Estimated density of Borrelia burgdorferi sensu stricto-infected Ixodes scapularis nymphs in the eastern United States.

Background: Most vector-borne disease cases reported in the United States are caused by pathogens spread by blacklegged ticks, Ixodes scapularis. Of these, a majority are Lyme disease cases caused by Borrelia burgdorferi sensu stricto (s.s.). Because most human infections are associated with nymphal tick bites, the density of host-seeking B. burgdorferi s.s.-infected I. scapularis nymphs (DIN) is often used to estimate risk of Lyme disease cases. DIN combines estimates of nymphal infection prevalence with estimates of densities of host-seeking nymphs, making it a costly metric to obtain through tick surveillance. Thus, field-derived estimates of DIN are limited.

Methods: To fill these gaps, we sought to estimate DIN across all counties in the eastern United States. We first estimated B. burgdorferi s.s. prevalence in host-seeking I. scapularis nymphs using generalized additive models and historical tick surveillance data reported to the Centers for Disease Control and Prevention's ArboNET database (2004-2023). We then combined prevalence estimates with previously estimated densities of host-seeking nymphs to estimate DIN. We validated these model-based estimates against data reported to ArboNET: field-derived county estimates of prevalence and DIN as well as county records of B. burgdorferi s.s. presence and collection of host-seeking nymphs.

Results: We estimated higher average nymphal prevalence (20-30%) in the Upper Midwest and Northeast and lower prevalence (0-5%) throughout the Southeast. Categorizing estimated DIN as minimal or elevated, we identified areas in the Upper Midwest and Northeast as elevated, with the majority of the Southeast and Great Plains as minimal risk.

Conclusions: Our resulting risk map can be used to raise awareness of Lyme disease in communities at elevated risk and aid in the promotion of tick-bite prevention practices.