A Winn Feline Foundation Health Article On ...
Feline Infectious Peritonitis
Susan Little DVM, Diplomate ABVP (Feline)
One of the most poorly understood and enigmatic feline viruses is the feline coronavirus - the virus responsible for feline infectious peritonitis (FIP). It is no exaggeration to say that this is one of the most feared diseases in catteries. Many catteries that remain operative for several years will have a brush with FIP. Despite the fact that this disease is a shared experience in the cat fancy, affected catteries are wrongly feared and ostracized. All breeders need to make efforts to understand this disease and how to control it.
While the first description of feline infectious peritonitis was reported by Dr. Jean Holzworth in 1963, there are reports of clinical cases that are likely FIP going back to1914. Even though we have known about this virus for a long time, we know frustratingly little about it. However, much new research has been done in the past 5 years that is slowly shedding more light on this ever-present feline health problem. This article is designed to present some of the newer information and change some of the older ideas still found in print and other media.
Feline coronavirus operates differently from any other feline virus in several important ways: a) systemic antibodies have no protective function for the cat and may play a role in the disease FIP itself b) antibody titres are meaningless for diagnosis of FIP or prognosis c) a vaccine is available, but there is no consensus on its efficacy or safety
First, some notes on terminology.
FIP is the term for clinical disease associated with feline coronavirus infection.
The common benign form of feline coronavirus is referred to as FECV (feline enteric coronavirus).
When FECV has mutated into a disease-causing form, it is then referred to as FIPV (feline infectious peritonitis virus).
Feline coronaviruses in general are referred to as FCoV.
FECV is a very common, highly infectious feline virus. It belongs to the genus Coronavirus, which has members that infect other species (man, swine, cattle, birds, dogs). The majority of cats with FECV (about 95% or more) remain healthy. But in a small number of cases, FECV infection is the first step in a chain of events leading to FIP. This happens because coronaviruses are made of large numbers of nucleotides, the basic unit of genetic material, and they are very prone to mutations. As a virus reproduces itself, errors are made in copying these nucleotides. The more nucleotides, the more errors are possible. While most of these errors are harmless, some will have the effect of giving FECV the ability to cause disease. These mutant FECV strains are called FIPV.
Recent research has shown that mutant FECVs arise within an individual cat. Thus, we now know that the vast majority of cats do not "catch" FIP, but they develop it themselves from their own mutant FECV. Transmission of FIP from cat to cat is considered to be rare. This fact has caused leading FIP researchers to state that cats who are ill with FIP are unlikely to be a risk to other cats and thus do not need to be isolated.
It has been estimated that in multi-cat households where FECV has been introduced, 80-90% of all the cats will be infected. Catteries are especially likely to be FECV positive since traffic of cats and kittens in and out of the establishment is common. However, the incidence of cases of FIP is quite low in comparison. Generally, most catteries experience far less than 10% losses to FIP over the years. Rare instances have been documented where an apparent epidemic of FIP is associated with mortality rates of over 10% in a short period of time. One possible factor in these epidemics is the shedding of virulent virus, an uncommon situation. Usually, losses are sporadic and unpredictable. The peak ages for losses to FIP are from 6 months to 2 years old (with the highest incidence at 10 months of age). Age-associated immunity to FIP appears to be possible. Transmission of FIP from a queen to her unborn kittens has not been shown to occur.
What are the factors that predispose a small percentage of cats with FECV to the development of FIP? Research is currently trying to find more answers to this question, but some facts are becoming clear. Dr. Janet Foley and Dr. Niels Pedersen of the University of California at Davis have identified three key risk factors: genetic susceptibility, the presence of chronic FECV shedders, and cat-dense environments that favour the spread of FECV.
A genetic predisposition to the development of FIP was identified by Drs. Foley and Pedersen in 1996. They examined pedigree and health data from 10 generations of cats in several purebred catteries and found that the heritability of susceptibility to FIP could be very high (about 50%). It is likely a polygenetic trait rather than a simple dominant or recessive mode of inheritance. Inbreeding, by itself, is not a risk factor. Selecting for overall disease resistance is a helpful tool for breeders. The likely defect in immunity to FIP is in cell-mediated immunity. Therefore cats that are susceptible to FIP are also likely susceptible to some other infections as well, especially fungal and viral infections. This finding gives breeders the ability to achieve success in reducing the risk of FIP by using pedigree analysis to select breeding cats from family backgrounds that have strong resistance to FIP and other infectious diseases.
Research has shown that there are two main patterns that occur with FECV infection. Most cats will become infected and recover, but will not be immune. They are susceptible to reinfection the next time they contact the virus. A small number of cats become infected but do not recover. They become persistent shedders of FECV in the cattery and are the source of reinfection for the other cats. Therefore, the key to eliminating FECV (and thus the risk of FIP) in a cattery would be the identification and removal of chronic shedders. Currently, however, there is no easy way to determine which cats in a cattery are persistent shedders. The traditional antibody titre for FECV cannot be used to determine which cats are chronic shedders. The most effective and practical tool is PCR analysis of feces for the presence of FECV, a test which is not yet widely available.
In addition to selecting disease-resistant breeding stock, breeders can initiate husbandry practices that discourage the spread of FECV and development of FIP. Cat-dense environments favour the transmission of the highly contagious FECV. Dr. Diane Addie of the University of Glasgow, Scotland, recommends that the ideal way to house cats in catteries is individually. However, since this is not always possible, she recommends that they be kept in stable groups of no more than 3 or 4. Kittens should remain in groups of similar ages and not be mixed with adults in the cattery. Any measures that reduce environmental and social stress in the cattery population will have a beneficial effect.
