Complex Population Structure of Borrelia burgdorferi in Southeastern and South Central Canada as Revealed by Phylogeographic Analysis

Abstract

g Lyme disease, caused by the bacterium Borrelia burgdorferi sensu stricto, is an emerging zoonotic disease in Canada and is vectored by the blacklegged tick, Ixodes scapularis. Here we used Bayesian analyses of sequence types (STs), determined by multilocus sequence typing (MLST), to investigate the phylogeography of B. burgdorferi populations in southern Canada and the United States by analyzing MLST data from 564 B. burgdorferi-positive samples collected during surveillance. A total of 107 Canadian samples from field sites were characterized as part of this study, and these data were combined with existing MLST data for samples from the United States and Canada. Only 17% of STs were common between both countries, while 49% occurred only in the United States, and 34% occurred only in Canada. However, STs in southeastern Ontario and southwestern Quebec were typically identical to those in the northeastern United States, suggesting a recent introduction into this region from the United States. In contrast, STs in other locations in Canada (the Maritimes; Long Point, Ontario; and southeastern Manitoba) were frequently unique to those locations but were putative descendants of STs previously found in the United States. The picture in Canada is consistent with relatively recent introductions from multiple refugial populations in the United States. These data thus point to a geographic pattern of populations of B. burgdorferi in North America that may be more complex than simply comprising northeastern, midwestern, and Californian groups. We speculate that this reflects the complex ecology and spatial distribution of key reservoir hosts.

Evolutionary Aspects of Emerging Lyme Disease in Canada

“…1). In that study, we found that strains in some Canadian populations (those in Manitoba and western Ontario and the Maritimes) are frequently different from those in the United States (albeit with recent ancestors immediately to the south in the United States), while in one region (extending from southeastern Ontario to southwestern Quebec), the strains are almost all the same as those in the northeastern United States (49). Details of the geographic occurrence of these strains are shown in Fig.…”

“…This conclusion was reached because the novel strains in each site were mostly scattered about the phylogenetic tree and among clonal complexes (illustrated in Fig. 2) rather than comprising the distinct clades that would be expected if the new strains had arisen simply from recent founders or strains surviving in Canadian refuges (49). The increasing recognition that certain reservoir host species of B. burgdorferi such as the deer mouse (Peromyscus maniculatus) and the eastern chipmunk (Tamias striatus) may have survived glacial periods in northern refugia in North America may support the idea of the occurrence of multiple small refugia in the northern United States (61,62).…”

“…At the time of writing, 111 sequence types (STs) of B. burgdorferi have been identified by MLST in North America (49) and their occurrence shows clear geographic patterns-for the most part, different STs are found in the northeastern United States, in the midwestern United States, and in California (Fig. 1), and geographic and landscape barriers currently limit gene flow between these groups (50).…”

“…Either way, because the main clades are not associated with geographical isolation, we speculate that clades may relate broadly to some form of ecological "isolation," which for B. burgdorferi may be host species association. In this scenario, successful expansions involved strains particularly adapted to hosts that were abundant at that time and whose descendants may persist today (49). There is evidence that some B. burgdorferi strains may be more efficiently transmitted by, or more frequently associated with, some host species than others (52,53,54,55).…”

“…However, the main phylogenetic pattern of clades is not geographically defined, as illustrated in Fig. 2: clonal complexes (which mostly equate with clades in the phylogenetic tree [49]) comprise STs from multiple geographic locations, as illustrated in Fig. 2 by the color coding of STs according to the geographic location of origin described in the Fig.…”