Two Boundaries Separate Borrelia burgdorferi Populations in North America

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Abstract
Understanding the spread of infectious diseases is crucial for implementing effective control measures. For this, it is important to obtain information on the contemporary population structure of a disease agent and to infer the evolutionary processes that may have shaped it. Here, we investigate on a continental scale the population structure ofBorrelia burgdorferi, the causative agent of Lyme borreliosis (LB), a tick-borne disease, in North America. We test the hypothesis that the observed population structure is congruent with recent population expansions and that these were preceded by bottlenecks mostly likely caused by the near extirpation in the 1900s of hosts required for sustaining tick populations. Multilocus sequence typing and complementary population analytical tools were used to evaluateB. burgdorferisamples collected in the Northeastern, Upper Midwestern, and Far-Western United States and Canada. The spatial distribution of sequence types (STs) and inferred population boundaries suggest that the current populations are geographically separated. One major population boundary separated westernB. burgdorferipopulations transmitted byIxodes pacificusin California from Eastern populations transmitted byI. scapularis; the other divided Midwestern and Northeastern populations. However, populations from all three regions were genetically closely related. Together, our findings suggest that although the contemporary populations of North AmericanB. burgdorferinow comprise three geographically separated subpopulations with no or limited gene flow among them, they arose from a common ancestral population. A comparative analysis of theB. burgdorferiouter surface protein C (ospC) gene revealed novel linkages and provides additional insights into the genetic characteristics of strains.

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