Epidemiology

Rabies has a near global distribution. Approximately 40,000 people die each year worldwide from Rabies. Most deaths occur in China, Bangladesh and Pakistan, where rabies is endemic and healthcare is poor. Death as a result of rabies is rarer in developed countries such as the United States where there were only 3 cases per million in 1994. Rabies is the oldest recorded infection, dating back as early as 23 BC in Egyptian writings. The word Rabies comes from Sanskrit rabbahs, "to do violence". The Italian Girolamo Fracastoro described the disease in The Incurable Wound in1584. Rabies is still endemic due to the large reservoirs in global wild and domestic animal population.

Humans contract rabies through the bite of a rabid animal or through mucosal exposure. Once there are clinical symptoms, no treatment is available and death is eminent. Symptoms and pathogenesis include, malaise, fever, sore throat, acute neurological disorder thrashing, hydrophobia, paralysis, coma and death.

The epidemiology of rabies in the United States has changed substantially during the last half century, as the source of the occurrence of disease has shifted from domesticated animals to wildlife, principally raccoons, skunks, foxes, and bats. Human deaths in the U.S. caused by rabies have declined recently to an average of one or two per year. However the estimated costs required to achieve this decrease in deaths amount to hundreds of millions of dollars annually. In addition, post-exposure treatment primarily in developing countries of the subtropical and tropical regions is extremely costly.

The route of exposure of human to rabies has also changed considerably over the last 50 years. From 1946 to 1965, 70% to 80% of human rabies cases occurred after a known exposure (most often a dog bite), and 50% of the cases before 1975 occurred after treatment with sub-optimal vaccines. Over the last decade, 80% of rabies-related human deaths were among persons who had no definitive history of an animal bite, and none resulted from ineffective post-exposure prophylaxis. Almost all the recent human cases occurred after an animal exposure that was unrecognized by the patient as carrying a risk for rabies infection. The apparent source of human rabies has also changed: 14 of the 18 cases acquired in the United States between 1981-95 involved rabies variants associated with insectivorous bats instead of dogs.

The major terrestrial reservoirs include raccoons, skunks, foxes, and coyotes, and several species of insectivorous bats. Raccoons continued to be the most frequently reported rabid wildlife species (50.4% of all animal cases during 1996), followed by skunks (23.2%), bats (10.4%), foxes (5.8%), and other wild animals, including rodents and lagomorphs (2.1%) An epizootiologic transition began in the United States in the 1920s, when rabies prevention efforts were no longer focused exclusively on human vaccination but began to include programs for the control of rabies in dogs. Domestic animal cases gradually declined, largely as a result of local dog rabies control programs that included vaccination, stray animal removal, and leash and muzzle ordinances. However, as such cases decreased, surveillance systems designed to track the source of infection for residual domestic animal foci detected increased cases in wild species. By 1960, rabies was diagnosed more frequently among wildlife than among domesticated animals, and wild animals accounted for 93% of 8224 of animal rabies cases in 1994.

In 1971, rabies was reported for the first time from all 48 contiguous states and Alaska. Skunks (primarily the striped skunk, Mephitis mephitis) formed the major animal reservoir from 1961 to 1989, until they were unexpectedly supplanted by the raccoon during the rabies outbreak in the mid-Atlantic and north-eastern states. This epizootic is believed to have started during the late 1970s by the translocation of infected animals from a southeastern focus of the disease.

While dog rabies has been largely controlled, a region of southern Texas that borders Mexico has persisted as a focus of both dog and coyote rabies. The number of cases of coyote rabies has gradually risen in this area since the late 1980s, accounting for 46 of the 50 cases of coyote rabies reported in the United States during 1991.

Since the transmission of rabies by a bat was first reported in 1953, rabid insectivorous bats have caused an average of 700 to 800 cases annually, and have been found throughout the United States, excluding Alaska and Hawaii. Rabies virus variants maintained by insectivorous bats appear to be exchanged largely independently from those in terrestrial mammalian reservoirs. The distances between Africa, Eurasia, Pacific Ocean, and the New World mitigate against the dispersal, migration, and introduction of healthy bats without human intervention . However, several recent events illustrate the opportunity for the transoceanic transfer of rabies-infected bats. Commercial enterprises also serve as vehicles for the accidental translocation of animals infected with rabies virus.

References:

1. Bat photograph courtesy of Bats:http://rville.k12.mo.us/Cave/bat.html

2.Rabies case figure courtesy of the CDC Rabies Information Page: http://www.cdc.gov/ncidod/dvrd/rabies/

6. CDC Rabies Information Page: http://www.cdc.gov/ncidod/dvrd/rabies/

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