Feline infectious peritonitis (FIP) is an uncommon, fatal viral disease of cats caused by infection with feline coronavirus (FCoV). Infections with FCoV are in fact very prevalent in cats throughout the world, and in most cases the virus will cause no clinical signs or only mild transient disease such as self-limiting diarrhoea. Uncommonly, infection will result in the development of the progressive fatal manifestation of disease known as FIP - one of the most serious of the viral infections of cats not only because of its fatal nature, but also because of the difficulties in achieving an ante mortem diagnosis, and the problems associated with control of this disease.
Feline coronaviruses (FCoV) belong to a group of closely related coronaviruses that cause enteric and respiratory disease in dogs and pigs, and the common cold in human beings. There is no evidence that the cat virus is transmitted naturally to any other species. An important characteristic of coronaviruses is that they are prone to genetic change (mutation) and variant viruses arise in infected animals that may be more pathogenic than, or antigenically different from an original virus.
From a clinical perspective, it is recognised that there are two 'biotypes' of FCoV. One is the common, or ‘enteric', form of FCoV that is transmitted in nature between cats and represents the vast majority of FCoVs in the population. This biotype causes no overt disease, or only transient, self-limiting gastrointestinal signs. The virus is spread mainly by the faecal-oral route, virus being shed in the faeces of an infected cat and subsequently ingested by a susceptible cat. The virus mainly replicates within the cells that line the intestine, and may cause local damage that results in the diarrhoea. Infected cats shed the virus for a variable period of time, usually a few months, and then stop. The cats produce antibodies to the virus, which can be detected in the blood, and the quantity declines when shedding of virus ends. After a cat has cleared an infection, it may become re-infected from another cat with the same or another strain of FCoV and begin shedding again. A small number of cats may become persistent, life-long shedders of the virus.
The second biotype of FCoV is the one that causes FIP. It is believed that most clinical cases of FIP arise through the emergence of this virulent virus strain by mutation during an existing infection with the relatively avirulent common ‘enteric' biotype. A smaller number of FIP cases involve viruses that arise through recombination between FCoV and the closely related canine coronavirus (CCV), which suggests that CCV is occasionally transmitted from dogs to cats. Whatever their origin, FCoVs of the FIP-causing biotype do not appear to be commonly transmitted in nature and usually die out with their host. However, the occurrence of outbreaks of FIP in individual colonies of cats suggests that direct transmission of a FIP-causing biotype is occasionally possible.
How the variant viruses cause FIP is not yet known. It has been suggested that the main difference between the FIP-causing viruses and the related ‘enteric' FCoV strains is that the former can invade the body and are not restricted to the intestinal tract like the latter, possibly because they are able to more readily infect and grow in macrophages. However, it is now clear that in cats infected with FCoV that do not develop FIP, virus also invades beyond the intestine and is found in their blood. Irrespective of the precise changes that occur in the virus, the development of clinical disease (FIP) will also depend on an ineffective immune response where the infected cat fails to adequately control replication of the virus resulting in inflammation in virtually any site in the body, thus producing a wide range of clinical signs.
Therefore, the outcome of the infection in an individual cat depends on the result of a complex interaction between the strain and dose of virus, and the age and immune competence of the animal. Potential sources of FCoV infection for a susceptible cat include asymptomatic carrier cats, cats with FCoV-induced enteritis, clinical cases of FIP, and environmental contamination with virus.
Infection with FCoV is ubiquitous in domestic cat populations throughout the world. Surveys have demonstrated that typically 25 to 40 per cent of household pet cats are FCoV seropositive (ie, have antibodies against FCoV detectable in their blood indicating current or previous infection with the virus), whilst this figure generally rises to 80 to 100 per cent for cats from breeding catteries and other large colonies. Yet despite this high level of FCoV infection in cat populations, FIP remains a relatively uncommon disease (see below), with most infections appearing to be with low-virulence 'enteritis-producing' strains of the virus.
FCoV infection is thus endemic in cat populations, and highly prevalent in large colonies of cats such as multicat households, rescue and breeding catteries. Where the virus is endemic in a group of cats, some adults may be carriers and continually shed virus in their faeces, while others may become re-infected over time and shed virus only intermittently. Studies have suggested that as few as six cats sharing the same environment are sufficient to maintain persistent FCoV infections.
Kittens born in an environment where FCoV is prevalent will likely be protected initially by maternally-derived immunity (antibodies absorbed from the mother's milk in the first 24 hours of sucking). This immunity typically wanes to undetectable levels at six to eight weeks of age, and most kittens become infected and shed FCoV in their faeces from five to 11 weeks of age. Following this, the kittens produce their own antibodies. Although coronaviruses are quite fragile and susceptible to most commonly used disinfectants (for information on disinfectants see: www.fabcats.org), some strains may remain viable in the environment for several weeks, suggesting that susceptible kittens may sometimes be infected from a contaminated environment as well as via contact with other cats.
