Friday, January 7, 2011
Feline Panleukopenia Virus
Feline panleukopenia (also called feline infectious enteritis, feline "distemper," and feline ataxia or incoordination) is a highly contagious viral disease of cats characterized by its sudden onset, fever, inappetence (loss of appetite), dehydration, depression, vomiting, decreased numbers of circulating white blood cells (leukopenia), and often a high mortality rate. Intrauterine (within the uterus) infection may result in abortions, stillbirths, early neonatal deaths, and cerebellar hypoplasia (underdevelopment of the cerebellum) manifested by incoordination (ataxia) in kittens beginning at two to three weeks of age. All members of the cat family (Felidae) are susceptible to infection with feline panleukopenia virus (FPV), as are raccoons, coatimundis, and ringtails, in the family Procyoniclae. Many excellent vaccines are available to protect cats against panleukopenia. In unvaccinated populations, however, panleukopenia remains the most severe and destructive disease of cats.
THE CAUSE. Feline panleukopenia virus is a very small and very stable virus classified in the parvovirus group. The genetic material of the virus is composed of a single"strand of DNA. The virus is highly resistant to most disinfectants ether, chloroform, acid, alcohol, and heat (56'C, or 132.8'F, for thirty minutes)but is susceptible to Clorox bleach. Replication (reproduction) of the virus in the host occurs in cells that are themselves actively reproducing.
OCCURRENCE AND TRANSMISSION. Although it can affect cats of all ages, feline panleukopenia is primarily a disease of kittens. The characteristics of the disease may vary considerably from population to population and from outbreak to outbreak. In susceptible populations, the disease may affect nearly 100 percent of individuals; in other populations, only a few animals may be affected. Panleukopenia rarely occurs in populations in which vaccination is routinely practiced.
There is a seasonality to the occurrence of panleukopenia that usually coincides with the production of new populations of susceptible kittens. This seasonal effect may vary according to geographic location. In the northeastern United States, most cases of panleukopenia are seen during the summer and early fall. However, outbreaks of panleukopenia may occur at any time of the year.
Transmission of FPV occurs most commonly by direct contact with infected cats or their excretions. During the early stages of the infection, virus is shed in feces, urine, saliva, and vomitus. In addition, fleas may transmit FPV from infected to susceptible cats during the acute stage of the disease. The virus may also be spread by contact with contaminated objects, such as food bowls, litter pans, bedding, and cages, or by persons (on hands or clothing, for example).
The remarkable resistance of FPV to environmental conditions requires thorough cleansing and disinfection of the premises before the introduction of new cats. Once infection with FPV has occurred on the premises, however, the infectious virus may persist for months to years. New cats should be vaccinated against panleukopenia at least two weeks before their introduction to infected premises.
PATHOGENESIS. The virus usually enters orally, with infection occurring primarily in the lymphoid tissues of the oropharynx (tonsillar area) and intestine. Within twenty-four hours of infection, virus is present in the blood, which distributes it throughout the body. Within two days of infection, nearly every body tissue contains significant amounts of virus. As circulating antibodies appear, the amount of virus present gradually decreases. Small quantities of virus may persist for up to one year in certain tissues, but the strong immune response of the host usually neutralizes the virus as it is shed, so that most persistently infected kittens are not infectious.
The most severely damaged tissues in the infected newborn cat are those undergoing rapid cell division -the thymus (lymph organ in the chest) and the cerebellum (rear of the brain). Cells of the small intestine, which have a slow turnover rate in neonates (newborns), are not damaged, although the virus is present within them. In older kittens, the development of the disease also depends on the reproductive activity of the various tissues within the body. Lymphoid tissues, bone marrow, and the surface cells of the intestine are the most severely affected.
CLINICAL SIGNS. The severity of the clinical signs exhibited can vary tremendously from case to case. Many cats undergo a subclinical infection and do not show signs at all; the only method of diagnosis would be viral isolation or serology (detection of antibody in the blood). Others may experience a very mild clinical infection, in which a mildly elevated temperature, slight inappetence, and a borderline drop in the white blood cell count are observed. in the "typical" case of panleukopenia, clinical signs develop suddenly. The animal may have a rectal temperature of 104oF or greater and may be severely depressed and not eating. Vomiting usually occurs, and severe diarrhea may develop in twenty-four to forty-eight hours. If vomiting and diarrhea continue, severe dehydration and electrolyte imbalances occur.
