Changing geographic ranges of ticks and tick-borne pathogens: drivers, mechanisms and consequences for pathogen diversity

Abstract

The geographic ranges of ticks and tick-borne pathogens are changing due to global and local environmental (including climatic) changes. In this review we explore current knowledge of the drivers for changes in the ranges of ticks and tick-borne pathogen species and strains via effects on their basic reproduction number (R 0), and the mechanisms of dispersal that allow ticks and tick-borne pathogens to invade suitable environments. Using the expanding geographic distribution of the vectors and agent of Lyme disease as an example we then investigate what could be expected of the diversity of tick-borne pathogens during the process of range expansion, and compare this with what is currently being observed. Lastly we explore how historic population and range expansions and contractions could be reflected in the phylogeography of ticks and tick-borne pathogens seen in recent years, and conclude that combined study of currently changing tick and tick-borne pathogen ranges and diversity, with phylogeographic analysis, may help us better predict future patterns of invasion and diversity.

Keywords: Ixodes; Lyme; genetic diversity; invasion; range change.

Figure 1

Pairwise mismatch distribution of 55 mtDNA Cox1 haplotypes of I. scapularis in Canada (Obs) compared against the expected distribution for a population under expansion (Exp). This graph represents a re-analysis of the data in Mechai et al. (2013) using only one example of each haplotype, to clarify the similarity of observed and expected frequency distributions.

Figure 2

Spatial cluster analysis of 55 haplotypes of mtDNA Cox1 sequences from Ixodes scapularis (see Mechai et al., for details) and MLST sequence types of B. burgdorferi within these ticks collected in passive surveillance in southern Canada. Three significant clusters of Cox1 haplotypes were identified respectively in south and south-western Quebec and Southern Ontario (red circles). One small-scale cluster of one B. burgdorferi ST (ST-01) was also found in south eastern Ontario (hatched circle). Because this ST has been found previously in the northeastern US, and because of the small radius of the cluster, this cluster likely represents a founder event.

Figure 3

Geographic distribution of B. burgdorferi-infected Ixodes scapularis ticks collected in passive surveillance in Canada in which the ospC major groups were identified in Ogden et al. (2011). A significant spatial cluster of ticks infected with B. burgdorferi carrying ospC major group I is indicated by the red circle. Reproduced with permission from Ogden et al. (2011).