Feline infectious peritonitis is predominantly a disease of young cats occurring from the post-weaning period onwards. The peak age for development of disease is between six months and two years, although it can occur in cats of any age. Probably the risk of the generation of a mutation of FCoV that is able to produce FIP in an infected cat is greatest when the amount of virus replication in the body is very high. Replication of the virus may be enhanced by the poorer immune response of the young cat, especially when this is combined with stress factors such as rehoming, neutering, vaccination and intercurrent disease. There is a much higher prevalence of disease in cats from breeding catteries and other large multicat households, presumably largely reflecting the fact that these are environments where exposure to FCoV is difficult to avoid. Indeed, the proportion of cats in a cattery chronically shedding FCoV in their faeces, and the frequency of FCoV shedding has been shown to be linked to the risk of FIP cases developing. However, other factors may also be involved such as genetic susceptibility to disease, as some breed predilections for development of FIP have been suggested (eg, Persians and Burmese).
There are no accurate figures for the incidence of FIP, but it has been suggested that the overall mortality due to FIP in mature household cats kept in low numbers (one or two individuals per household) is approximately 1:5000 (0.02%), whereas in colonies of cats with endemic disease, it is typically up to 5 per cent (with tremendous variation between different colonies). Typically, even in colony situations, the disease is sporadic with outbreaks occurring where mortality rates may reach around 10 per cent. Occasionally small epidemics of disease are seen where up to 40 per cent of high risk (young) cats may die, but these are rare. Commonly, FIP may develop in several or all of the kittens from an individual litter over a period of time.
Most kittens infected with FCoV are infected with ‘enteritis'- producing strains and remain asymptomatic, or develop mild to moderate self-limiting diarrhoea (and occasionally vomiting) of a few days' duration. On only one occasion has FCoV-induced enteritis been reported to be fatal (in the absence of FIP).
On the other hand, FIP is a disease with extremely diverse clinical manifestations, and clinical signs that tend to be progressive and ultimately fatal. Unfortunately there are no clinical signs associated with FIP that are unique for this disease, and the diverse nature of the signs means that FIP must be considered a possible cause of many different feline disease syndromes.
In general, the disease takes on one of two broad forms - effusive (or 'wet') FIP and non-effusive (or 'dry') FIP. In effusive disease, the virus causes inflammation of the blood vessels resulting in the leakage of protein-rich fluid from the blood into body cavities (most commonly the abdomen in a form of peritonitis). In non-effusive disease inflammatory lesions (granulomas) develop around blood vessels within organs and other structures leading to more diverse and often more vague clinical signs, depending on what site(s) of the body are affected. The most common places to find granulomas are the abdominal organs (liver, kidneys, intestines, lymph nodes), the eyes and the central nervous system (CNS). Effusive disease is more common than non-effusive and in general probably accounts for around 60 to 70 per cent of cases.
The clinical course of FIP varies from several days to several months. Generally effusive disease tends to have a somewhat shorter duration than non-effusive disease and most cases have a course of a few weeks. The early clinical signs in FIP are nonspecific and common to both forms of the disease. These include fluctuating pyrexia, lethargy, inappetence, weight loss, diarrhoea and occasionally mild upper respiratory tract signs. The severity and duration of these signs vary, but even where quite severe, their vague nature makes an early diagnosis of FIP extremely difficult. As the disease progresses, often more overt signs of either effusive or non-effusive disease develop, although in some cases this may take many weeks or months to occur.
Accumulation of fluid in the abdomen and obvious abdominal distension is one of the most common manifestations of FIP, but the fluid will sometimes be present in the chest rather than the abdomen (causing difficulty in breathing), and sometimes at both sites. Inflammatory lesions in the eyes and the CNS (causing neurological signs), and the liver (causing jaundice) also occur in a proportion of cats with 'effusive' disease. The common overt signs of effusive disease (abdominal distension and/or difficulty breathing) are often accompanied by a waxing and waning nonresponsive pyrexia (fever) and the other non-specific signs seen early in the course of disease. However, some cats remain remarkably bright despite the presence of severe disease.
In non-effusive disease, clinical signs can be extremely vague (persistent pyrexia, progressive weight loss and lethargy are often dominant signs) and prolonged. In around 50 per cent of cases, lesions will affect the CNS (causing progressive neurological signs, with ataxia - a 'wobbly' gait - being probably the single most common manifestation) and/or the eyes (causing changes such as uveitis or retinitis, detectable on ocular examination). Jaundice is also a relatively common finding. Some cats with this type of FIP do also have small amounts of effusion present in their abdomen or chest which, while not severe enough to cause distension or interfere with breathing, may be helpful in making a diagnosis (see below).
