Genotypic Diversity of an Emergent Population of Borrelia burgdorferi at a Coastal Maine Island Recently Colonized by Ixodes scapularis

The recent range expansion of Ixodes scapularis has been accompanied by the emergence of Borrelia burgdorferi. The development of genetic diversity in B. burgdorferi at these sites of emergence and its relationship to range expansion is poorly understood. We followed colonization of I. scapularis on a coastal Maine island over a 17-year period. B. burgdorferi's emergence was documented, as was expansion of ospC strain diversity. Ticks collected from rodents and vegetation were examined for the presence of B. burgdorferi. Sequencing and reverse line blot were used to detect B. burgdorferi ospC major groups (oMG). No I. scapularis were found until year four of the study, after which time they increased in abundance. No B. burgdorferi was detected by darkfield microscopy in I. scapularis until 10 years into the study, when 4% of adult ticks were infected. Seven years later, 43% of adult ticks were infected. In 2003, one oMG accounted for 91% of B. burgdorferi strains. This "founder" strain persisted in 2005, but by 2007 was a minority of the 7 oMGs present. Given the island's isolation, gene flow by avian introduction of multiple strains is suggested in the development of B. burgdorferi oMG diversity.