Throughout developed countries, rabies remains a disease primarily affecting and maintained by wildlife populations (Figure 1). During 2005, wild animals accounted for more than 92% of all rabies cases reported to the CDC. The most frequently reported rabid wildlife remain raccoons, skunks, bats, and foxes; however, their relative proportions have continued to fluctuate because of epizootics of rabies among animals infected with several distinct rabies virus variants (Figure 2).1
Rabies control programs, including exensive vaccination campaigns implemented during the 1940s and 1950s, caused 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) by the 1960s. During the late 1970s and early 1980s, a canine rabies virus variant reemerged in southern Texas. This virus had been maintained historically in coyotes (Canis latrans) and transmitted to unvaccinated dogs. Oral rabies virus vaccination programs were initiated to interrupt transmission of this rabies virus variant. With cooperation by Mexico via the Border Infectious Disease Surveillance project, this variant has probably been eliminated in the United States.2–5 Rabies cases associated with a second canine 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
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 human beings.8 However, emerging rabies cases in human beings have resulted from infection with rabies virus variants that are associated with bats.9,10 Rabies control in bats is difficult by conventional methods. Prevention of rabies in human beings resulting from infection with bat-associated rabies virus variants is further challenged by the frequent absence of documented exposure histories involving a bat bite. Since 2000, 15 of 17 cases of indigenously acquired rabies in human beings were associated with rabies virus variants maintained by bats, as determined by genetic analysis or epidemiologic investigation. Only 3 of these cases involved a report of a definite bat bite (4 received organ transplants from a rabies virus–infected donor).6,10 Two cases of bat-associated rabies were reported to have no known exposure to a bat. The remaining 6 cases indicated some prior contact with a bat (eg, awakening to find a bat on the body or picking up a grounded bat). The most likely route of infection with rabies virus (excluding inoculation via infected transplant material) remains transmission by a bite that either was ignored or went unnoticed during an interaction with a bat.
Rabies virus infections of terrestrial animals in most areas of 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 intraspecific transmission. Once established, enzootic virus transmission within a species can persist regionally for decades or longer.
Rabies virus variants can be identified by reaction with panels of monoclonal antibodies11 or by comparing patterns of nucleotide substitution determined by genetic analysis.1,12 Spatial boundaries of enzootic rabies in reservoir species are temporally dynamic (Figure 3). Affected areas may expand and contract through virus transmission and population interactions.13,14 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.1,6,7,12–14
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 human beings of infected raccoons from the Southeast.15 Although identifiable as separate foci prior to 1994, the mid-Atlantic and south-eastern fronts merged in North Carolina in 1995. Rabies is now enzootic in raccoons 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.16 Two rabies virus variants are in geographically limited populations of gray foxes in Arizona and Texas. Enzootic rabies among canids in southern Texas had been the result of long-standing interactions between unvaccinated domestic dogs and coyotes at the Texas-Mexico border.2–5 However, only 2 dogs (believed to have been translocated from outside the United States) have been reported infected with the dog/coyote rabies virus variant in Texas since 2001. On the island of Puerto Rico, another wildlife rabies reservoir exists in mongooses.17,18 Rabies virus maintained and circulated by mongooses is periodically transmitted to unvaccinated dogs and cats.6
Despite the threat of rabies transmission from wild terrestrial carnivores, the use of population-reduction programs to control rabies among such animals is not desirable. Programs in Europe and southeastern Canada have used modified-live or recombinant virus vaccines for the oral inoculation of free-ranging wildlife reservoir species (predominantly foxes) to control the disease. During the past 2 decades, more than 100 million doses of vaccine-laden bait have been distributed over 6 million square kilometers in Europe,19 with promising results for controlling rabies in red foxes. The use of ORV in Switzerland during the past 30 years resulted in a declaration of rabies-free status for that country in 1998, and similar strategies in France led to rabies-free status being declared in 2000.20 Substantial decreases in the number of reported cases of rabies in fox populations in southern Ontario strongly support the hypothesis that rabies virus associated with red foxes can be eliminated by vaccination.16 Distribution of a V-RG recombinant vaccine targeting raccoons in the eastern United States21–23 and gray foxes and coyotes in Texas5 has shown promise as a complement to traditional rabies control methods (eg, parenteral vaccination of domestic animals). Products used in oral vaccination programs are self-replicating, and the unintentional exposure of nontarget species, including human beings, must be minimized and monitored.24,25
Overlaying the patterns of rabies virus maintenance among terrestrial mammals are multiple, independent reservoirs for rabies virus in several species of insectivorous bats. Rabies virus transmission among bats appears to be primarily intraspecific, and distinct virus variants can be identified for 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, Ari, to skunks (M mephitis) in an area previously naive for terrestrial rabies.26 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 readily detectable, as would be sustained transmission of a bat-associated rabies virus variant in a terrestrial mammal population.
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 1955. For this report, short-term trends were determined by comparing reported cases from 2005 with those from 2004 and by examining seasonal patterns for selected species.
Summaries of 2005 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 2006 is also included.
Oral rabies virus vaccination
Direct immunofluorescent antibody
USDA Wildlife Services
Georgia, Alabama, and Tennessee
ArcMap, version 8.3, ESRI, Redlands, Calif.
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