Epidemiology

Rabies is primarily a disease of animals.5 The epidemiology of human rabies is a reflection of both the distribution of the disease in animals and the degree of human contact with these animals.5 In those parts of the world where canine rabies has been controlled (i.e., the United States, Canada, and Europe), dogs account for less than 5 percent of the cases in animals. Where canine rabies has not been controlled (i.e., most developing countries of Asia, Africa, Latin America, and South America), dogs account for 90 percent or more of reported cases in animals. The major wildlife vectors of rabies are: dogs (major vector of rabies throughout the world, particularly Asia, Latin America, and Africa); foxes (Europe, Arctic, and North America); raccoons (eastern United States); skunks (midwestern United States, western Canada); coyotes (Asia, Africa, and North America); mongooses (yellow mongoose in Asia and Africa; Indian mongoose in the Caribbean Islands); and bats (vampire bats from Northern Mexico to Argentina, insectivorous bats in North America and Europe). In 1996, 49 states, the District of Columbia, and Puerto Rico reported 7124 cases of rabies in animals.15 Wild animals accounted for almost 92 percent of the reported cases: raccoons (50.4 percent); skunks (23.2 percent); bats (10.4 percent); foxes (5.8 percent); and other wild animals including rodents and lagomorphs (2.1 percent). Rabid domestic animals included cats (3.7 percent); cattle (1.8 percent); dogs (1.6 percent); horses and mules (0.65 percent); sheep and goats (0.22 percent); and other animals such as ferrets (0.06 percent).

Animal reservoirs for rabies are found worldwide with the exception of some islands (e.g., Hawaii, Great Britain), Australia, and Antarctica. Rabies infections of mammalian species occur in geographically discrete outbreaks. Disease transmission within an outbreak is primarily intraspecific and involves a single, distinctive rabies variant. The geographic boundaries of current North America reservoirs for rabies are: 6

1. An expanding reservoir for raccoon rabies that now encompasses the southeastern, mid-Atlantic, and northeastern United States (>3000 cases in 1996).

2. A long-standing reservoir in red and arctic foxes in Alaska, that has spread to include foxes across Canada as far east as Ontario, Quebec, and the upper parts of the northern New England states (usually <100 cases reported each year).

3. Three separate variants that infect striped skunks in long-standing reservoirs in California, the north central states, and the south central states (several thousand cases per year).

4. Two different variants in gray foxes in small but long-standing reservoirs in Arizona and Texas (~10 to 20 cases per year).

5. A recently recognized outbreak of rabies in coyotes in south Texas as a result of spillover infection from domestic dogs in a long-standing reservoir at the Texas-Mexico border.

Transmission of rabies virus usually begins when the contaminated saliva of an infected host is passed to a susceptible host. The most common mode of rabies viral transmission is through the bite of a rabid animal. Other routes of documented transmission have included contamination of mucous membranes (i.e., eyes, nose, mouth), aerosol transmission during spelunking in bat-infested caves or while working in the laboratory with rabies virus, corneal transplants, and iatrogenic infection through improperly inactivated vaccine.14 Fomites have not been implicated in transmission. The risk of developing rabies following an animal bite or scratch by a rabid animal depends on whether a bite or scratch occurred, the number of bites, the depth of the bites, and the location of the wounds. In the absence of postexposure prophylaxis, the risk ranges from 80 to 100 percent (multiple severe bites around the face), to 15 to 40 percent (single bites), to 5 to 10 percent (superficial bites on the extremities). Contamination of open wounds with saliva carries a risk of approximately 0.1 percent.

The epidemiology of human rabies in the United States between 1946 and 1996 has been well described in a series of reports from the CDC. —I6,! and l8 In the United States, deaths in humans caused by rabies totaled 99 in the 1950s, 15 in the 1960s, 23 in the 1970s, 10 in the 1980s, and 22 from 1990 through 1996. 18 The median age of these patients was 27 (range: 4 to 82 years of age) and 20 (63 percent) were male. Cases were reported from 20 states; 7 cases (22 percent) were reported from California and 6 from Texas. Eleven patients were exposed to rabies in eight foreign countries. The onset of illness occurred in all months and with no apparent seasonal pattern.

Of the 32 patients that died of rabies between 1980 and 1996, a definite history of animal exposure was identified in only 7 (22 percent); 6 resulted from a dog bite received in a foreign country and 1 was from a bat bite received in the United States. Contact with an animal was identified in 12 persons; 8 with a bat, 2 with a dog, 1 with a cow, and 1 with a cat. For the remaining 13 patients, a potential source of animal exposure was not identified.

Current epidemiologic patterns of rabies in the United States have been summarized as follows:18 The annual reports of rabies in wildlife far exceed those of rabies in domestic animals; rabies variants in bats are associated with a disproportionate number of infections in humans (53 percent), although bats constitute only about 10 percent of all reported rabies cases in animals annually; most other cases of human rabies diagnosed in the United States are attributable to infections acquired in areas of enzootic canine rabies in outside of the United States; most persons with a case of rabies that originated in the United States have no history of an animal bite; and rabies is diagnosed after death in more than one-third of the latter group.

Bites of squirrels, hamsters, guinea pigs, gerbils, chipmunks, rats, mice, rabbits, hares, and other small rodents almost never require antirabies postexposure prophylaxis. This author is at variance with T§bJ,e.,141-,2. in that he would treat unprovoked attacks by woodchucks, beavers, and other large rodents.

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