North-South change in Ixodes scapularis host-seeking behavior contributes to profound regional variation in Lyme disease prevalence in the eastern United States

In the northeastern United States, the main vector of Lyme disease is the nymphal life-stage of the blacklegged tick (Ixodes scapularis). For example, the majority of Borrelia burgdorferi-infected ticks removed from humans in northeastern states by the Department of Defense (DoD) Tick Test program are nymphs. Consequently, in these states there is a very strong association between the late spring to early summer peak in nymphal host-seeking and the timing of human disease onset (e.g., Falco and Fish 1999).

A very different pattern is evident, however, in southeastern states. Despite robust populations of I. scapularis in coastal habitats there, a very small proportion of the ticks biting humans are I. scapularis, and those few that do attach are almost entirely the adult life-stage. We discuss several lines of evidence -- including DoD Tick Test program data, adult:nymph ratios in field ‘flagging’ surveys, and behavioral observations on captive ticks -- that support our conclusion that a dramatic change in nymphal I. scapularis questing behavior helps explain why human Lyme disease case reports from southeastern states are two orders of magnitude lower than in the Northeast.