Comparative population genetics of Lyme disease spirochete (Borrelia burgdorferi) and its tick vector (Ixodes scapularis) in the United States

In the present dissertation, the population structure of Borrelia burgdorferi, a spirochete that causes Lyme disease, was investigated across its major geographic range of distribution in North America. Local populations of Borrelia were first sampled from a small endemic area of Lyme disease, eastern Long Island, New York for three consecutive years from 1994 to 1996. Using the genetic variation at the outer surface protein A (ospA) locus, the Borrelia populations were found to be highly polymorphic within local populations but homogeneous across the region in any particular year. The genetic structure of B. burgdorfieri also differed between spirochetes infecting nymphal ticks and those infecting adult ticks.;The geographic homogeneity of B. burgdorferi was then seen across virtually all of its range of distribution in northeastern and mid-Atlantic coasts. Local populations of B. burgdorferi were genetically highly diverse, however, there is little genetic differentiation among B. burgdorferi populations in the northeastern and mid-Atlantic region. A biogeographic break in Borrelia prevalence was identified in a coastal region between southern Maryland and northern North Carolina.;Geographic range expansion of the blacklegged ticks (I. scapularis ) is implicated in the emergence of Lyme disease and other tick-borne diseases. A comparative investigation of the genetic structure of I. scapularis was therefore conducted for the same geographic samples of the Borrelia study, using PCR-SSCP analysis of a 300 basepair portion of tick mitochondrial 16S ribosomal RNA gene. In contrast to the geographic homogeneity of Borrelia, I. scapularis ticks showed a strong geographic structure along the East Coast.;Three main conclusions can be drawn from the present comparative molecular population genetic survey. First, geographic range expansion of a phylogenetically relatively homogeneous lineage of I. scapularis, carrying with it a phylogenetically likewise homogeneous lineage of B. burgdorferi - B. burgdorferi sensu stricto (relative to the overall Borrelia diversity in North America), is underlying the emerging epidemics of Lyme borreliosis and other tick-borne infectious diseases in the recent two decades in the northern United States. Second, there is a broad correspondence between the geographic range of distribution of the northern I. scapularis populations with the range of high prevalence of B. burgdorferi sensu stricto. Third, the geographic homogeneity of B. burgdorferi and genetic differentiation among local tick populations suggest either balancing selection in Borrelia or high level of Borrelia gene flow mediated by some long-ranging reservoir hosts. (Abstract shortened by UMI.).