FIP is one of the most difficult diseases to diagnose in cats. The wide range of clinical signs makes FIP a potential consideration in many different clinical cases and there is no routine laboratory test that can be used to definitively confirm the clinical suspicions of a diagnosis. In deciding whether FIP is a likely or probable diagnosis in any individual case, consideration needs to be given to the background of the cat and historical features, to the clinical signs the cat is showing, and to the results of a variety of routine laboratory tests. These may lead to sufficient evidence for a presumptive diagnosis of FIP to be made, but if significant doubt exists confirmatory testing of tissue biopsies may be necessary.
A complete assessment of the clinical picture and results of routine laboratory tests is needed to enable a presumptive diagnosis of FIP to be made.
Background
Cases of FIP are far more common in young cats (less than two years of age), in pedigree cats or cats from multicat households. Older cats and cats from single cat households also develop FIP, but these cases are less common, and with appropriate clinical signs and supportive laboratory data, the index of suspicion will be higher in cats known to have come from a higher risk background.
Clinical signs
In cases of suspected FIP, a thorough clinical examination is performed to try and identify any signs or changes that are consistent with the diagnosis. This includes a thorough ocular and neurological examination whether effusive or non-effusive disease is suspected and careful examination (eg, radiographs or ultrasound examination) for detection of the presence of small amounts of effusion. If an effusion is present, laboratory analysis of the fluid is one of the most useful of the routine laboratory tests. Similarly if abdominal effusive disease is suspected, thoracic (chest) radiographs to detect a pleural effusion can be valuable, as few other diseases produce an effusion with the characteristic features of FIP in both the abdominal and thoracic cavities.
As FIP is a progressive disease, the clinical manifestations will change over time. Reaching a diagnosis in the early stages when clinical signs are vague may be extremely challenging, but in most cases, as time goes by, more classical signs of FIP will develop. However, this does mean that repeat and thorough clinical examinations (and blood tests) may be required in ongoing cases in order to detect the development of these changes.
Routine laboratory tests
Results of routine laboratory tests are useful in either supporting a presumptive diagnosis of FIP, or suggesting that some other cause for the clinical signs is more likely. On routine haematology, the most consistent features are a lymphopenia (low lymphocyte count), a neutrophilic leucocytosis (high neutrophil count) that may be accompanied by a left shift (presence of immature neutrophils), and a mild to moderate non-regenerative anaemia. All these are nonspecific changes that can occur with other disease.
Similarly, serum biochemistry changes are non-specific in FIP, with the most common abnormalities being a hyperproteinaemia (high blood protein concentration) due to hyperglobulinaemia (high globulin levels), often accompanied by low or low to normal serum albumin concentrations. Hyperproteinaemia has been reported in 50 per cent to 80 per cent of FIP cases, but varies from mild to very marked (due to the presence of inflammatory proteins and production of antibodies/immunoglobulins). Other biochemical changes seen with FIP include jaundice or hyperbilirubinaemia (in approximately 25 per cent of cases) and elevated liver enzymes.
Although all the haematological and biochemical changes seen with FIP are non-specific, and can occur with many other diseases, they are still valuable in making a presumptive diagnosis of the disease. In a cat showing appropriate clinical signs, if there are multiple haematological and biochemical abnormalities typical of the changes seen with FIP, this increases the likelihood that FIP is the underlying cause of the disease. Conversely, if few or none of the typical changes are observed, FIP is an unlikely underlying cause. Results of these tests can be misleading though, as occasionally cases of confirmed FIP show no abnormalities on routine haematology and biochemistry, whereas other diseases may at times lead to all of the changes classically associated with FIP!
Coronavirus serology
Coronavirus serology (detection of antibodies to the virus) is frequently performed as part of the routine screening of cats with suspected FIP. Excessive reliance should not be placed on this test, and it is of no more value than results of routine haematology and biochemistry. No test is able to distinguish between the different strains of FCoV and thus a positive result simply shows that a cat has been exposed to a strain of FCoV. The presence and magnitude of the antibody titre cannot distinguish the type of FCoV strain to which a cat has been exposed, whether the infection is current or previous, or whether the cat is susceptible or resistant to the development of FIP.
Interpretation of coronavirus titres thus requires great care, and a positive titre in light of the background of a high prevalence of seropositivity in the general pet cat population (typically 25 to 40 per cent are seropositive) and a very high prevalence of seropositivity in multicat households where typically 80 to 100 per cent are seropositive. In general, cats with FIP tend to have higher FCoV antibody titres than healthy cats, but there is complete overlap in the magnitude of titres between these populations so that interpretation of results in an individual is extremely complex. Furthermore, a minority of cats with confirmed FIP also lack detectable FCoV antibodies!
