Rabies is an acute encephalomyelitis
with an extremely high fatality rate. It is primarily a viral infection
of nonhuman carnivores.
The etiologic agent is the rabies virus, a 100-nm to 150-nm RNA
virus classified as a rhabdovirus. Infection is relatively rare
in humans, and when it occurs, it is usually the result of an animal
bite that introduces infectious saliva; it is not known to be introduced
through intact skin.
The virus probably binds initially to the acetylcholine receptor
and then travels centripetally, via the peripheral nerves, toward the
central nervous system. After infection of the brain, virus travels
via the sensory and autonomic nervous system to the eyes, salivary
glands, skin, and viscera. The principal pathologic changes are
confined to the central nervous system; they consist of neuronal necrosis
and nonsuppurative encephalitis. Changes are most pronounced in
the thalamus, hypothalamus, substantia nigra, pons, and medulla.
The most distinctive histologic feature of rabies infection is the
presence of the pathognomonic Negri bodies, acidophilic inclusion
bodies found most commonly in the hippocampus.
The vast majority of animal rabies in the United States occurs
in wildlife, not pets. Bats, skunks, wolves, coyotes, foxes, raccoons, and
many other species are implicated; for purposes of prophylaxis, bats
and skunks should be considered to be rabid unless proven otherwise.
Rabies is not endemic in rodents or lagomorphs. Dog-associated rabies
is relatively uncommon in the United States but remains a problem
in many parts of Mexico, Latin America, Asia, and Africa. Dog bites
continue to be a predominant reason for postexposure prophylaxis.
In the United States, approximately 40,000 individuals receive postexposure
prophylaxis per year.1
The World Heath Organization estimates that annually there are 55,000
deaths secondary to rabies worldwide; the majority occur in Asia,
Africa, and the Middle East.2 Rabies may occur
in any climate or season, and susceptibility does not seem to vary
with age, sex, or race. The incidence of rabies infection is highest
in children, probably because of their friendliness toward animals
and their inability to defend themselves. In the United States
since 1990, there have been approximately 50 reported cases.3,4 Of
these, close to 20% were associated with dog bites occurring
in travelers outside of the United States; the remaining cases followed
exposure to indigenous reservoirs, most commonly insectivorous bats.
In many cases, there is exposure to bats without a recognized bite.3,4
Rare cases are reported after organ transplantation or graft.5,6
The attack rate in persons bitten by rabid animals is difficult
to estimate, and it depends on the location of the wound, the depth
of the bite, the presence of saliva infected with virus, and the
protection afforded by clothing. Administration of both active and
passive immunization dramatically reduces the risk of disease in
persons bitten by rabid animals.