Spatial distribution and clustering of medically important tick species in Illinois: Implications for tick-borne disease risk

Abstract

The geographic range of medically significant tick species has expanded across the United States, with Illinois experiencing increased tick populations and incidence of tick-borne diseases (TBDs) in recent decades. This study investigated the spatial distribution of three tick species: Amblyomma americanum (lone star tick), Dermacentor variabilis (American dog tick), and Ixodes scapularis (blacklegged tick) in Illinois, using data collected through active surveillance from 2018 to 2022. A total of 476 records of 1414 ticks were analyzed. Spatial analytical techniques, including species distribution mapping, Inverse Distance Weighted (IDW) interpolation, Hot Spot (Getis-Ord Gi*), and Cluster and Outlier (Anselin Local Moran’s I) analyses, were applied to identify distribution patterns and statistically significant local clusters. The species distribution mapping results revealed distinct geographic patterns: A. americanum was most prevalent in southern, D. variabilis in central and southern, and I. scapularis in central and northeastern Illinois regions, respectively. With agreement between Hot Spot and Cluster-outlier analysis, counties with significantly high tick prevalence were identified, including 10 counties for A. americanum in southern Illinois, four counties for D. variabilis in central and southern Illinois, and seven counties for I. scapularis in central and northeastern Illinois. The study results correspond with incidence of TBDs reported to the Illinois Department of Public Health (IDPH), including ehrlichiosis, Rocky Mountain spotted fever, Lyme disease, and anaplasmosis. These findings provide insights into the geographic variability of tick distributions, emphasizing the importance of targeted public health strategies and tick control efforts to mitigate the burden of TBDs in Illinois.