The Role of Vertebrate Hosts in Tick-Borne Infections

Ticks require vertebrate hosts for survival and reproduction and can acquire pathogens when they ingest blood. Reservoir competency, however, varies greatly. Some rodents, such as white-footed mice (Peromyscus leucopus), are efficient reservoirs for Borrelia burgdorferi infections and infect attached larval and nymphal Ixodes scapularis ticks. Although white-tailed deer (Odocoileus virginianus) probably do not efficiently pass Lyme disease spirochetes to ticks when these ectoparasites feed, they are reservoirs for Ehrlichia chaffeensis (the agent for human monocytic ehrlichiosis). Deer are important hosts for I. scapularis and Amblyomma americanum adults, and as the number of deer has increased over the past four decades, there has been a corresponding sharp rise in abundance of both tick species. Birds transport immature ticks to new sites, even from one continent to another, and in some cases can pass B. burgdorferi to attached larvae and nymphs. Increased populations of ticks, deer, rodents, birds, and other wildlife species and the rising prevalence of tick-borne pathogens are linked, in part, to the reversion of agricultural lands to woodlands during the past century. When human dwellings are built in or near tick-infested habitats, such as in suburban developments near forests, and when people engage in more outdoor activities (e.g., camping, hiking, hunting, and fishing), there are greater risks for acquiring tick-transmitted pathogens.