Geographic Variation in the Relationship between Human Lyme Disease Incidence and Density of Infected Host-Seeking Ixodes scapularis Nymphs in the Eastern United States

Prevention and control of Lyme disease is difficult because of the complex biology of the pathogen's (Borrelia burgdorferi) vector (Ixocles scapularis) and multiple reservoir hosts with varying degrees of competence. Cost-effective implementation of tick- and host-targeted control methods requires an understanding of the relationship between pathogen prevalence in nymphs, nymph abundance, and incidence of human cases of Lyme disease. We quantified the relationship between estimated acarological risk and human incidence using county-level human case data and nymphal prevalence data from field-derived estimates in 36 eastern states. The estimated density of infected nymphs (mDIN) was significantly correlated with human incidence (r = 0.69). The relationship was strongest in high-prevalence areas, but it varied by region and state, partly because of the distribution of B. burgdorferi genotypes. More information is needed in several high-prevalence states before DIN can be used for cost-effectiveness analyses.