Genetic diversity of Borrelia burgdorferi sensu stricto in Peromyscus leucopus, the primary reservoir of Lyme disease in a region of endemicity in southern Maryland

Abstract

In the north central and northeastern United States, Borrelia burgdorferi sensu stricto, the etiologic agent of Lyme disease (LD), is maintained in an enzootic cycle between the vector, Ixodes scapularis, and the primary reservoir host, Peromyscus leucopus. Genetic diversity of the pathogen based on sequencing of two plasmid-located genes, those for outer surface protein A (ospA) and outer surface protein C (ospC), has been examined in both tick and human specimens at local, regional, and worldwide population scales. Additionally, previous studies have only been conducted with tick or human specimens at the local population level in areas with high LD transmission rates. This study examined the genetic diversity of circulating borreliae in the reservoir population from a large region of the western coastal plains of southern Maryland, where moderate numbers of human LD cases are reported. Six ospA mobility classes, including two that were not previously described, and eight ospC groups were found among the P. leucopus samples. Twenty-five percent of all specimens were infected with more than one ospA or ospC variant. The frequency distribution of variants was homogeneous, both locally and spatially. The spirochete diversity found in Maryland was not as high as that observed among northern tick populations, yet similar genotypes were observed in both populations. These results also show that mice are important for maintaining Borrelia variants, even rare variants, and that reservoir populations should therefore be considered when assessing the diversity of B. burgdorferi.

FIG. 1.

Representative ospA SSCP analysis of all MCVs observed. Numbers represent MCVs as defined by Guttman et al. (18). m = profile of a specimen infected with two or more Borrelia MCVs on the basis of ospA. L = 100-bp ladder.

FIG. 2.

SSCP analysis of predominant ospC groups observed among P. leucopus mice collected in southern Maryland. ospC group D is not shown. L = 100-bp ladder.

FIG. 3.

Maximum-likelihood tree based on ospA gene fragment alignments derived with PAUP. Maximum-likelihood bootstrap values (1,000 replicates) are listed for each supported node. Bb, B. burgdorferi; Bv, B. valaisiana; Ban, “B. andersonii”; Bbi, “B. bissettii”; Bg, B. garinii; Bj, B. japonica; Ba, B. afzelii; Bt, B. turdi; Bl, B. lusitaniae.

FIG. 4.

Unrooted maximum-likelihood phylogenetic tree based on nucleotide sequence alignment of ospC fragments. Maximum-likelihood bootstrap values, based on 1,000 replicates, are listed at each supported node. Groups are based on the ospC allele nomenclature of Wang et al. (73). Maryland samples were found within encircled clades. All sample names are followed by a T, R, or H indicating the source of the material (tick, rodent, or human, respectively).

FIG. 5.

Relationship of the B. burgdorferi strains based on ospA combining the findings of Guttman et al. (18) and Qiu et al. (54) and mobility classes defined in this study (italicized). Dashed lines indicate recombination events rather than mutation events (solid lines).