Fluid analysis
The analysis of effusions remains one of the most valuable routine tests for the diagnosis of FIP. This is the reason why even in suspected cases of non-effusive disease, it is worthwhile undertaking investigations to assess whether a small amount of effusion is present and can be aspirated. The fluid in cases of FIP is often, but not invariably, straw-coloured and viscous due to its high protein content. In FIP effusions, the total protein content of the fluid is in excess of 35 g/l (with levels of around 60 g/l being typical) and the globulin content is invariably 50 per cent or more of the total protein content. The cell count of effusions is usually less than 20 x 106/ml. Few other diseases produce effusions similar to those seen in FIP, and in the presence of other supportive evidence (clinical signs and laboratory data) this can provide strong evidence for a presumptive diagnosis. The finding of this type of effusion in more than one body cavity (eg, thorax and abdomen) provides even more compelling evidence.
Other tests
Advances in molecular biology have led to the development of sensitive techniques to detect the genetic sequences of viruses (including FCoV) in specimens of blood, for example via the method known as polymerase chain reaction (PCR). Experience with this test shows that as with antibody testing, PCR is unable to distinguish the strain of FCoV to which a cat has been exposed, and very many healthy cats are positive using this test (probably reflecting infection with non-virulent strains of FCoV). Results of studies therefore suggest this test has little, if any value, in the routine diagnosis of FIP and at best has no more value than serology. Detecting the presence of virus by PCR in effusions (in ‘wet' FIP) or in CSF (in cases of neurological disease) may be of more diagnostic value, though these results again can be misleading.
In cats with neurological manifestations relating to FIP, analysis of cerebrospinal fluid (CSF - the fluid that surrounds the brain and spinal cord) can give valuable additional information, both to support the diagnosis of FIP, and to rule out other potential causes of the clinical signs. With FIP, CSF samples typically have marked elevations in both total white cell counts (with a predominance of neutrophils), and total protein levels.
The results of the investigations outlined might, in many cases, give sufficient grounds for a presumptive diagnosis of FIP to be made. However, not uncommonly a definitive diagnosis is desirable prior to a decision on whether euthanasia should be performed, and where the presence of disease has implications for other in-contact animals.
The only way of making a definitive diagnosis of FIP is by histopathological examination of affected tissues collected either at post mortem examination, or collected ante mortem from appropriate sites (affected tissue - eg, lymph node, liver, kidney etc). Thus where significant doubts over the diagnosis of effusive FIP remain, exploratory surgery may be indicated to collect biopsies of affected tissue for histological analysis. Similarly in suspected non-effusive disease, evidence can be sought for involvement of disease at sites where biopsy collection of material would be possible, by surgery, or by per-cutaneous needle biopsy using ultrasound guidance if necessary. Ultrasound examination of the abdomen can be particularly useful in this instance, for detecting granulomas in the liver, kidneys or spleen, or for detection of enlarged lymph nodes.
Microscopic examination of tissues affected by FIP yields characteristic signs of inflammation, but if there is any doubt as to whether FIP has caused the inflammation, additional confirmation can be sought by detecting the presence of FCoV within the lesions using immunohistochemistry (special staining procedures to detect the virus).
Once clinical signs of FIP develop, it is generally an incurable and fatal disease. Although exceptional cases of spontaneous recovery do occur, these are extremely rare and the disease is usually both progressive and fatal. Treatment of affected cats is therefore generally supportive and symptomatic. Fluid therapy, nutritional support and symptomatic treatment with corticosteroids are probably the most common therapeutic interventions used, and this may produce temporary alleviation of clinical signs.
However, concerns exist in multicat households as a cat with clinical disease clearly has the potential to be shedding virulent FCoV thus posing a potential risk to other in-contact cats. In severely affected cats, euthanasia is usually the most humane course of action to avoid suffering. Attempts have been made to treat FIP diseased cats with a combination of immunomodulating and antiviral drugs in the hope of inhibiting viral replication while stimulating a cell-mediated immune response. Such an approach is attractive, but has not actually proved to be beneficial so far. There have been some reports of potential response to the use of interferon in some affected cats but again, substantial data to confirm the value of this type of therapy is lacking.
Management strategies to prevent cats developing FIP are hampered in many respects. The strains of virus causing FIP are uncommon mutations of a ubiquitous virus present in virtually all cat populations. Infection with pathogenic strains of the virus, and differentiation from non-pathogenic strains is impossible in the clinical situation, and making a definitive diagnosis of the disease itself is problematic. Nevertheless, management strategies have been developed to help prevent the likelihood of FIP developing, which are to a large extent aimed at preventing or reducing the prevalence of the underlying infection with FCoV.
Household pets
In general, FIP is uncommon to rare in household pets (except where a new kitten has been obtained from a colony where FCoV infection is endemic). Ideally, to minimise the risk of this occurrence, household pets should be obtained from a source where there are relatively few cats, or where cats/kittens are kept in small stable groups. If there is a particular concern, it may be possible to obtain kittens known to be seronegative to FCoV.
