Rabies is endemic in several insectivorous bat species throughout the Americas. The disease also is endemic in raccoons from southeastern Canada to the Gulf of Mexico and from the East Coast to Ohio and the Appalachian Mountains. Four rabies variants are found in striped skunks throughout the midwestern United States; the Flagstaff, Ariz., area; and in mid-to-southern California. Rabies in foxes is endemic in Alaska and the southwestern United States and in mongooses in Puerto Rico. In each of these mammals, unique rabies viral variants are exquisitely adapted to and efficiently perpetuated animal-to-animal. These variants cause rabies in other mammals, but the probability of sustained transmission is lower than in the reservoir mammals. (Hawaii is the only rabies-free U.S. state or territory.)

An important consideration in evaluating risk based on recorded cases is the level of surveillance. If the index of suspicion and infrastructure support for animal management and testing is low, such that few or no domestic and wildlife species are tested, endemic cases might be present in a locality but not be reflected in numbers of positive cases. The definitive test for rabies requires postmortem testing of the brain.

The risk of rabies exposure for horses is higher in geographic localities where the disease is endemic in raccoons, skunks, foxes, or other terrestrial reservoirs. Rabies transmission can also occur from a rabid bat to a horse or secondarily from terrestrial carnivores infected with bat rabies. Fortunately, only about 50 horses are diagnosed with rabies every year in the United States. However, even a single case can have significant public health ramifications. These cases then trigger extensive investigations to identify potentially exposed humans and other animals.

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