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Feline infectious peritonitis (FIP) arises as a mutation of feline coronavirus (FCoV), an enveloped RNA virus. In contrast to the notorious parvoviruses, infection with FCoV is very common in shelter cats and other densely housed feline populations, while mortality due to feline infectious peritonitis (FIP) is generally low. This is fortunate, as many of the tools that help us control panleukopenia and parvovirus are simply not available for management of FCoV and FIP: vaccination is unreliable at best; diagnosis is rarely straightforward; and the murky understanding of transmission and development of disease makes set quarantine policies impossible. One might think this would not matter given the endemic nature of FCoV infection in multicat populations and the fact that FIP is not thought to be transmitted cat-to-cat per se. However, outbreaks involving substantially increased mortality due to FIP do occur in groups of unrelated cats in shelters and catteries.
Disease course and transmission of FCoV/FIP
Feline coronavirus is shed extensively in the feces of infected cats and is very readily spread by fomite transmission. Viral shedding may begin within just a few days of infection, and antibody titers will develop within 7-14 days. Although most cats eventually resolve their infection, some cats are chronic shedders. Shedding may be intermittent. The only method to detect shedders is PCR analysis of feces. A minimum of three samples obtained a month apart are recommended to consider a cat a negative shedder. Most often no clinical signs of FCoV infection are seen; mild diarrhea or respiratory signs may occur but will generally be indistinguishable from other common conditions in feline populations.
Feline infectious peritonitis generally results from a mutation of FCoV which occurs anew in each individual cat that develops the disease. This mutated virus is cell-associated and thus is not commonly transmitted directly from cat to cat. Disease generally develops within a few weeks to 18 months after infection with FCoV, often subsequent to a stressor such as rehoming or spay/neuter surgery. Disease most commonly occurs in cats < 18 months of age, and in cats > 12 years. There appears to be a genetic component to risk of FIP; thus littermates of kittens that have developed FIP are at substantially increased risk. Unfortunately, there is no way to predict, out of a group of seropositive cats at risk for FIP, which ones are more likely than others to develop the disease.
Diagnosis of FIP
A positive diagnosis of FIP can be difficult to make, particularly in the absence of characteristic effusion. Diagnosis must generally be made based on a combination of signalment, clinical signs, blood work, titer results and specific tests. The most common signs of FIP in young cats are cyclic, antibiotic non-responsive fever, lethargy, unexplained weight loss and failure to grow. Common laboratory abnormalities include elevated total protein (mainly globulin), increased numbers of total white blood cells and neutrophils, decreased numbers of lymphocytes and anemia. However, none of these abnormalities are present in all cats with FIP; and there are other conditions which can lead to any of these findings – therefore, FIP can not be definitely diagnosed or ruled out based on these tests alone. Definitive diagnosis is accomplished by visualization of FCoV within macrophages in effusion fluid, on biopsy or necropsy. However, such tests may not always be financially practical in a shelter.
Coronavirus titers: Feline coronavirus titers may be used as an adjunct to diagnosis, however these only indicate whether a cat has been exposed to the near-ubiquitous FCoV. There are no titer tests specific for FIP. If the titer is negative at <1:25, it is likely the cat is truly negative and does not have FIP; be aware that not all laboratories test down to such a low dilution, and negative at higher dilutions (e.g. 1:400) is not meaningful. Very high titers (>1:16,000) are strongly suggestive of FIP; however most cats with FIP do not have titers this high. Rising titers are less informative than they would be for other diseases, because cats with benign FCoV and those with FIP both cycle up and down in titer level. This is particularly likely in a shelter cat, who can be presumed to have suffered a recent onslaught of viral exposure. Diagnosis of FIP, or risk for developing FIP, should never be made based on titers alone.
Characteristic effusion: Wet FIP can often be diagnosed with reasonable confidence based on presence of yellow to clear modified transudate effusion (protein levels > 3.5 g/dl, low cellularity of < 5000 cells per microliter). Protein content can be readily assessed in-house using a refractometer. The Rivalta Test can be done in-house to further assess suspicious fluid: A test tube is filled with distilled water and one drop of 98% (glacial) acetic acid is added (this can be obtained from chemical supply companies). One drop of effusion is added to this mixture, watch carefully while doing so: if the drop dissipates (disappears), the test is negative. If the drop retains its shape, the test is positive. A negative Rivalta’s test is 97% accurate in ruling out FIP. A positive test is 86% accurate in ruling in FIP. Further evaluation of the fluid at a laboratory can include cellular count and characterization and albumin to globulin ratio. However, other rule outs for a modified transudate in the abdomen are virtually all serious conditions; therefore in a shelter that must make euthanasia decisions about some cats, it may be reasonable to euthanize cats based on a strong suspicion of an untreatable condition rather than investing resources on further testing.