If household pets are kept singly or in small, stable groups, the risk of developing FIP is very low.
Other than choosing the source of new kittens with some care (healthy kittens from a healthy background), and avoiding having large numbers of cats sharing one living space, specific measures to reduce the risk of FIP in household pet cats are difficult to recommend and probably not warranted due to the low frequency of FIP.
Breeding catteries with endemic FIP
In breeding catteries with a history of FIP, two management approaches have been advocated: either eradication of infection or taking measures to try and reduce the frequency of disease.
Eradication of infection is laborious, time-consuming and requires a considerable commitment. Furthermore, given the ubiquitous nature of FCoV infections, maintaining a virus-free colony can be extremely challenging. This system is thus unsuitable for use in most household or breeding cattery situations.
A more practical approach is to consider elimination of FCoV infection in kittens born in an FCoV-endemic environment, thus providing the opportunity of selling or re-homing kittens free of FCoV infection. This approach has been shown to work well in several studies but may not always be completely successful. If pregnant queens are isolated when they are one to two weeks pre-partum, and if the queen is subsequently kept isolated with her kittens (whilst employing good hygiene procedures to prevent environmental spread of infection to the kittens), a substantial proportion of these kittens will remain FCoV antibody negative once any maternally-derived antibody has waned. Following weaning, the queen can be removed and the kittens still kept isolated and tested at 12-16 weeks of age for FCoV antibodies. If the litter is seronegative, the isolation procedure will have been successful and they can be re-homed as FCoV negative kittens.
This procedure sometimes fails if the queen herself is shedding FCoV and thus passes it on to the kittens. It is thought that this may be less likely if an older (greater than two years of age) queen is used, but can be helped by early weaning of the kittens (at five to six weeks of age when maternally-derived immunity is still protective) and removing the queen from the environment. Good hygiene is also an important part of the control of spread of FCoV to the kittens in these situations.
Although these procedures have been proven to be successful, they too require considerable commitment from breeders, and there are genuine concerns about the behavioural and social development of kittens when they are reared in isolation up to the age of four months. Behavioural development will be better where the queen is kept with the kittens, but even this represents a compromise for the kittens.
As eradication of FCoV infection or even just eradication of infection in the kittens is difficult, and may compromise their development in other respects, often it may be more appropriate to accept that there is endemic FCoV infection, and to institute measures to try to minimise the prevalence of this infection, and minimise its impact. Practical control measures have been recommended bearing in mind that the faecal-oral route is the major way in which the virus is spread:
- Having at least one litterbox for every two cats, located in easy to clean/disinfect areas
- Litterboxes should be kept away from food and water bowls to prevent cross-contamination
- Faeces should be removed from litterboxes at least once daily, and litter should be completely changed as often as possible (at least weekly and ideally daily) accompanied by disinfection of the trays
- Cats should be kept in small stable groups of four or fewer - minimising cross-contamination within a household
- Breeding programmes with more than eight to 10 cats (including kittens) should not be undertaken in a normal household.
- Larger numbers require some purpose-built facilities to enable proper hygiene and care to be maintained
- Regular brushing of the coat, particularly of long-hair cats is desirable to reduce contamination with faeces and litter
- Isolation of queens and their kittens can be recommended as a means of controlling spread of FCoV to the kittens
New entrants to a colony of cats pose a potential risk, as they may be FCoV shedders. Short-term quarantine is probably not helpful as virus shedding may persist for many weeks to months. Introducing such a cat to a small, stable group of mature cats (see above) is likely to carry the lowest risk. If an individual cat is to enter a high-risk household, it would be sensible to perform FCoV serology first, and if the cat has a low or negative titre to undertake vaccination prior to its introduction. However, an efficacious vaccine is not yet available in the UK.
Cat shows and stud cats are also possible sources of infection, although hygiene at cat shows is probably sufficient to minimise this. Similarly the risk can be reduced for stud cats and visiting queens by limiting the time spent together and not allowing them to share food, water and litter trays; however this may not always be practical.
Breeding catteries with no history of FIP
In most cases, the FCoV status of cats in a cattery will be unknown. Serotesting of cats, in the absence of disease is of limited value as most catteries will still have endemic FCoV infection and thus most cats would test seropositive. Nevertheless, there are some exceptions to this and some catteries may contain cats known to be seronegative. As seronegative cats are unlikely to be shedding virus, in such situations efforts should be made to maintain the colony free of infection (through hygiene precautions and testing cats prior to being admitted to the colony) whilst recognising that long-term maintenance of the FCoV-negative status is extremely difficult.
Rescue catteries
Control of FIP in rescue catteries should follow similar lines to the control in breeding catteries where FIP is endemic. Adequate hygiene and avoiding overcrowding are essential strategies for minimising the risk of FIP in such situations. Cats should be housed individually, or if this is not possible, batched on arrival, and kept in small stable groups.
