As is the case in many developed countries, wild animals accounted for the majority (93%) of all rabies cases in the United States reported to the CDC during 2007. The most frequently reported rabid wildlife remain raccoons, bats, skunks, and foxes; however, their relative proportions have continued to fluctuate because of epizootics of rabies among animals infected with several distinct rabies virus variants.1
Rabies virus infections of terrestrial animals in the United States occur in geographically definable regions where virus transmission is primarily between members of the same species. Spillover infection from these species to other animals occurs but rarely initiates sustained transmission in other species. Once established, enzootic virus transmission within a species can persist regionally for decades or longer.
Rabies virus variants can be identified antigenically by reaction with panels of monoclonal antibodies2 or by comparing patterns of nucleotide substitution determined by genetic analysis.1,3 The spatial boundaries of enzootic rabies in reservoir species are temporally dynamic (Figure 1). Affected areas may expand and contract through virus transmission and population interactions.4,5 Population increases and emigration result in the expansion of rabies-infected areas, whereas natural barriers, such as mountain ranges and bodies of water, may restrict animal movements or sustain lower population densities that slow the spread of rabies. Unusual animal dispersal patterns and human-mediated translocation of infected animals have resulted in more rapid and unexpected introductions of rabies into new areas.1,3–7
Rabies control programs, including extensive vaccination campaigns implemented during the 1940s and 1950s, resulted in a substantial decline of rabies in domestic animals in the United States and eliminated the circulation of the major canine variants of the rabies virus in dogs (Canis lupus familiaris) by the late 1960s (Figure 2). During the late 1980s, a canine rabies virus variant reemerged in south Texas. This virus had been maintained historically in coyotes (Canis latrans) and transmitted to unvaccinated dogs. Oral rabies vaccination programs were initiated to interrupt transmission of this rabies virus variant. No cases of animals infected with this rabies virus variant have been reported since 2004.8 After more than 10 years of oral vaccination, this variant has now been eliminated from the United States.9–12 Rabies cases associated with a second canid rabies virus variant found mainly in gray foxes (Urocyon cinereoargenteus) in west and central Texas have similarly been reduced. Regulations in place in Texas and other states prohibiting the translocation of certain wild animal species for hunting and restocking purposes may have reduced the likelihood of accidental introduction of rabies virus variants into unaffected areas.1,6,7
Raccoons (Procyon lotor) have been recognized as a major reservoir for rabies in the southeastern United States since the 1950s. An outbreak that began during the late 1970s in the mid-Atlantic states was attributed to the translocation by humans of infected raccoons from the Southeast.13 Although identifiable as separate foci prior to 1994, the mid-Atlantic and southeastern fronts merged in North Carolina in 1995. Raccoon rabies is now enzootic in all of the eastern coastal states as well as in Alabama, Ohio, Pennsylvania, Tennessee, Vermont, and West Virginia.
Three rabies virus variants are responsible for disease in skunks (primarily Mephitis mephitis) in California and the north central and south central United States. In Alaska, a long-standing reservoir for rabies virus exists in red and arctic foxes (Vulpes vulpes and Alopex lagopus, respectively). Rabies spread during the 1950s among red foxes across Canada and, intermittently, to foxes in adjoining areas of the New England states. Although rabies persists in foxes in Alaska, reports of rabid foxes have declined in Canada, in part because of ORV programs.14 Two rabies virus variants are in geographically limited populations of gray foxes (U cinereoargenteus) in Arizona and Texas. On the island of Puerto Rico, another wildlife rabies reservoir exists in mongooses (Herpestes javanicus).15,16 Rabies virus maintained and circulated by mongooses is periodically transmitted to unvaccinated dogs and cats.6
Distribution of an oral V-RG recombinant vaccine targeting raccoons in the eastern United States17–19 and gray foxes and coyotes in Texas12 has shown promise as an important adjunct to traditional rabies control methods (ie, parenteral vaccination of domestic animals). Biologics used in oral vaccination programs are self-replicating, and the unintentional exposure of nontarget species, including humans, must be minimized and monitored.20,21
Multiple, independent reservoirs for rabies virus exist in several species of insectivorous bats, overlaying the patterns of rabies virus variants maintained among terrestrial mammals. Rabies virus transmission among bats appears to be primarily intraspecific, and distinct virus variants can be identified and associated with different bat species. In contrast to maintenance cycles in terrestrial animals, however, the greater mobility of bats precludes definitive range-mapping of different variants, other than the geographic ranges of the implicated host bat species. Because bat species known to be reservoirs for rabies virus are found in all areas of the continental United States, every state except Hawaii is considered enzootic for rabies.
Although transmission of rabies virus from bats to terrestrial mammals occurs, such transmission rarely results in sustained, independent, intraspecific cycles among terrestrial animals. Such occurrences represent significant shifts in host adaptation and the emergence of rabies virus variants in a new host species. In 2001, this rare phenomenon was demonstrated by the adaptation of a rabies virus variant associated with big brown bats (Eptesicus fuscus) in Flagstaff, Ariz, to skunks (M mephitis) in an area previously naive for terrestrial rabies.22 Prior genetic analysis indicated a net difference of 15% to 20% between rabies virus RNA sequences in bats, compared with those in terrestrial mammals. Thus, instances of spillover transmission of rabies virus from bats are detectable, as is sustained transmission of a bat-associated rabies virus variant in a terrestrial mammal population.
Various public health activities, including vaccination of companion animals, vaccination programs targeting wildlife, and ongoing education programs, have contributed to the reduction in transmission of rabies virus from terrestrial animals to humans.23 However, the majority of cases in humans have resulted from infection with rabies virus variants that are associated with bats.24,25 Rabies control in bats is difficult by conventional methods. Prevention of rabies in humans resulting from infection with bat-associated rabies virus variants is further challenged by the frequent absence of documented exposure histories involving a bat bite.
This report is prepared annually to inform veterinarians and public health officials of the current status of rabies in the United States. Information is provided on the geographic distribution of rabies and long- and short-term temporal patterns for reported cases of rabies in various species. Long-term trends for reported cases of rabies in animals in the United States are generated by examining reports beginning in 1957. For this report, short-term trends were determined by comparing reported cases from 2007 with those from 2006 and by examining seasonal patterns for selected species.
Summaries of 2007 surveillance data are provided for Canada and Mexico because of common borders and frequent travel between the United States and these countries. A brief update on cases of rabies and other related activities reported to the CDC during 2008 is also included.
Direct immunofluorescent antibody
Oral rabies vaccination
Pan American Health Organization
USDA Wildlife Services
ArcMap, version 9.1, ESRI, Redlands, Calif.
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