Chapter 321

Rabies is an acute encephalomyelitis with an extremely high fatality rate. It is primarily a viral infection of nonhuman carnivores.

Epidemiology and Pathophysiology

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.

Clinical Manifestations

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