Population structure of the lyme borreliosis spirochete Borrelia burgdorferi in the western black-legged tick (Ixodes pacificus) in Northern California

Abstract

Factors potentially contributing to the lower incidence of Lyme borreliosis (LB) in the far-western than in the northeastern United States include tick host-seeking behavior resulting in fewer human tick encounters, lower densities of Borrelia burgdorferi-infected vector ticks in peridomestic environments, and genetic variation among B. burgdorferi spirochetes to which humans are exposed. We determined the population structure of B. burgdorferi in over 200 infected nymphs of the primary bridging vector to humans, Ixodes pacificus, collected in Mendocino County, CA. This was accomplished by sequence typing the spirochete lipoprotein ospC and the 16S-23S rRNA intergenic spacer (IGS). Thirteen ospC alleles belonging to 12 genotypes were found in California, and the two most abundant, ospC genotypes H3 and E3, have not been detected in ticks in the Northeast. The most prevalent ospC and IGS biallelic profile in the population, found in about 22% of ticks, was a new B. burgdorferi strain defined by ospC genotype H3. Eight of the most common ospC genotypes in the northeastern United States, including genotypes I and K that are associated with disseminated human infections, were absent in Mendocino County nymphs. ospC H3 was associated with hardwood-dominated habitats where western gray squirrels, the reservoir host, are commonly infected with LB spirochetes. The differences in B. burgdorferi population structure in California ticks compared to the Northeast emphasize the need for a greater understanding of the genetic diversity of spirochetes infecting California LB patients.

FIG. 1.

Neighbor-joining distance phylograms for partial 16S rRNA gene (observed differences) and flaB (Tajima-Nei method) sequences of selected Borrelia species and B. burgdorferi strains. The upper panel includes relapsing-fever species as the outgroup. Nodes with bootstrap support (1,000 iterations) of ≥50% by distance criteria (above the line) and by maximum-likelihood criteria (below the line) are shown. CA8 is the representative strain expressing ospC H3. All B. burgdorferi strains, with the exception of CA8 and HB19, were subjected to whole-genome sequencing, and the sequences were obtained from the GenBank.

FIG. 2.

Relative frequency distribution of B. burgdorferi ospC alleles H3, E3, and I3 found in I. pacificus nymphs across 74 woodland collection sites in Mendocino County. Circle diameters correspond to relative frequency values (displayed as percentages). The largest city in the county, Ukiah, is indicated by a star.