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.

Clinical Signs

"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.

Pathology

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.

Diagnosis

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.

Significance

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.

Rabies. Beware of Rabies. Be Aware of Rabies.

Brochure from the Michigan Department of Community Health, July 2000

Controlling The Spread of Rabies in Wildlife and Transmission to Humans
Michigan Department of Natural Resources, Wildlife Division Issue Review Paper 6 May 10, 1999