Deer migration, deer density, tick distribution and incidence of a tick‐borne zoonosis

Our understanding of how migratory wildlife populations affect incidence of infectious diseases spilling over into humans is limited. Ticks are expanding their distribution towards northern latitudes, causing emergence of tick-borne diseases. Deer serve as reproduction hosts for adult ticks, supporting the tick populations. However, in northern areas, deer populations are partially migratory, and migrants occupy high elevation summer ranges that are unsuitable for ticks. Migration can thus theoretically lead to escape from exposure to ticks and to deer feeding fewer ticks, which lower disease hazard to humans. Combining data on Ixodes ricinus tick abundance with GPS-tracking data of red deer (Cervus elaphus), we quantified the distributional overlap of ticks and red deer along elevational gradients in Norway. Furthermore, we correlated both deer density and the proportion of migratory deer with the incidence of Lyme disease in humans. We found that migratory deer summer ranges had colder climate and overlapped less with tick distribution than those of resident deer. Deer density consistently increased Lyme disease incidence in humans. However, we found only weak evidence that deer migration negatively affected Lyme disease incidence. Our study provides a rare quantification of how host availability, in terms of both host density and migratory movement, affects risk of a zoonotic disease.