Affected cats often assume a typical "hunched" posture with the head between the forepaws. Sometimes the head will hang over a water bowl or food dish. They often act as though they would like to drink and may even take a lap or two of milk or water, but they are unable or reluctant to swallow. The hair coat becomes rough and dull and there is a loss of elasticity of the skin due to the dehydration. The third eyelid (the haw in the corner of the eye toward the nose) often appears. The abdomen is painful, and touching it will elicit a pain response. The lymph nodes in the abdomen are enlarged, and the digestive tract contains excessive amounts of gas and liquid. Cats that are not going to survive develop a subnormal temperature, with coma and death following in a few hours.
The mortality rate in an outbreak of panleukopenia may vary from 25 to 75 percent. Acute deaths may occur, with kittens showing no warning signs, often causing the owner to suspect poisoning. More commonly, deaths occur within the first five days of illness in uncomplicated cases, or later in cases subsequently complicated by other problems. if a cat survives the first five days of illness, and secondary complications such as bacterial infections or dehydration do not develop, then recovery should follow fairly rapidly. it usually requires several weeks, however, for the animal to regain its lost weight and condition.
Signs in kittens infected in utero or just after birth go unnoticed before sudden death, or until the development of ataxia at about two weeks of age when the kittens begin ambulating. The incoordination is exemplified by rolling or tumbling as the cat attempts to walk, by an involuntary twitching of the head, or by swaying of the body. If they are coordinated enough to obtain food, the kittens will survive; however, the ataxia will persist throughout life with little, if any, improvement or compensation as they grow older.
DIAGNOSIS. A presumptive diagnosis of feline panleukopenia can be made by the veterinarian on the basis of the history, the clinical signs, and the presence of leukopenia. The diagnosis can be confirmed by necropsy (autopsy) examination, virus isolation (growth of the virus in cells in the laboratory), identification of the virus in infected tissues, or by demonstration of an increase in circulating antibodies by testing paired (acute and convalescent) serum samples taken several weeks apart (the increase in antibodies indicates recent infection).
TREATMENT. Panleukopenia. normally has a high mortality rate, but with diligent effort and good nursing care this can often be reduced. The main objective is to keep the affected animal alive and in reasonably good health until the natural defenses take over (i.e., the appearance of antibodies and an increase in number of circulating white blood cells). Antibodies usually appear about three to four days after the first signs of illness; two to three days later, the sharp "rebound" in white blood cell number can be expected to occur. Thus, if the patient can be supported for five to seven days after onset of the disease, the chances of recovery usually are good. Veterinary supportive care is aimed at the vomiting, diarrhea, and dehydration, which may dangerously upset fluid and electrolyte balance, and at preventing secondary bacterial infections. Secondary viral respiratory infections are common complications of panleukopenia. The FPV infection may act to trigger a latent respiratory virus, such as feline viral rhinotracbeitis virus or feline calicivirus. Simultaneous FPV and respiratory virus infections usually produce a more severe illness than if either virus alone had infected the animal.
PREVENTION. There are several excellent vaccines available to immunize cats against panleukopenia. These vaccines are highly effective and produce long-lasting immunity. Because panleukopenia is an entirely preventable disease, one cannot overemphasize the importance of proper immunization.
Immunization should be initiated by the veterinarian when kittens are eight to ten weeks of age. A second vaccination should be given four weeks later. In areas where the prevalence of infection is high, and for maximal protection, a third vaccination may be indicated at sixteen weeks of age. if a kitten is twelve weeks of age or older at the time of initial vaccination with a modified livevirus vaccine, a booster vaccination need not be given until it is at least one year of age. Immunity produced by FP vaccines is long-lasting, perhaps for life. Revaccinations every year would not seem to be necessary from a scientific standpoint, but the vaccines are licensed for only 3 years' protection.
Immunity acquired from the queen via colostrum (initial breast milk) must be considered when establishing a routine vaccination program. interference by maternally acquired (passive) immunity is the most common cause of vaccine failure. There exists a direct correlation between the FPV antibody level of the queen at the time of birth and the duration of passive immunity in the kitten. This passive immunity, if of sufficient strength, will not only protect the kitten against virulent FPV but will also react with the vaccine virus and interfere with immunization. Vaccination must be performed after kittens have lost most or all of their maternally derived immunity.
The use of FPV antiserum (clear blood liquid containing antibody) to immunize cats passively is indicated if an unvaccinated animal has been exposed to the virus or is likely to be exposed before vaccine induced immune responses can develop. Antiserum is also indicated for colostrum-cleprived or orphaned kittens. The routine use of antiserum in unexposed kittens is not recommended, however; instead, kittens should be vaccinated during their first visit to the veterinarian's office, and revaccinated as indicated.