Evolution of northeastern and midwestern Borrelia burgdorferi, United States

Abstract

The per capita incidence of human Lyme disease in the northeastern United States is more than twice that in the Midwest. However, the prevalence of Borrelia burgdorferi, the bacterium that causes Lyme disease, in the tick vector is nearly identical in the 2 regions. The disparity in human Lyme disease incidence may result from a disparity in the human invasiveness of the bacteria in the Northeast and Midwest caused by fundamentally different evolutionary histories. B. burgdorferi populations in the Northeast and Midwest are geographically isolated, enabling evolutionary divergence in human invasiveness. However, we found that B. burgdorferi populations in the Northeast and Midwest shared a recent common ancestor, which suggests that substantial evolutionary divergence in human invasiveness has not occurred. We propose that differences in either animal ecology or human behavior are the root cause of the differences in human incidence between the 2 regions.

Figure

Phylogeny of Borrelia burgdorferi isolates in the northeastern and midwestern United States based on intergenic spacer (IGS) sequence. operational taxanomic unit names beginning with IGS were isolated in the northeastern United States (10); all other isolates are from patients in the Midwest. The letter after period designates the outer surface protein C (ospC) major allele of the isolate. Colored isolate names highlight isolates with the same ospC major group that cluster in different clades, which suggests horizontal gene transfer. The ospC of several strains is not linked to the IGS ribosomal spacer type (RST) to which it is commonly linked in the Northeast (10,34). AB indicates differences between the ospAB tree and the IGS tree. This tree is midpoint rooted. Scale bar indicates number of substitutions per site.