A commercial FIP vaccine has been developed based on the intranasal administration of a FCoV isolate that only replicates in the tissues of the upper respiratory tract and is designed to provoke a good immune response.
This vaccine is available in many parts of the world, but not currently in the UK, and questions have been raised over its efficacy with different studies providing very different results. It appears that the vaccine may have efficacy in some situations, but that it provides considerably less than 100 per cent protection. In addition, the vaccine is currently only licensed for use in kittens over 16 weeks of age, and in endemic situations most kittens of this age will already be infected with FCoV.
Nevertheless there are some situations in which the vaccine may be of value - if kittens have been reared under conditions to prevent exposure to FCoV, they could be vaccinated prior to introduction to a high risk environment, and similarly if other seronegative (or low titre) cats are being introduced to a high-risk environment (eg, where FIP is endemic) these too may benefit from vaccination.
However, as FIP is not a common disease (estimated to affect no more than one in 5000 pet cats), and the vaccine is considerably less than 100 per cent efficacious, routine vaccination of cats cannot be recommended.
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Feline infectious peritonitis (FIP) is an infection of cats caused by a virulent strain of feline coronavirus (FCoV). Cats are very commonly infected with FCoV, but most strains of this virus cause little or no disease. Only rarely do strains emerge that have the potential to cause fatal FIP. Thus FIP is an uncommon disease manifestation of a very common viral infection of cats. In FIP the virus can replicate throughout the body causing a wide variety of clinical signs. Unfortunately, in virtually every case, once clinical signs of disease develop, the cat will go on to die from FIP, or will need to be euthanased due to progression of the disease. The disease is called ‘infectious peritonitis', because peritonitis is one of the most common (and first-recognised) manifestations of the infection. However, a wide variety of different signs can be seen depending on the sites affected by the virus.
The clinical signs of FIP are extremely diverse. Broadly two forms of disease are recognised, so called 'wet FIP' and 'dry FIP'. In wet FIP the most obvious signs are due to accumulation of fluid in body cavities - usually either the abdomen (eventually causing marked swelling and distension of the abdomen) or the chest (around the lungs) causing difficulty breathing. In cases of dry FIP, the disease mainly manifests as areas of inflammation within various tissues of the body. This can lead to very diverse and often complex clinical signs including neurological disease, ocular disease, liver disease, kidney disease and intestinal disease. Some cats also have a mixture of the 'wet' and 'dry' forms of the disease. Affected cats are often quite ill, becoming lethargic with a poor appetite and weight loss and there is often fluctuating pyrexia. The course of the disease can vary from a few days to a few months. In the early stages, some cats remain quite bright, but over time there is an inevitable deterioration. Unfortunately, none of the clinical signs with FIP are diagnostic for this condition - many other diseases can cause similar signs.
The diagnosis of FIP is very difficult. The development of typical clinical signs may lead to a suspicion of the disease, but these alone are not diagnostic. On routine blood tests, FIP will commonly cause a variety of changes, including changes in haematology (red and white blood cell counts) and biochemistry (blood proteins, tests of liver and kidney function etc.). However, again, although these changes may suggest or support a diagnosis of FIP, they cannot confirm a diagnosis as the changes seen are not confined to cases of FIP. Where fluid develops in the chest or abdomen, retrieving a sample of this fluid is a simple procedure and analysis of this fluid is very valuable. In cases of FIP the fluid has a very high protein and globulin (one type of protein) concentration. The finding of a fluid typical of FIP is strongly supportive of a diagnosis, although again there are some other diseases that can cause accumulation of fluid with similar characteristics. At present, the only way to confirm a diagnosis of FIP is to look at histological (microscopic) changes in affected tissues (obtained by biopsy or at post-mortem examination). Where there is significant doubt about a diagnosis, or where it is important to be sure whether FIP is present or not, examination of tissues in this way is essential.
Coronavirus serology is a laboratory test that detects antibodies in a cat's blood sample against feline coronavirus (FCoV). However, this blood test does not have the ability to distinguish between different strains of FCoV, and cannot differentiate between strains that cause FIP and those more common strains that do not. Additionally, the presence of antibodies does not necessarily imply that the cat is currently infected with FCoV, although in practice it appears that in most antibody positive cats FCoV can be found. Although this blood test is often used as part of the investigation of a suspected case of FIP, it is not particularly helpful as a very large number of perfectly healthy cats have positive test results reflecting the high level of exposure to FCoV in the cat population (most strains being unlikely to cause any significant disease). In a healthy cat, the finding of antibodies against FCoV in a blood test is neither surprising nor alarming. This is common (as is the finding of antibodies against numerous infectious agents) and no special precautions are needed unless the test is being performed as part of a programme to try to eliminate or control FCoV infection in a cattery situation.
