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Review Article: Feline Infectious Peritonitis

Feline coronavirus infection is extremely common in our domestic cat population and its implications are concerning for cat owners, breeders and veterinarians alike. In this article we take an in-depth look at this virus and, in particular, consider how by understanding it better we can hope to reduce infection rates and limit its sometimes devastating consequences. 


Feline Infectious Peritonitis (FIP)

FIP Disease

Feline infectious peritonitis (FIP) is an invariably fatal disease caused by feline coronavirus (FCoV) infection. The ‘wet’ or effusive form occurs when immune complexes aggregate in blood vessel walls, leading to vasculitis and subsequent leakage of fluid. The ‘dry’ or non-effusive form is characterized by multiple granulomas or pyogranulomas which occur in various sites (eg. lungs, liver, kidneys, intestines and CNS) and ultimately lead to overwhelming disease. However, these disease types are not distinct entities and affected cats often have a combination of both with one form predominating over the other.

fig. 1 - pendulous abdomen may be apparent on inital presentation

The clinical signs are initially non-specific and include lethargy, anorexia, pyrexia and weight loss/poor growth. Cats with ‘wet FIP’ later go on to develop body cavity effusions. The most common site for this is the abdomen, leading to the classical presentation of a thin cat with distended, pendulous abdomen (figure 1) but fluid may also accumulate within the chest leading to dyspnoea. In cases of ‘dry FIP’ the clinical signs tend to be more vague and weight loss (or failure to gain weight) may be the predominant finding (figure 2). However, neurological signs such as behaviour changes, gait abnormalities or seizures may be present. In both ‘wet’ and ‘dry’ FIP, jaundice may be noted. Additionally, perivascular granulomatous lesions may be detected on retinal examination (figure 3), so ophthalmoscopy is an important part of the clinical examination. FIP affects many organ systems throughout the body and ultimately leads to organ failure. The disease course varies from a few weeks to months (tending to be longer in non-effusive patients) but is invariably fatal.

Transmission and risk factors

fig. 2 - weight loss may be readily apparent

Cats that are infected with feline coronavirus excrete viral particles (figure 4) in their faeces. The virus itself has a protective envelope, which allows it to survive in the environment for several weeks. Other cats become infected when they ingest the viral particles either during mutual grooming or from the environment. Once ingested the virus will multiply in the gut and the cycle of excretion in the faeces will start again.

Feline coronavirus infection is widespread in the domestic cat population and is particularly prevalent in situations where many cats are housed together. In multi-cat households infection rates are often as high as 80-100%. Infection rates in cat shelters vary dramatically depending on the type of housing (individual or group), the age of the cats, and the length of time they have spent in the shelter. Luckily, the vast majority of cats that become infected with feline coronavirus do not get FIP. Most infected cats show no clinical signs or develop only a mild gastro-enteritis which is self limiting within a few days. A very small percentage of cats infected with feline coronavirus will actually go on to develop FIP.

fig. 3 - granulomatous lesions may be visible on retinal examination

Any cat has the potential to develop FIP but older cats appear to be protected. Over 80% of patients are young cats (<2yo) and many of these are kittens. Purebred cats are at the highest risk for development of the disease. This is thought to be due to a genetic predisposition although management factors are also likely to play a role. Stress appears to increase a cat’s susceptibility to FIP and the disease can sometimes be triggered after a stressful event such as re-homing or neutering. At the moment it is still unclear why some cats develop FIP while others do not. Most people believe it is down to a combination of factors including the inability of an individual cat’s immune system to fight the infection, along with mutation of the virus causing it to become more pathogenic.


fig. 4 - electronmicrograph of coronavirus

The diagnosis of FIP is often not straightforward. The only definitive pre-mortem test is histological and immunohistochemical evaluation of tissue biopsies, which is not always practical. However, a tentative diagnosis can be based on a combination of appropriate signalment, suggestive clinical signs and supportive laboratory results. In ‘wet FIP’ the presentation of a young, thin cat with a swollen abdomen is very suggestive. Serum biochemistry, routine haematology and fluid analysis should then be done to look for changes consistent with FIP (figure 5). Fluid analysis is often the most rewarding laboratory test and even visual inspection can be suggestive (FIP effusions are usually straw coloured and are highly proteinaceous which tends to make them viscous and frothy when shaken – figure 6). In ‘dry FIP’ the clinical signs can be very vague and diagnosis is much more difficult.

fig. 5 - reference for clinicopathology findings to aid diagnosis of FIP

The findings of the clinical examination and blood tests must be carefully considered and other conditions must be ruled out. If neurological signs are present then csf analysis can certainly be helpful (figure 5) and diagnostic imaging (radiography/ultrasonography/MRI) can be used to look for evidence of granulomatous lesions. Serum antibody titres for feline coronavirus can be measured but are often unhelpful since titres in cats with FIP can be extremely wide ranging. A very high titre may increase the suspicion of FIP but a negative titre does not rule it out. It is also extremely important to remember that many healthy cats will have positive antibody titres and this does not mean that they are going to develop FIP!


fig. 6 - frothy appearence of peritoneal fluid

The most frequently used therapy for cats with FIP is anti-inflammatory doses of prednisolone. Many cats with FIP will survive for weeks or sometimes months with treatment but it is important to counsel owners that there is no cure for this disease and that it is invariably fatal. The most important consideration is keeping patients comfortable in the time they have remaining. Particular attention should therefore be paid to ensuring adequate nutrition (+/- appetite stimulants and anti-emetics) and providing other supportive therapies such as intravenous fluids and analgesia as required. Feline interferon omega is a widely used therapy but as yet there are no controlled trials confirming whether or not it is of any clinical benefit.