FECV is spread primarily by the fecal-oral route and, to a lesser degree, through saliva or respiratory droplets. The virus can persist in the environment in dried feces on cat litter for 3 to 7 weeks, so scrupulous cleaning of cages and litter pans is important to reduce the amount of virus in the environment. It is important to have adequate numbers of litter pans available and that they be scooped at least daily and dumped and disinfected at least weekly. Litter pans should be kept away from food bowls and spilled litter should be regulary vacuumed up from the floor.
Dr. Addie has also described a method for early weaning and isolation of kittens born to FECV positive queens. It involves rigorous barrier nursing techniques to prevent the spread of the highly contagious FECV, and so is not for every breeder or cattery. The procedure involves first isolating the pregnant queen in a separate area to have the kittens. When they are 5-6 weeks old (at the time when their maternal immunity to FECV is waning), the kittens are removed from the queen and isolated by themselves. Some of the difficulties with this method involve the strict infection control procedures needed and possible difficulties in socializing kittens. When properly practiced, not only can FECV-negative kittens can be produced, but the kittens are often less prone to respiratory diseases and other common kitten ailments.
As with so many aspects of FIP, testing remains problematic. To date, there is no way to screen healthy cats for the risk of developing FIP. Antibody titres are poorly correlated with risk of FIP and should not be used to screen cats. As well as problems with interpretation of these antibody tests, there are problems with laboratory quality control. There are newer DNA-based tests offered by a few labs that are purported to be FIP-specific. However, these tests are considered unvalidated by experts as they have not been subjected to scientific scrutiny by researchers outside of the labs that offer them. In addition, there are no published studies that have identified the genetic difference between FECV and FIPV. An article was published by the Cornell Feline Health Center in 1998 warning that the so-called FIP-specific ELISA tests are based on insufficient data and the assays are not yet validated.
The fact remains that we have no screening test for FIP in well cats. Neither do we have a fool-proof way to diagnose FIP in a sick cat. The gold standard remains a biopsy or findings at necropsy. Recently, Dr. Andrew Sparkes and his colleagues at the University of Bristol, England, have suggested that combining several test results (globulin levels, lymphocyte counts) with clinical findings and antibody titre can help rule in or rule out FIP with some degree of certainty. It remains true, however, that a negative antibody titre does not rule out FIP. Neither does a positive antibody titre rule in FIP as a diagnosis. One benefit of the new DNA-based tests may be their use on body fluids, such as effusions from the chest or abdomen of a sick cat. If virus is found in these fluids, it strongly supports the presumptive diagnosis of FIP.
Probably one of the most controversial areas in any discussion of FIP is Primucell FIP, the vaccine made by Pfizer Animal Health, available since 1991. The vaccine is a modified-live termperature-sensitive viral mutant licensed for intranasal use in cats at least 16 weeks of age. The manufacturer recommends annual revaccination although no duration of immunity studies are available. The vaccine stimulates local immunity and will also produce an antibody titre. Evaluation of the risks and benefits associated with this vaccine is a difficult venture and has engendered much controversy.
Since FIP is a severe and fatal disease, the safety of any vaccine is a paramount consideration. Dr. Fred Scott of the Cornell Feline Health Center, concluded in a recently published paper, that the risks associated with the Primucell FIP vaccine are minimal in most situations. He notes that the vaccine has been in use for 7 years with no increase in the incidence of FIP. Troubling reports of a phenomenon called "antibody-dependent enhancement" (ADE) of infection arose from several labs, where cats vaccinated with FIP vaccines and challenged experimentally with virus developed accelerated disease instead of being protected. It is not known whether the phenomenon of ADE occurs in the real world and there is no easy way to find out. If it does occur, it is likely an uncommon event, but the possibility remains troubling.
On the other side of the issue, the benefits of the Primucell FIP vaccine appear to be small. The best reported efficacy for the vaccine is seen when FCoV negative cats at least 16 weeks old were vaccinated twice (3 weeks apart), in a study by Dr. Nancy Reeves published in 1995. In this study, FCoV antibody-negative cats were vaccinated before entering a large cat shelter where FIP was endemic. The vaccinated cats experienced a significantly lower mortality rate than unvaccinated cats. The efficacy of the vaccination was calculated to be 75% (preventable fraction).
In catteries where FIP is endemic, studies have shown the vaccine had no effect on the incidence of disease. One reason may be that most kittens in catteries are infected between 6 and 10 weeks of age, long before the 16 weeks of age the vaccine is licensed for. Once a cat is infected with FCoV, the vaccine has no benefit. Some cattery owners have been using the vaccine at ages younger than 16 weeks to get around this problem. Dr. Johnny Hoskins has outlined a vaccination protocol for catteries experiencing FIP losses in kittens under 16 weeks of age. He recommends giving the vaccine at 9, 13 and 17 weeks with annual revaccination afterward. Use of this protocol must be made with the knowledge that no controlled studies have been done on kittens under 16 weeks of age and that this is an off label use. It would appear that the use of the vaccine according to the manufacturer’s directions is limited to the vaccination of FCoV antibody-negative cats entering high risk situations, such as catteries and shelters.