A high-virulence clone of Borrelia burgdorferi sensu stricto is distributed widely in Europe and North America

Abstract number: P2147

Xu Y., Bruno J., Luft B.

Objective: Among the multiple Borrelia burgdorferi species causing Lyme disease, only B. burgdorferi sensu stricto is found in both North America and Europe. We investigated the evolutionary link between the European and North American populations of B. burgdorferi sensu stricto by comparing the DNA sequences of isolates from the United States.

Methods: We sequenced one chromosomal locus (16S-23S ribosomal RNA intergenic spacer, IGS) and three highly variable plasmid loci (ospC on cp26, dbpA on lp54, and BBD14 on lp17) from U.S. (n = 36) and European (n = 32) isolates. Cluster analysis of multilocus sequences revealed 19 intra-specific genomic groups, 16 of which were strictly endemic lineages. One genomic group, the B31 clonal type, is the most frequent group in both continents. One single-nucleotide substitution and one gene-conversion over a span of 3,800 nucleotides distinguished the European and North American B31-type isolates. Sequence variations of the outer surface protein gene (ospC) were associated with the divergence of incipient genomic groups in both continents, corroborating the lineage-defining role of ospC variations. Linkage breakdowns between ospC and genomic groups were observed in one European isolate and 50% of the midwestern U.S. isolates, suggesting stable population sizes in those regions, in contrast to the rapid clonal diversification in the northeastern U.S.

Conclusions: We conclude that the bi-continental distribution of B. burgdorferi sensu stricto is due to post-glacial vicariance, indigenous diversification, and accelerated migration in modern times. The broad host- and vector-species range of a small number of intra-specific lineages (e.g., the B31 type) may underline their high pathogenecity in humans, and the introduction and preferential growth of these clones may have played a role in the rapid emergence of Lyme disease in the United States in recent decades.