Vaccination for Feline Coronavirus/FIP
There are inherent challenges to creating a truly reliable vaccine for FCoV. Even natural infection does not convey lasting immunity, and a vaccine is unlikely to do better than this. There is currently only one vaccine available for feline coronavirus, a modified live intranasal product labeled for use in cats > 16 weeks of age, to be given as a series of two vaccines 3-4 weeks apart. Results of studies regarding the efficacy of this vaccine have been variable, some showing no efficacy and others showing limited efficacy under certain circumstances. One study showed a significantly decreased risk of FIP for cats that were seronegative at the time of vaccination. Although there may be some benefit to giving the vaccine to cats that have never before been exposed to a multi-cat environment (and are therefore relatively likely to be seronegative), most shelter cats will have long since been exposed by the time the recommended booster vaccine can be administered. Unlike the parvoviruses, vaccination will be an adjunct at best and will not be sufficient in itself to control outbreaks of FIP.
What is an outbreak of FIP?
The frequency of FIP in “coronavirus endemic” catteries, multiple cat homes, rescue or hoarder homes is reportedly about 5-10% most of the time1,2. “Outbreaks” of FIP in > 10% of cats occurred in 4/7 catteries followed over five years. However, these are all environments where cats are chronically housed in a high-risk multiple cat environment. In a research cattery following introduction of coronavirus the rate of FIP was only 0.8% of 1000 exposed cats3, and in a study of cats adopted from an open-intake shelter where cats were only in the environment relatively briefly, the rate of FIP was simlarly low at less than 0.6%4. Therefore, rates higher than ~1% in a shelter are cause for concern.
Given that FIP itself is not generally transmitted cat-to-cat and coronavirus is so commonly present in multiple cat populations without causing apparent harm, what account for FIP outbreaks? Although the dynamics of outbreaks are poorly understood, several factors likely contribute:
The difficulties in diagnosing infection and predicting which cats will get ill; the lack of a reliable vaccine; the ease with which FCoV can be spread; and the poorly understood basis of FIP outbreaks combine to create unique challenges for control. Shelters and catteries must decide what to do in the face of an outbreak; whether to quarantine all exposed cats, depopulate, or continue business as usual. The same questions apply on an individual or household basis when a group of cats have suffered a known exposure to FIP. It is commonly agreed that cats remaining in a household from which a cat developed FIP, or those cats casually exposed to a cat with FIP (for instance, at a veterinary clinic) are not at appreciably increased risk of developing infection themselves. However, the risk of maintaining these cats in a shelter in which they are exposed to an ongoing parade of vulnerable new intakes is unknown. While the risk of introducing cats from such an environment into a home containing naïve cats is likely low, this also poses some problems as described below.
Can all exposed cats be effectively and safely quarantined? There is no way to predict which cats from a shelter having FIP problems are going to get FIP, although littermates of affected kittens are known to be higher risk. There is no realistic way to quarantine cats possibly exposed to a virulent coronavirus: if they are going to get FIP, it could happen any time in the subsequent couple of years. Even if they are not going to get FIP, they may continue cheerfully shedding coronavirus while under “quarantine” and beyond, which may place other cats (especially kittens) entering the facility in which the cats are quarantined at risk. Spread via fomites is so easily accomplished, it is unlikely that true quarantine can be achieved in a shelter facility, even if housed in a separate room from other cats (virus can be easily spread via clothing and shoes).
Risk to adopter’s pets: Most likely, if cats from an outbreak-environment are adopted out to homes with resident cats, the other cats in the household will not get FIP as a result of this exposure. This is true even if the newly adopted cat happens to be shedding a relatively virulent strain of feline coronavirus or itself eventually succumbs to FIP: the resident pets are likely under less stress than newly admitted shelter cats, and therefore at lower risk from exposure, and development of FIP is even less likely if all cats in the adopter’s household are over 2 years of age, and therefore in a lower risk period for development of FIP. In general, the risk of FIP is lower in households with less than five cats, as shedding of coronavirus is less likely to be chronically maintained. However, occasionally a cat in an adopter’s home will happen to get FIP following adoption of a cat from a shelter having FIP problems. This may be due to transmission of coronavirus from the newly adopted cat and mutation of that virus to FIP in the resident cat; although uncommon this is possible. Or, it may be that the resident cat already harbored a coronavirus infection from a prior exposure, and the stress of adding a new pet to the household, by an unfortunate coincidence, triggered the manifestations of FIP. Regardless of the reason, this obviously puts the shelter in a difficult and painful position of wondering if a fatal disease was transmitted to an adopter’s pet.
Keeping these difficulties in mind, there are three general strategies for responding to an outbreak of FIP in a cat population:
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3. Hickman M, Morris Q, Rogers R, et al. Elimination of feline coronavirus infection from a large experimental specific pathogen free cat breeding colony by serologic testing and isolation Feline Practice 1995;23:34-39.
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