RABIES INFECTIOUS AGENT
Rabies is caused by a rhabdovirus of the genus Lyssavirus. Serologic and molecular biologic techniques can distinguish among strains which are geographically distinct and/or are maintained in various animal populations, e.g. bat, raccoon, and skunk.
SIGNS AND SYMPTOMS
Rabies in animals can take a variety of forms, but a common prodromal feature is a change in behavior. Normally docile animals may become aggressive, and vice versa. Animals may next progress through a "furious" phase and then a "paralytic" phase, although not all rabid animals exhibit all of the stages. The appearance of nocturnal animals such as raccoons and skunks during daylight hours and without fear of humans may indicate rabies infection. It is possible for an animal, especially a wild animal, to be infected and not show any obvious clinical signs.
Rabies in humans usually begins with generalized anxiety with tingling, pain and or itching at the site of inoculation. This stage is followed by a nondescript flu-like illness. Next, an excitative stage, characterized by hypersensitivity to external stimuli and by hydrophobia , appears in most patients. Finally, a paralytic phase ends with death, usually due to respiratory paralysis.
Rabies virus is transmitted in nature through the introduction of virus-containing saliva from an infected animal into a susceptible animal or human by way of a bite or scratch, a break in the skin, or a mucous membrane. Rabies virus may be present in the saliva of animals even during the prodromal period. Human-to-human transmission has been reported as a result of corneal transplantation. Aerosol transmission to humans in bat caves has also been suggested.
The incubation period in humans is usually 3 to 8 weeks. Incubation periods of months to over a year are not uncommon, however.
The incubation period in animals varies. In dogs, it is usually 3 to 8 weeks, although incubation periods longer than 6 months have been reported.
PERIOD OF COMMUNICABILITY
The period of communicability is variable and depends on the species of animal infected, among other things. In dogs, virus shedding in the saliva occurs at or a few days before the development of prodromal signs and continues until death. About 50% of field-infected dogs had rabies virus in their saliva at death. In other species of domestic and wild animals, the period of communicability is not well known. Some animals may shed virus without exhibiting any obvious clinical signs of disease.
Rabies in animals and humans is normally diagnosed by direct fluorescent antibody (FA) testing of the brain. Other tests include mouse inoculation, FA test of frozen skin sections, and serology.
All animal bites and other potential exposures to rabies virus should be investigated thoroughly and without delay. Every effort should be made to determine the circumstances and degree of exposure and to locate and capture the biting animal. The vaccination history of biting dogs, cats and ferrets should be noted, as well as whether the bite was provoked or not. (See Animal Bites section.)
Most human rabies cases in the United States are the result of direct contact with infected wildlife or indirect contact with wildlife through cats, dogs and other domestic animals. Thus, avoiding contact with wildlife, control of stray dog and cat populations, and vaccination of all dogs and cats (per Indiana law) would greatly reduce the risk of rabies infection for humans. In areas with epidemic wildlife rabies such as the eastern seaboard states and Texas, wildlife vaccination projects have been initiated. For the present and near future, Indiana has been spared the need for such programs.
Pre-exposure immunization is appropriate for persons, such as conservation officers, animal control workers and veterinarians, at high risk of exposure to rabies-infected animals. The primary series consists of 3 injections on days 0, 7 and 21 or 28. Post-immunization serologic testing and/or booster doses are used depending on the level of continuing risk of exposure.
Post-exposure immunization (injections on days 0, 3, 7, 14, and 28, plus rabies immune globulin on day 0) should be given under the following circumstances: 1) bites, scratches or contamination of broken skin or mucous membranes with the saliva of a known rabid animal or with any wild carnivore unavailable for testing, 2) encounter with a bat in which the possibility of a bite, scratch or contact with saliva can not be ruled out, and 3) bites or scratches from a domestic animal, usually a cat or dog, that is unavailable for testing. In 3), the circumstances of the exposure and the incidence of rabies in the geographic area should be taken into consideration when deciding whether to proceed with post-exposure immunization. For rabid dogs, cats and ferrets only, virus in the saliva will precede death by no more than 10 days. Thus, if a dog, cat or ferret bites a human, the animal may be observed for 10 days. If it does not become ill or die within that time, it can be presumed to have not been infectious at the time of the bite, and post-exposure immunization will not be necessary. There is no known risk associated with delaying the start of post-exposure immunization until after the 10 observation period. The 10 day observation period is not applicable to any other species. An unwanted dog, cat or ferret that exposes a human should be humanely killed and the head sent for testing. (See the flow chart at the end of this section for more information on post-exposure immunization decision-making.)
Dogs, cats and ferrets with a current vaccination status and which have been bitten or otherwise exposed to a known rabid animal, carnivorous wildlife species or bats should be revaccinated and observed for 45 days. An unvaccinated dog, cat or ferret that is exposed to a known rabid animal, carnivorous wildlife species or bat should be euthanized immediately. If the owner is unwilling to have this done, the animal should be placed in strict isolation for 6 months and vaccinated 1 month prior to release.
The 10 day observation period applies only to dogs, cats and ferrets.
Most of the human rabies cases in the U.S. in recent years have been due to the bat strain of the virus. In many of these cases, there was no known bat bite, and in a few cases, no known bat exposure at all. Thus the current recommendation is that if bat exposure has occurred and a bite or other contact can not be ruled out, then post-exposure immunization is warranted.
Bites by rodents and rabbits almost never require testing or treatment. Ground hogs, opossums and livestock are rarely infected with rabies in Indiana and decisions on whether to initiate post-exposure immunization for humans exposed to these species should be based on the individual circumstances.
There has not been a rabid dog or cat diagnosed in Indiana in over since 1989 and 1984, respectively. Thus, the risk of rabies transmission from these species is low but not zero.
Information on vaccine can be obtained from Merieux Institute at 1-800 VACCINE or the Chiron Corporation at 1-800-CHIRON8.
It is very important to ensure that dogs, cats and ferrets being held under observation for 10 days are kept in secure facilities (kennel, clinic, pound, home etc.), that the persons responsible for keeping the animal under observation understand their responsibilities, and that someone from the local health department or other agency determines the status of the dog or cat after the 10 days are up. The fact that the animal did not become ill and thus is not rabid should be communicated to the bite victim as soon as possible.