Understanding the ecology and epidemiology of tick-borne diseases requires detailed knowledge of the complex interactions among the tick vector, the microorganisms they carry and the vertebrate hosts used by ticks, as well as the environmental conditions experienced by all three groups of organisms in this triad. In this thesis, I addressed questions relating to the biology and vector ecology of the Rocky Mountain wood tick (Dermacentor andersoni) and the American dog tick (Dermacentor variabilis). Comparisons were made of the distribution of both tick species, the vertebrate hosts used by immature ticks, and the types and prevalence of bacteria in individual ticks from multiple localities near the northern extent of their geographic ranges in western Canada. The results revealed that the distributions of both D. andersoni and D. variabilis have expanded since the 1960s, and there is now a broad zone of sympatry in southern Saskatchewan. In this zone of sympatry, D. andersoni and D. variabilis immatures were found to use the same species of small mammals as hosts and, in some cases, the same host individuals. This provides for the possibility of cross-transmission of bacteria from one tick species to the other. Bacteria of several genera (e.g. Rickettsia, Francisella, Arsenophonus and Anaplasma) were detected in D. andersoni and/or D. variabilis, some of which represented new tick-bacteria associations. However, most bacterial species were highly host (tick)-specific, except for three examples of apparent host switching from one tick species to the other at localities where the two tick species occurred in sympatry. The findings of this thesis provide a basis for understanding microbial transmission, the structure of tick-borne microbial communities, the risk of tick-borne disease in humans and animals, and the vector potential of D. andersoni and D. variabilis in geographical areas where they have not been studied previously.

Understanding the ecology and epidemiology of tick-borne diseases requires detailed knowledge of the complex interactions among the tick vector, the microorganisms they carry and the vertebrate hosts used by ticks, as well as the environmental conditions experienced by all three groups of organisms in this triad. In this thesis, I addressed questions relating to the biology and vector ecology of the Rocky Mountain wood tick (Dermacentor andersoni) and the American dog tick (Dermacentor variabilis). Comparisons were made of the distribution of both tick species, the vertebrate hosts used by immature ticks, and the types and prevalence of bacteria in individual ticks from multiple localities near the northern extent of their geographic ranges in western Canada. The results revealed that the distributions of both D. andersoni and D. variabilis have expanded since the 1960s, and there is now a broad zone of sympatry in southern Saskatchewan. In this zone of sympatry, D. andersoni and D. variabilis immatures were found to use the same species of small mammals as hosts and, in some cases, the same host individuals. This provides for the possibility of cross-transmission of bacteria from one tick species to the other. Bacteria of several genera (e.g. Rickettsia, Francisella, Arsenophonus and Anaplasma) were detected in D. andersoni and/or D. variabilis, some of which represented new tick-bacteria associations. However, most bacterial species were highly host (tick)-specific, except for three examples of apparent host switching from one tick species to the other at localities where the two tick species occurred in sympatry. The findings of this thesis provide a basis for understanding microbial transmission, the structure of tick-borne microbial communities, the risk of tick-borne disease in humans and animals, and the vector potential of D. andersoni and D. variabilis in geographical areas where they have not been studied previously.