FIP in cats


What is FIP and how does infection with FCoV?

FIP stands for feline infectious peritonitis and is caused by feline Coronavirus (FCoV).

Coronavirus infection is extremely common in cats, especially in high population areas. It is estimated that 25-40% of household cats are positive for FCoV (however this is closer to 80% in colonies). The virus replicates in the intestinal tract and is shed in the faeces where it can survive for several days and infection occurs after ingestion of the virus, generally by licking.


How does FCoV cause FIP?

Infection is generally limited to the gastrointestinal tract (with very limited viral replication elsewhere). While replicating in the intestine, the virus can, occasionally, undergo a spontaneous mutation which is capable of causing FIP. This new strain varies from the original in that it is no longer able to replicate within the intestine, preferring to use macrophages (one of the important cells of the immune system). When this begins, the virus is spread around the body, and if not met by an appropriate immune response, FIP will develop.

What are the clinical signs of FIP?

The clinical signs of FIP vary greatly and, unfortunately, can mimic other disease processes. Early signs are generally vague but can include fluctuating fever, lethargy and reduced appetite.
There are two main forms of the disease, known as ‘wet’ or ‘effusive’ disease, and ‘dry’ or ‘non-effusive’ disease. Many cats may in fact have a mix of these two types:

What is dry FIP?

The dry form of the disease is thought to develop due to a partially effective immune response which limits but does not stop viral replication, this is generally enough to stop fluid accumulation however not enough to prevent disease. This form often causes the development of chronic inflammatory lesions around the blood vessels and in other organs known as pyogranulomatous inflammation. In 30% of cases this inflammation can affect the eyes or brain but can also affect he kidneys, liver, lungs and skin meaning a wide range of signs may be observed.

In a number of cats, signs may develop that are a combination of both wet and dry disease.

What is wet FIP?

This form of the disease is characterised by an accumulation of fluid within the abdomen and/or chest cavities caused by inflammation (called vasculitis) which can result in distension of the abdomen and development of peritonitis or breathing difficulty. In wet FIP, the fluid that accumulates typically has a very high protein content and is often a clear-yellowish colour.

Which cats are most at risk from FIP?

FIP can occur in any cat of any age however it is most common in young cats, with 80% of cases occurring in cats less than two years of age. FIP is also most common in breeding colonies and high population households, possibly, in part, due to the stress of over-crowding dampening the immune response.

How is FIP diagnosed?

FIP can be a challenging disease to diagnose, in part based on the non-specific clinical signs and in part due to the lack of a simple diagnostic test.

Your Vet may become suspicious of FIP if a pet shows elevated white blood cells, increase globulin concentrations (protein in the blood), elevated liver enzymes and jaundice (a yellow discolouration to the skin).

If there is fluid present within the abdomen, then a sample of this can be taken and assessed for protein levels. Other tests that may be helpful

include analysis of the blood proteins for the presence of specific markers. Unfortunately, these tests cannot distinguish between the different types of coronavirus (FIP causing and non-mutated FCoV), therefore, they can be used in combination with clinical signs to increase suspicion, however they cannot give a definitive answer.

The best test to confirm diagnosis is to collect a tissue sample called a biopsy from affected tissues to isolate the virus, however, many cats are too sick to undergo surgery for this purpose.

Can FIP be treated?

Since 2021 a treatment for FIP called Remdesivir (an injection) and GS-44154 (a tablet) became licenced in the UK. This is administered daily for 12 weeks, starting with intravenous administration or injections under the skin for the first few days in very sick cats, transitioning to oral tablets once the cat is feeling better. For cats which are still eating and don’t require being in the hospital, treatment can commence with tablets only.  

Side-effects are uncommon but may include pain at the time of injection, nausea following intravenous injection and worsening of fluid around the lungs for the first 24 hours. Approximately 80-95% of cats are successfully treated, however relapse rates are currently unknown. 

How can FIP be prevented?

In some countries, a commercially available vaccine is available to protect against FCoV. The main indication for use of this vaccine would be in high risk cats (breeding households or colonies), however cannot be given in cats under 16 weeks of age, by which point the cats would have most likely already have been exposed to FCoV.


Reducing risk in multi-cat and breeding households

The risk of FIP can be minimised by obtaining your pet from a source with relatively few cats. Eradication of FCoV in breeding colonies can be extremely difficult and a more practical approach would be to use measures to prevent mutation into FIP.

Good practice to minimise risk would include: avoiding large groups of cats or having multiple litters in one go, keeping small isolated groups separate, having numerous, easy to clean litter trays Keep litter boxes away from food or water bowls, avoiding stress and maintaining good hygiene.

Cat Health Information

Find out more

To assist owners in understanding more about a health conditions that are specific to cats we have put together a range of information sheets to talk you through some of the more common health concerns seen and treated by our General Practice Vets.