Identifying counties at elevated acarological risk for reporting high incidence of Lyme disease in the eastern United States

Lyme disease, caused primarily by the bite of a nymphal blacklegged tick ( Ixodes scapularis ) infected with Borrelia burgdorferi sensu stricto (Bbss), is the most common vector-borne disease in the United States. Counties reporting high Lyme disease incidence (LDI) continue to expand beyond historic foci in the Northeast and Upper Midwest. Identifying areas of expansion aids in targeting efforts to improve Lyme disease awareness in affected communities. Tick surveillance is conducted to track changes in acarological risk for tickborne diseases; the density of host-seeking Bbss-infected I. scapularis nymphs (DIN) is considered the most predictive metric for human LDI. We aimed to identify a threshold in DIN that accurately separated high (≥10 average annual cases/100,000 and ≥3 cases/year; 2022–2023) from low LDI counties in the eastern US. Using previously reported model-derived DIN, we identified 0.98 Bbss-infected nymphs per 1000 m 2 as the threshold. Based on this threshold, we classified 18% of eastern counties (487/2695), primarily in the Northeast and Upper Midwest, with both high LDI and elevated acarological risk and 72% (1935/2695) with both low LDI and acarological risk. Another 8% of counties (220/2695), primarily in Michigan, Illinois, Indiana, and Ohio, were predicted to pose elevated acarological risk, but reported low LDI and may reflect areas of emergence that could benefit from expanded tick surveillance and public health outreach. The remaining 2% of counties (53/2695), mainly along the Appalachian Mountains, reported high LDI, but were misclassified as low acarological risk, highlighting the need for additional tick surveillance to improve acarological risk assessments.