RabiesJeffrey E Barlough, D.V.M., Ph.D., Diplomate, American College of Veterinary Microbiologists
The specter of rabies has been known throughout Asia and Europe since the days of antiquity. Today, rabies can be found on all continents of the world except Australia and Antarctica. it is ordinarily a disease of bats and carnivores, including the domestic dog and cat and many wild species. Despite the availability of excellent human and animal rabies vaccines, rabies remains a spe7cial cause of concern among human populations, especially in the developing world.
In the developed nations, canine rabies vaccination programs have all but eliminated cases of rabies among human beings. Animal rabies, however, still occurs with varying frequency. Among domestic species in the United States, cats now surpass dogs in the number of rabies cases reported each year. Although those numbers remain small, they serve as a reminder of the continual presence of this ancient scourge.
THE CAUSE. Rabies is caused by a bullet-shaped, virus belonging to the family Rhabdoviriclae. Rhabdo viruses are enveloped viruses and hence are relatively easily destroyed by common household soaps and detergents.
OCCURRENCE AND TRANSMISSION. Rabies is maintained in nature by wild and domestic carnivores and by certain other wildlife species. in the United States, skunks play a major role in spreading the disease, especially in the Midwestern portion of the country, where they are now the primary reservoirs of infection. Raccoons are important in transmission in the southeastern states and have recently begun to spread the disease to northern areas of the Atlantic seaboard. Wild foxes are important reservoir hosts in Europe and to a certain extent in North America as well. In the Caribbean and much of the Americas, bats are important reservoirs. In Latin America, vampire bats are particularly notorious for spreading rabies. The only rodent species of any importance in rabies transmission appears to be the woodchuck, in the mid-Atlantic and Midwestern United States. Among domestic species, only dogs and cats are important carriers of the infection. in most developing nations today, dogs remain the primary reservoir of the disease and the principal source of human exposure. Rabid animals excrete vast numbers of rabies virus particles in their saliva-a fact that accounts for the primary means of rabies virus transmission, the bite of an infected animal.
PATHOGENESIS. The incubation period-the time between exposure to the rabies virus and the development of signs quite variable, ranging from one week to one year. Most of this variability appears to reflect the length of time the rabies virus spends within muscle cells at the site of the bite, prior to gaining access to the nervous system. Once the virus has entered nerve endings, however, it advances relentlessly up the nerve bodies until it reaches the spinal cord and eventually the brain. From there, it can spread to other tissues important in transmission of the virus-the salivary glands, respiratory system, and digestive tract. To date, the actual mechanism by which the virus produces locomotion and cerebral derangement, with eventual death of the host, remains unclear.
CLINICAL SIGNS. In general, signs of rabies in cats are similar to those observed in other domestic species, but the signs seen in each individual case may vary widely. Two principal forms of rabies are recognized: an excitatory or "furious" form, and a paralytic or "dumb" form. In actuality, most rabid animals exhibit some manifestations of both forms. The paralytic form of rabies always represents the terminal or end stage; however, some animals may die during the convulsive seizures of the furious stage without exhibiting the final stage. Some will show few or no signs of excitement, the clinical picture reflecting instead the effects of paralysis.
During the furious stage, which lasts variably for one to seven days, affected animals become wild and aggressive. Rabid cats are extremely dangerous animals because of their viciousness and quickness of action. Rabid animals frequently snap at imaginary objects and may attempt to bite any animals or humans that approach them. If restrained, an animal may chew viciously on metal chains or the bars of its cage. It may break its teeth, lacerate its mouth and gums, and drool a ropy saliva tinged with blood.
Within a short time, these signs give way to those of the final or paralytic stage, which lasts only for a day or two. The paralysis usually appears first in the muscles of the head and neck, the most characteristic sign being difficulty in swallowing. Signs of localized paralysis are quickly succeeded by more generalized paralysis, with death following usually within two to four days of onset.
For both animals and humans, rabies is an inevitably fatal disease once clinical signs have appeared (only three human survivors are documented in the medical literature). Therefore, utmost care must be taken if one suspects that a pet has been exposed to the rabies virus.
DIAGNOSIS. A definitive diagnosis of rabies can be made only by laboratory examination of brain material from an affected animal. Considering the grave prognosis for recovery from rabies once clinical signs have appeared, it is imperative that an accurate diagnosis be obtained. Any wild or domestic mammal that has bitten a human being and is showing signs suggestive of rabies should be humanely destroyed, and the head submitted to a qualified rabies laboratory for diagnostic testing. In addition, any bat or wild carnivore, regardless of signs manifested, that has bitten a human being should be destroyed immediately and the brain examined for the presence of the rabies virus (this latter action is necessary because of the variable period of salivary virus-shedding that can occur before clinical signs appear). Any healthy domestic animal that has bitten a human being should be confined for at least ten days and observed for the development of clinical signs of rabies.
An unvaccinated domestic animal that has been bitten by or exposed to a known rabid animal should either be destroyed or placed in quarantine for six months and vaccinated for rabies one month prior to release. A rabies vaccinated domestic animal that has been bitten by or exposed to a known rabid animal should be given a rabies booster immunization immediately and observed by the owner for ninety days. if signs of rabies appear during the time of observation, the animal should be humanely destroyed and the brain examined to confirm the diagnosis of rabies.
Currently, three methods are available for the laboratory diagnosis of rabies: immunofluorescence microscopy, the most rapid and accurate method, in which slides of brain tissue are examined for the presence of the rabies virus using special antibodies and a fluorescent microscope; histopathology, in which sections or smears of brain tissue are examined for the presence of Negri bodies, intracellular inclusion bodies seen in many (but not all) cases of rabies; and mouse inoculation, which is frequently used to confirm positive results or to investigate further suspected cases that have proven negative by other methods.
TREATMENT. Because of the potential risk of exposing susceptible humans to the rabies virus, treatment of animals suspected of having rabies is strongly discouraged. Treatment of humans exposed to a rabid animal, however, must be aggressively applied. Treatment of humans consists of thorough flushing and cleansing of the bite wound with soap and water (the importance of this simple step cannot be overemphasized); administration of rabies immune globulin (rabies virus antiserum) to exposed individuals who have never been vaccinated against rabies; and administration of the human diploid-cell rabies vaccine in five doses, given on days 0, 3, 7, 14, and 28 postexposure.
PREVENTION. In the United States and most other countries of the world, effective rabies vaccines are available for use in domestic animals. Mass immunization of dogs has been employed for many years to control the spread of rabies by creating an "immunological barrier" between wildlife reservoirs of rabies and human populations. Several countries, including England, Iceland, Japan, and the Scandinavian nations, have succeeded in eradicating rabies by implementing control programs and very strict quarantine regulations. It is recommended that all dogs and cats be vaccinated for rabies at three months of age and revaccinated as required by vaccine specifications. At present, there are no rabies vaccines licensed for use in wild animals; however, genetically engineered rabies vaccines are being tested in selected wildlife populations.
PUBLIC HEALTH SIGNIFICANCE. The signs and course of rabies in humans are similar to those seen in animals. Both excitatory and paralytic symptoms may be manifested. The incubation period, as in animals, is quite variable from about two weeks to as long as one year-but on the whole it averages between three and six weeks. The course of the disease is short-only a few days-and the mortality rate is essentially 100 percent. For the safety of humans and their pets, cat and dog owners should see to it that their animals are routinely vaccinated against this most deadly and uncompromising of viral diseases.
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