Bat Rabies in the United StatesSources of U.S. Human Rabies Infection
James A. Wilkerson III, MD
In the late 1940s and through the 1950s, most human rabies in the United States resulted from bites by dogs or cats. Of 146 infections in the years 1946 to 1961 for which a source of exposure could be identified, dogs were responsible for 120 and cats for nine. Other animals involved included foxes (seven), skunks (five), and bats (five).
When rabies in domestic animals was controlled, human rabies resulting from bites by pets disappeared. Since 1966, all but two of the human rabies infections resulting from documented exposures to rabid dogs have been incurred outside of the continental United States. Prior to 1965, the Centers for Disease Control and Prevention (CDC) had recorded no human rabies occurring within the United States that had been acquired outside the country.
Until recently, the number of infections acquired outside the United States has been steadily increasing: three of fifteen (20%) between 1965 and 1970, six of twenty-three (26%) in the 1970s, seven of ten (70%) in the 1980s, but only five of twenty-eight infections (21%) from 1990 through 2000. Although lack of knowledge about the risk of rabies in developing countries has led some travelers to disregard animal encounters and not obtain rabies immunoprophylaxis, some of these infections have been in children who did not inform their parents of the animal contact.
Identifying the source of human rabies infections in the United States is a long-standing problem because no animal exposure incident ó or even an opportunity for animal exposure ó can be identified for many infected persons. An animal exposure could not be identified for 35% (eighty-four of 230) of the human rabies infections occurring in the United States between 1946 and 1961, for 16% (six of thirty-eight) of the human infections between 1960 and 1970, or for 60% of the infections (six of ten) between 1980 and 1989. However, in recent years, including the 1980 , 1989 decade, monoclonal antibody typing or nucleotide analysis of the infecting organisms has allowed the source of human rabies infections to be determined when no animal exposure incident can be identified. Such studies have clearly demonstrated that bats are now the major source of human rabies in the United States.
Rabies in Bats
The incident that drew widespread attention to bat rabies in the United States occurred in Tampa, Florida, on June 23, 1953. The seven-year-old son of a ranch hand was looking for a baseball near some shrubbery when a lactating female yellow bat suddenly flew out of the bushes and bit the boy on the chest, remaining firmly attached until knocked off by the boy's mother. The ranch owner had heard of rabies in vampire bats in Mexico and insisted the bat be examined for infection. Negri bodies were found in smears of the brain, and the diagnosis was confirmed by mouse inoculation of brain tissue. The boy was treated and survived, and the publicity given this event led to many more bats being submitted for rabies examination.
Subsequently, rabies has been found in bats in every state, except Hawaii, and in eight Canadian provinces. Large areas of the United States, particularly the Pacific Northwest report rabies in bats but in no other species. The estimated incidence of rabies in all bats is 0.5 to 1.0 %; the incidence in bats that appear ill or injured is much higher, 7 to 50%.
Transmission of Bat Rabies to Humans
Twenty-six of the twenty-nine human rabies infections acquired within the United States since 1980 have been produced by virus strains associated with bats, eighteen by the strain associated with silver-haired and eastern pipistrelle bats. Five were produce by Brazilian (Mexican) free-tail bat strains, one by a big brown bat strain, and the associated species for two could not be determined.
The information available about these incidents is conflicting, but two of the individuals had a definite history of being bitten by a bat. A second may have been bitten. Ten to twelve of the victims had a history of apparent contact with a bat, but no bite is known to have been inflicted. In seven of ten individuals no exposure to bats could be identified. No instance of more than one bat-associated rabies infection in a single household has been reported.
The etiology of all of the infections was established with monoclonal antibodies or by nucleotide analysis of viral isolates. The route of transmission of the infection remains an enigma. Vampire bats have long been known to bite sleeping persons without waking them. The small, very sharp teeth of other bats may be capable of inflicting similar bites.
Preventing Rabies Transmission by Bats
The problem of rabies transmission by bats is best addressed through education. Reducing the bat population is not an acceptable approach, is probably impossible, and would almost certainly be an ecological disaster because bats play such a major role in insect control. The following measures have been recommended:
- Dwellings should be "bat-proofed"by carefully covering all possible entrances, particularly roof ventilation openings, with wire screens. Protection from bats in unscreened dwellings or when sleeping outdoors can be achieved with mosquito netting.
- Contact with bats should be assiduously avoided, particularly bats that are behaving unusually. Bats are nocturnal and any activity during daylight hours should be considered abnormal. Diseased bats often are unable to fly. Bats should be caught with nets; disabled bats should be scooped into a container. These animals must never be picked up or handled with unprotected hands.
- Any person who has contact with a bat,regardless of whether a bite is thought to have been inflicted, should receive postexposure prophylactic therapy unless the bat can be caught and tested for rabies.
- Any person who awakens from sleep and finds a bat in their room should receive postexposure prophylaxis.
To learn more about bats, contact your local wildlife conservation agency or the Bat Conservation International at www.batcon.org, or visit the Center for Disease Control at www.cdc.gov.
Jim Wilkerson is a long time member of WMS. He has just finished chairing the Annual Meeting in Park City and is the past chair of the Continuing Medical Education Committee. Jim has a pathology practice in Merced, California.
Wilderness Medicine Letter, Volume 18, Number 2, Spring 2001