Distribution of Dermacentor andersoni Stiles and Dermacentor variabilis (Say) (Acari: Ixodidae) in western Canada based on active surveillance.

A total of 454 unique sites in western Canada were drag sampled for host-seeking Dermacentor species. Dermacentor andersoni Stiles was detected in British Columbia, Alberta, and Saskatchewan, while D. variabilis (Say) was detected in all four provinces. Dermacentor similis (Lado et al.) was collected from two sites in British Columbia. Dermacentor andersoni was detected as far east as -103.53° and as far north as 52.38° at elevations from 500 m to 1,231 m. Dermacentor variabilis was detected as far west as -110.9° and as far north as 54.78° at elevations from 200 to 1,200 m. Logistic regression indicated that prevalence of D. andersoni decreased with latitude, increased in the west, and increased with elevation to 900 m, then, declined. Prevalence of D. variabilis decreased with latitude, increased eastwards, and declined as elevation increased. Relationships with environmental variables were developed and indicated that mean winter temperature (Jan.-Mar.), summer temperature (Jul.-Sept.) and total winter precipitation affected both species. Dermacentor andersoni occurred at sites with mean winter temperatures between -10.4 °C and 1.4 °C, while most D. variabilis occurred at sites with winter temperatures between -15.0 °C and -7.9 °C. Prevalence of both species increased as summer and winter temperatures increased. Prevalence of D. andersoni also declined as total summer precipitation increased, consistent with it being a xerophilic species. Prevalence was also influenced by ecoregion and soil variables. Models had good fit, low bias, and good discrimination. Results will be useful for forecasting distribution and presence of these tick species throughout western Canada.