccinated, they are introduced to other cats that may not be friendly, they are spayed or neutered. Sometimes these events can quite literally happen in the space of a few days and if the cat is already infected with coronavirus, then this is the time when the virus will mutate, when the cat’s defences against such a thing are down.

FIP in Pedigree Cats

Pedigree cats are more at risk than cross breeds and also work is progressing on identifying certain genetic lines which seem to be more susceptible to others, but this may turn out to be of purely statistical interest as numbers affected remain relatively low. With all of that said, some older cats in non-stressful situations also experience the mutation of the virus into FIP, so these factors are not the only ones. Viruses can mutate for no reason – the flu virus is a good example with which we are all familiar.

What are the Signs of FIP?

The signs are very vague but quite severe and so your immediate reaction will be to take your cat to the vet. It will be very lethargic, with difficulty breathing due to the build-up of fluid in the abdomen. It may have lesions on the eyes and have a wobbly walk or some quite extreme behaviours, showing that the animal is hallucinating. Diagnosis is quite difficult as there is no definite blood test for FIP, but by using a combination of visual signs and various other blood tests such as proteins and bilirubin (these are usually high) and a test for the coronavirus antibodies can make the diagnosis more secure.

Is there any Treatment?

Sadly, there is no proven treatment for Feline Infectious Peritonitis. Some vets give anti-inflammatory drugs and appetite stimulants to improve the cat’s quality of life, and there have been a few unproved examples of anti-virals making a difference to life expectancy, but in practice the kindest thing if your cat has been definitely diagnosed with FIP is euthanasia, always a difficult choice but in this case wholly justified as the cat will be feeling very ill indeed and will not improve until death intervenes.

Can FIP be Prevented?

As such, FIP cannot be prevented – there is a vaccine but it has not been passed for use and most vets do not use it as unproven. The best way of preventing FIP in your cat is to make sure that any new cat you introduce into your home is from a small, stable community. FIP is actually quite rare in household pets where the number of animals is less than five. It spreads and mutates much more quickly in large communities such as breeding catteries and rescue centres. In a household situation keeping stress factors – such as worming, rehoming, spaying or neutering when the cat is otherwise unwell – to a minimum should make sure that your pet or pets remain unaffected by the mutated virus. This is the only preventative measure you can take but it is remarkably effective.


In catteries, it is inevitable that a large number of the cats will have coronavirus, increasing the risk of mutating forms being present. Keeping stress levels down and splitting large numbers of cats into smaller stable groups can help minimise it. Checking a pregnant female for coronavirus is also a good measure to take. If she is negative, it is vital to remove her from the group and employ stringent hygiene measures to keep her that way. Then she will not pass the virus on to her kittens and without the initial viral contamination, FIP cannot follow. If she has coronavirus, early weaning and removal of kittens can help – until four weeks of age, they are protected by her antibodies.

How Worried should I be?

FIP is a nasty disease and fatal in almost every correctly diagnosed case. It should be borne in mind, though, that its incidence is about 1 in 5000 for households with one or two cats. With around 19% of households owning a cat, this means that – without bothering you will all the maths – in a town with around 50,000 people, you would have one case of FIP. So the bottom line is, it is serious, but mercifully quite rare.


You can now get advice online 24/7 from a qualified vet: Ask a Vet Online Now

Close Bitnami banner