Rabies (from Latin rabere-to rage) is an acute infectious disease of the central nervous system caused by a Rhabdo virus. It generally persists in nature as a salivary gland infection of carnivorous animals. The virus is usually transmitted from animal to animal and from animal to man by a bite (exposure). All warm-blooded animals are susceptible. Once signs of illness appear, rabies is 100% fatal; however, proper post-exposure treatment is nearly 100% effective.
Rabies is one of the oldest recorded infectious diseases, having been reported in Asia as early as 2000 BC. It was probably imported into North America from Europe in the eighteenth century in domestic dogs. One of the first North American records was of an epizootic in foxes in Massachusetts in the early nineteenth century.
It is currently found in every continent except Australia; most countries free of the disease are islands.
Rabies has been identified in the contiguous United States, Alaska, the District of Columbia and Puerto Rico. In 2005, (the most recent nationwide data) rabies was reported in 49 states and Puerto Rico. During that year, 92.3% of the rabies cases were diagnosed in wild animals (5,923 cases), with only 7.7% (494 cases) in domestic animals. The total number of rabies cases reported in non-human animals was 6,417 with one case being reported in a human in 2005. The number of wildlife cases reported in 2005 decreased among skunks, rodents/lagomorphs, foxes and raccoons. The relative contributions of the major groups of wildlife to the total number of non-human animals rabies cases were as follows: raccoons (36.7%; 2,534 cases), skunks (23.8%; 1,478 cases), bats (21.9%; 1,408 cases) and foxes (5.9%; 376 cases), accounting for 90.3% of all reported rabies cases in wildlife. Number of rabid raccoons decreased in 10 of 20 eastern states in which raccoon rabies is enzootic. Rabies in bats is widely distributed throughout the United States with cases reported from all 48 contiguous states. In Michigan, the most common wildlife species to contract rabies are bats, accounting for 67% to 100% of the rabies cases in the state during the period 1990 to 2006. Skunks and red foxes are the next most likely species to contract rabies and there have been 3 cases of the bat-strain rabies in raccoons (1986, 1989, and 1997).
Maps of Rabies Positive Animals in Michigan
Transmission and Development
Until 1960, dogs were the most important animal host of rabies in the United States. After the institution of mandatory rabies vaccinations for dogs, wildlife species have been the most frequently infected animals with rabies. This has also been true in Michigan.
In the U.S., rabies in terrestrial mammals can be linked to distinct virus strains. Each strain is maintained primarily by intraspecies (within species) transmission although spillover infection of other species may occur. Nationwide there is one strain in raccoons that has been identified in 19 states and the District of Columbia; 3 skunk-strains; 4 fox-strains; and 1 coyote-strain in Texas. Bat rabies has had distinct strains identified but geographic boundaries cannot be defined.
In Michigan, bat rabies is the prevalent strain identified.
Rabies is usually transmitted by the bite of an infected animal. Transmission has occurred by non-bite routes however in 3 instances. The first involved the presumed inhalation of virus particles in a bat cave, a second involved laboratory workers creating an aerosol and inhaling virus particles while using a power saw to cut the tops off skulls of rabies suspects, and thirdly, cornea transplants where the cornea donor had rabies. These routes of infection are not considered of general importance.
"The atypical is typical" describes rabies symptomatically in any species of animal. Animals with furious rabies exhibit aggressive signs early in the disease and then become paralyzed. Those with dumb rabies simply become paralyzed and die shortly thereafter. Animals with furious rabies usually have an excitation phase lasting several days. The animal is restless and soon becomes vicious, biting at anything and everything. This action gradually subsides; incoordination and tremors are often apparent. Convulsions, paralysis and prostration occur just prior to death.
An important consideration in reaching a clinical diagnosis of rabies in animals, especially wild ones, is that no sign (or series of signs) is typical or characteristic. Signs of other diseases such as distemper, hepatitis, listeriosis, tetanus, botulism and some parasitic diseases are similar to those of rabies. Encephalitic syndromes can also be caused by plant or chemical toxins. These clinical signs are so varied and overlapping that limited confidence should be placed on a clinical diagnosis of rabies. The only sure way to diagnose rabies is with laboratory tests.
There is no gross pathology evident in animals which have died of rabies. Microscopic lesions of the central nervous system are inflammatory and similar to those seen in other virus infections. Negri bodies (inclusion bodies in the cytoplasm of neurons of infected animals) are positive proof of rabies infection.
In Michigan, rabies diagnosis is done by the Michigan Department of Community Health in Lansing. Dogs and cats are quarantined for 10 days for observation. If no illness becomes apparent within that time, they are considered not to have rabies. Wild animals that have bitten or exposed humans or unvaccinated domestic animals should be killed immediately and their heads submitted to the proper health authorities as soon as possible. Bats should be sent in whole. Care must be taken when killing such animals to avoid damaging the brain. The individual handling the dead animal should wear rubber gloves as a precautionary measure.
Veterinarians and local health departments should have mailing containers to submit the head or body for testing. If none of these are available the specimen should be placed in several plastic bags, cooled and driven to the Rose Lake Laboratory for handling.
At the Community Health Laboratory, diagnosis is made by placing a small amount of brain tissue on a microscope slide and treating it with a fluorescent dye. When viewed under a microscope equipped with ultraviolet light, rabies-positive specimens will fluoresce, or glow.
Reports of positive findings are immediately sent to the health official under whose name the specimen has been submitted. It is his responsibility to notify the exposed persons of the findings. Only a physician should advise the person as to what course of medical treatment should be followed.
Treatment and Control
Rabies in domestic animals can be controlled by vaccination and quarantine. These methods are difficult and expensive, but not impossible to use on wild species. Rabies in wildlife (raccoons) has been successfully controlled in some parts of the United States through the use of oral rabies vaccination programs. In these programs packets of vaccine are distributed for consumption by these terrestrial rabies vector species. The technique has also been successfully used on coyotes in Texas.
If bitten, one should wash and flush the wound with soap or detergent and water, or even water alone. Several specific steps can then be followed, under the direction of a physician. Anti-rabies serum can be administered in cases of severe exposures (bites on head and neck), whereas milder exposures (bites on trunk, arms or legs) can be treated satisfactorily with vaccination. Generally, combined serum plus vaccine gives the best protection possible. Contrary to common belief, vaccination reactions are very rare, due to improved manufacturing techniques. Rabies vaccination should not be viewed with fear or undue concern.
Once clinical symptoms develop, there is no known treatment for preventing death from rabies.
The human health hazard from rabies is obvious. Less well recognized is the economic loss to farmers when their cattle and horses are bitten by rabid animals.
The following are links to additional rabies information available on other websites:
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For questions about wildlife diseases, please contact the Michigan DNR Wildlife Disease Laboratory.