Historical distribution of Lyme disease vectors in the US

Since the late 1980s, three major foci of Lyme disease in the US have been identified and account for the vast majority of cases reported each year (Barbour and Fish, 1993). The most significant in terms of case numbers is in the northeastern and mid-Atlantic regions, followed by the upper midwestern region of

Wisconsin and Minnesota. Disease transmission in both areas is due to the presence of I. scapularis. A third focus, encompassing northern California, Oregon, and Washington, is characterized by a transmission cycle involving a different vector, I. pacificus, the western black-legged tick, which has a relatively low level of infection with B. burgdorferi. I. pacificus feeds frequently on the western fence lizard (Sceloperus occidentalis), which is an incompetent reservoir for the spirochete (Lane et al., 1991), so transmission of the agent to ticks is infrequent. Furthermore, there is little overlap of cohorts (Padgett and Lane, 2001) during the tick's lifecycle - a trait that may impede spirochete transmission from one generation to the next. Since the number of reported cases of Lyme disease in this focus tends to be only about 1 percent of the total cases nationally (CDC, 2004), the following discussion will not address I. pacificus.

In reviewing data on northern populations of I. scapularis, Spielman et al. (1993) noted that relict populations of this tick originally could be found along the terminal moraines that formed as glaciers pushed their way south during the last ice age, then receded about 15,000 years ago (Davis, 1983). The remaining tick populations were isolated and limited to coastal portions of the northeastern US and to the northern Great Plains. In Europe and Asia, terminal moraines also marked areas primarily affected by Lyme disease on those continents (Spielman et al., 1993; Steere, 1994), though the vector tick species are different in these locations.

It is unclear whether I. scapularis populations might once have been more widely distributed in the United States than at present, possibly expanding and declining with white-tailed deer numbers (Spielman et al., 1985), but as recently as the early twentieth century populations of this tick were small. Studies at the time revealed the presence of I. scapularis and f. muris in just a few sites in the northeastern US (Bishopp and Smith, 1937; Cobb, 1942; Smith and Cole, 1943). Later work by Hyland and Mathewson (1961) and Good (1972, 1973) indicated that tick numbers were relatively low and probably had remained localized. For instance, Larrouse et al. (1928) found I. scapularis on Naushon Island in 1926 when they attempted to control American dog ticks (Dermacentor variabilis) there using a parasitic wasp, Ixodiphagus (Hunterellus) hookeri, though I. scapularis was apparently absent, or at least very rare, on the neighboring islands of Martha's Vineyard and Nantucket (Hertig and Smiley, 1937; Spielman et al., 1979). Several museum specimens of I. scapularis collected from New Haven, Connecticut, and from various sites on Long Island, New York, in the 1930s and 1940s also suggest that the species may have been fairly widespread geographically, but with small, isolated populations (Anastos, 1947; Collins et al., 1949; Ginsberg, 1993).

Although cases of Lyme disease likely occurred before the outbreak that began in the 1970s, they were relatively few in number and would have represented localized phenomena, given the highly focal distribution of I. scapularis. Persing et al. (1990) documented the presence of B. burgdorferi DNA in museum specimens of I. scapularis collected on Montauk Point, New York, in the 1940s, and cases of "Montauk knee" and "Montauk spider bite" were probably due to tick bites (Ginsberg, 1993). Marshall et al. (1994) found evidence of B. burgdorferi DNA in mouse specimens collected in Massachusetts during the 1890s, suggesting the presence of an enzootic cycle that would have resulted in infected I. scapularis ticks. Ecological conditions, however, were not right for a widespread increase in Lyme disease cases until the 1970s, though the stage was being set for just such an event centuries earlier as humans opened up the wilderness that marked the landscape of early America. Because the emergence of Lyme disease in the northeastern and midwestern US is so closely associated with the presence of white-tailed deer (Wilson and Childs, 1997; Piesman, 2002), the role of deer in Lyme disease ecology should be reviewed.

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