The virus that causes FIP is in fact a variant (spontaneous mutation) of a very common infection in cats. Infection with feline coronavirus (FCoV) is extremely common (probably between 10 per cent and 30 per cent of all pet cats have been infected with this virus at some point), but in the vast majority of cases the virus will cause nothing more severe than transient diarrhoea. However, occasionally a variant of the virus emerges that is much more pathogenic and in this situation, unless the infected cat can control the infection with a good immune response, FIP will develop.
Feline infectious peritonitis is a relatively uncommon disease in cats, but is more common in colony cats and in young cats (less than one to two years old). Accurate figures are difficult to obtain, but it is estimated that the incidence of FIP in general household cats is in the order of 0.02 per cent (ie, around one death per year in every 5000 cats). However, where many cats are kept together in close confinement (where the virus can be readily transmitted) and where there are young cats present (which are more susceptible to developing FIP) the disease is much more prevalent. Thus in some multicat households or catteries, the incidence of FIP may be as high as five per cent.
Unfortunately there is no known treatment for FIP. In cats where the disease is diagnosed or suspected the immediate action depends on the severity of the disease. In some cases immediate euthanasia is necessary on humane grounds. In others, palliative treatment may be provided with anti-inflammatories (eg, prednisolone) for a period of time to make the cat comfortable. However, the disease will eventually progress and euthanasia will inevitably be required.
In most cases, the virus that causes FIP arises as a spontaneous mutation of an existing FCoV infection in a cat, and the FIPproducing virus is not generally spread from cat to cat. The risk of directly 'catching' FIP at a cat show is thus extremely low. However, FCoV infections could potentially be spread at cat shows. The virus is usually shed in the faeces, and may contaminate the haircoat. Thus if hygiene precautions are inadequate there is potential for the virus to be spread between cats. Careful disinfection of tables, hands and equipment, together with minimising the amount of handling will help to reduce this risk.
The most important action to take if you suspect your cat has FIP is to seek veterinary advice as soon as possible. A variety of investigations may need to be done to investigate the possibility of FIP (clinical examination, blood tests, radiographs etc.). Where a definitive diagnosis is needed then a tissue biopsy will probably be required. In most cases, cats that develop FIP will not be shedding the virulent FIP-causing virus and therefore will not be a risk to other in-contact cats. However, this will not invariably be the case and, as a precaution, where possible, it is probably best to try to keep a suspected clinical case separate from other cats.
Control of FIP is difficult, but as it is a relatively rare disease in the general pet cat population, specific control measures are generally not warranted. No effective vaccine is available in the UK, and although one is available in some other countries, it is not entirely reliable and the disease is not common enough to generally warrant vaccination of pet cats. If FIP is suspected in a pet cat, it should be isolated from other cats and appropriately investigated (see above). If disease is confirmed and there are other cats in the household, it is possible for the disease to be spread, but in general most adult cats are able to resist infection and are unlikely to go down with disease. If there are young cats, and especially kittens, in the household, these are at greater risk and should be monitored closely for signs of disease. Blood tests on a healthy cat are of no value in predicting whether it will develop disease. Good hygiene should be maintained. The virus is spread mainly by the faecal-oral route and it is quite fragile, not surviving long in the environment. The virus is susceptible to most commonly used disinfectants that are safe for cats (for information on disinfectants see www.fabcats.org).
As most cats that develop FIP do not shed virulent FIP-causing virus, the risk to any other cats is very low. However, some additional precautions may be taken to further reduce any potential risk:
- If there are no other cats in the household, waiting six to eight weeks before obtaining another cat will help ensure there is no or minimal risk from environmental contamination with the virus.
Disinfecting food and water bowls, litter trays and areas where the cat has been will also help as the virus is susceptible to all commonly used disinfectants (see www.fabcats.org).
- If there are other cats in the household, although they will probably all have been exposed to and infected with FCoV, there is only a small risk they will have been infected with, or developed, an FIP-causing strain of the virus, and again only a very small risk that they could pass this on to other cats. To minimise such risks further it may be advisable to
- Avoid bringing any new cats, and especially kittens, into the household for the following six months. This reduces the risk of new cases of FIP and also reduces the risk of rejection of a new cat coming onto the household.
- Test any remaining cats every few months to follow their antibody status (blood tests), and wait until they become antibody negative before introducing any other cats. The basis for this is that antibody negative cats are generally thought to be free of virus, and therefore new cats introduced would not be at risk of being infected with FCoV. However, some cats may remain persistently infected with FCoV and if there are several cats it may be necessary to separate them to prevent re-infection between them. If remaining cats are monitored until seronegative (antibody negative), any new additions should also be tested, and be seronegative, prior to introduction. Although feasible, this approach is rarely practical, especially given the low risk of further cases of clinical disease.