Prevention of FIP is difficult but steps can be taken to reduce the risk of disease. Although feline coronavirus is a fairly hardy virus it is easily destroyed by most household detergents. This means that good hygiene practices such as removing faeces from litter trays daily and disinfecting the tray at least weekly can be very effective in reducing the viral burden. In multi-cat households having several litter trays (one tray per cat if possible) also helps to minimise the viral burden and reduce cross contamination. Coronavirus is very easily transmitted from one cat to another and, because of this, reducing the number of cats in a household is a very effective way of controlling infection. In general it is advised that cats should be housed together in groups of no more than five. This means that, if possible, owners with more than five cats should separate them into smaller groups that are kept completely isolated from each other. In very young kittens the risk of coronavirus infection is usually highest at around five weeks of age as their maternally derived antibody levels wane. Because of this, early weaning and isolation policies can be of benefit in reducing the incidence of FIP in breeding establishments. This involves isolating the queen into a disinfected ‘kittening room’ one to two weeks before she is due to give birth. If the queen is antibody positive the kittens should be weaned at five to six weeks of age and placed into isolation. If the queen is antibody negative then she can remain with the kittens until they are older. Pfizer have developed an FIP vaccine called ‘Primucell’. However, the use of this vaccine remains a contentious issue amongst veterinary surgeons and it is not currently licensed for use in the UK. One major problem is that the vaccine is only licensed for kittens of over 16 weeks of age. Many kittens will be infected with feline coronavirus much earlier than this (from five weeks of age onwards) and for these cats the vaccine will have no benefit.

Case Report

Having a cat with FIP is always extremely distressing and it is made particularly difficult if there are other cats in the household. However, having a good understanding of feline coronavirus infection and of the control measures that can be put in place helps many owners to cope better with the disease and its implications. The following report follows the experiences of a multicat household with feline coronavirus infection. It illustrates the problems of having a cat with FIP and also highlights positive measures that can help to reduce the impact of the disease for the future.

Hopper, a 12 week old female Bengal kitten, was presented to her veterinary surgeon because she just seemed a little quiet and off colour. Her owner had adopted her only four weeks previously from a reputable breeder and at the time she had appeared to be in very good health. The owner had four other adult cats of various breeds and none of the older cats were unwell. When Hopper was examined by her vet she seemed a little small for her age but no other abnormalities were detected, and because her appetite was still good the owner was advised simply to keep a close eye on her for the next couple of days. Unfortunately Hopper’s condition deteriorated and over the following week she became inappetent and developed a distended abdomen. On returning to her vet blood samples were taken along with samples of ascitic fluid. The clinical signs and laboratory results strongly suggested that Hopper had developed FIP and palliative treatment with prednisolone tablets was started. Initially Hopper seemed brighter and her appetite improved but ten days later her condition deteriorated dramatically and her owner took the difficult decision to have her put to sleep.

This was a very sad time for Hoppers owner but as well as being devastated about Hopper she was also extremely concerned about the implications of feline coronavirus for her other cats. She was determined to do everything possible to minimise the chances of this happening again so she decided to have her remaining cats tested for feline coronavirus antibodies. All four cats were found to be positive and this was not unexpected given the high infectivity rates of the virus. Luckily FIP is usually sporadic and it is very rare for clusters of disease to occur together. When this does happen it is most often in related cats, which may have had a genetic susceptibility to development of the disease. The four remaining cats were all healthy, adult cats and their risk of developing FIP was therefore low. The owner was advised to maintain good hygiene practices, especially regarding disinfection of the litter trays, but no changes to the lifestyle of the cats was required and they remained in good health.

Given time it is likely that the coronavirus burden in the household would gradually reduce, although with four cats housed together there would be no guarantee of eliminating the coronavirus infection altogether. The owner was advised not to adopt another kitten for at least six months. However, if this had been a priority then arrangements could have been made to minimise the chances of a repeat FIP problem. In the case of re-homing a healthy adult cat the risk of FIP development is low and usually the only precaution necessary is to minimise stress for all the cats involved as far as is possible. However, in the case of re-homing kittens and cats with other diseases more care should be taken. In these cases it is wise to check the coronavirus antibody titres of the cats. If a coronavirus antibody positive cat enters a household with endemic coronavirus infection the risk of FIP development in any of the cats is low. However, if an incoming cat is found to be antibody negative then the best solution would be for it to be re-homed into a coronavirus negative household or alternatively into a single cat household.


In summary, it is certainly true to say that when FIP does occur the disease is dramatic and devastating. However, if the remaining cats are otherwise healthy and sensible precautions are taken, then the risk of FIP development in the other cats remains low in the vast majority of cases, despite high feline coronavirus titres.