Tick-borne diseases in Illinois (USA): A retrospective case analysis

Illinois is known to have established populations of four vector tick species of human health concern: Ixodes scapularis, Dermacentor variabilis, Amblyomma americanum, and Amblyomma maculatum. These ticks can transmit pathogens causing eight reportable tick-borne diseases (TBDs): anaplasmosis, babesiosis, ehrlichiosis, Lyme disease, spotted fever group rickettsioses (SFG rickettsioses), Powassan virus disease, Heartland virus disease, and Bourbon virus disease. The incidence of these diseases is spatially varied and has been changing over time. The purpose of this research is to describe factors associated with human incidence of the various tick-borne diseases in Illinois and to compare this to factors associated with canine seroprevalence to similar tick-borne diseases. All cases of tick-borne diseases in humans reported to the Illinois Department of Public Health (IDPH) between 2004 (when reporting began) and 2022 were reviewed (n = 6423), with all county-level seropositivity and canine test data reported by the Companion Animal Parasite Council between 2009 (when reporting began) and 2022. Descriptive statistics were performed to identify spatial and temporal variation. Comparison with known risk factors was conducted using zero-inflated spatiotemporal modeling for anaplasmosis, ehrlichiosis, Lyme disease, and SFG rickettsioses in humans and anaplasmosis, ehrlichiosis, and Lyme disease in dogs. Every county in Illinois reported at least one case of a human TBD from 2004 to 2022. Most reported cases were in males (61%), white (71%), and non-Hispanic (64%) residents over 40 years of age (56%). On average, the annual number of human cases increased by 23 cases every year (95% CI: 15, 31), despite large year-to-year fluctuations, with 343 in 2022 and 645 in 2021. The spatial hotspots were noted in southern Illinois for human TBDs associated with A. americanum, and D. variabilis, and for dog exposure associated with A. americanum. Hotspots were also noted in northern Illinois for diseases and exposure associated with I. scapularis for both humans and dogs and across the 2004-2022 study period. Case incidence was higher in rural counties, counties with higher deer harvests, and counties with lower median household income. These findings can be used to guide public health efforts that target self-prevention strategies to decrease the risk of a tick bite and tick-borne diseases in Illinois and are applicable in similar midwestern states with expanding TBD risk.