This is not an uncommon scenario. While it is impossible to know in an individual case how and where the kitten developed the infection, in most cases it will have been infected with FCoV before being sold, and subsequently a mutation in the virus will have allowed the development of the FIP-causing strain. It is important in this situation that wherever possible the diagnosis of FIP is confirmed and good communication between all parties involved (owner, breeder and veterinary surgeon) is very important to avoid misunderstanding. If FIP is confirmed it should be recognised that this may be a 'one-off' case or a very intermittent problem. However, it is likely that the breeding household does have endemic FCoV infection and this is dealt with under the section 'What should be done in breeding households with endemic FIP?'
Realistically it is extremely difficult and time consuming to eradicate FCoV infection from breeding catteries with an FIP problem, although it can be done. It requires regular testing of cats to segregate cats exposed to (and potentially infected with) FCoV from the other cats, and eventually probably rehoming some cats that are healthy but persistently infected with the virus from the colony. Even if this is achieved, maintaining a FCoV-free colony is challenging. It is important to have cases of suspected FIP confirmed – other diseases can mimic the appearance of FIP. More realistically, several measures can be advised to reduce the spread of FCoV and reduce the risk of FIP occurring (which is mainly a problem in the kittens and juvenile cats):
- Having at least one litterbox for every two cats, located in easy to clean areas, away from food and water bowls to prevent cross-contamination
- Faeces should be removed from litterboxes at least once daily, and litter should be completely changed as often as possible (at least weekly, and ideally daily) and the trays disinfected
- Cats should be kept in small stable groups, ideally of four or fewer to minimise spread and maintenance of the virus in a household
- Unless there are specially built and designed facilities, ordinary households where cats are bred should house a maximum of eight to 10 cats and kittens. Higher numbers of cats increase the risk of FCoV infection.
- Avoid breeding from any queens which repeatedly produce litters that develop FIP
If there are repeated cases of FIP, or if a breeder wishes to produce kittens where the risk of FIP is the lowest possible, there are means of attempting to breed kittens that are free from exposure to the virus. Pregnant queens can be isolated one to two weeks prepartum, and the queen is subsequently kept isolated with her kittens (whilst employing good hygiene procedures to prevent environmental spread of infection to the kittens). Following weaning (which can be performed early – at five to six weeks of age – if necessary), the queen can be removed and the kittens still kept isolated and tested at 12 to 16 weeks of age for FCoV antibodies. If the litter is seronegative, the isolation procedure will have been successful and they can be rehomed as FCoV negative kittens. Although potentially successful, this requires considerable commitment from breeders, and there are genuine concerns about the behavioural development of kittens when they are reared in isolation up to the age of four months. Also, experience suggests that this is not an invariably successful strategy and considerable time and expense can be spent with little benefit in some circumstances.
There is no justification for routine serotesting of cats in a rescue centre, and euthanasia should never be performed simply on the basis of a high antibody titre. However, because of the relatively large numbers and turnover of cats in a rescue shelter, this is a situation where there is a higher than normal risk of FCoV infection being spread and of FIP developing. Several recommendations can be made to reduce this risk:
- Wherever possible house cats individually and not in groups
- If individual housing is limited, as a priority keep this for
- Cats that require quarantine (sick cats or new arrivals)
- Cats that are pregnant
- Kittens and juveniles (which are at a higher risk of FIP and other infectious diseases)
- If some cats have to be group-housed, keep them in small groups (ideally fewer than four to six individuals per group) and do not mix cats from different groups.
- Use sneeze barriers and good cleaning/disinfection regimes to minimise the risk of spread of FCoV and other infectious diseases
- Use a separate litterbox for each pen that is only used in that pen and is regularly disinfected (ideally daily).
- Allow at least one litter tray for every two cats if they are in groups
- If cases of suspect FIP develop
- Where possible have the diagnosis confirmed (by tissue histopathology), as many other diseases can look like FIP.
- Do not immediately rehome any cats that were in direct contact with the affected individual. These cats are the ones most at risk, and ideally should be kept for two to three months before rehoming (the longer they remain healthy, the lower the probability that they will develop FIP).
- If good hygiene is being maintained and if cats are not allowed to mix then the risk to any other cats is extremely small
The risk of FIP in a boarding cattery should be very low assuming that good standards are maintained. Guidelines on cattery design and maintenance are available in the FAB Boarding Cattery Standard and the FAB Boarding Cattery Manual (see: www.fabcats.org). Spread of any infectious disease, including FCoV can be prevented if there are appropriate hygiene measures (individual litter trays, adequate disinfection), sneeze barriers, and no direct contact between cats.
Although the SARS virus that affects humans is a type of coronavirus, there is absolutely no evidence that this virus can be transmitted between humans and domestic cats. There is some evidence that Civet cats may be involved in the spread of this disease to humans, but these are not actually cats at all (not members of the Felidae family).
Updated May 2006
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