Tick-borne illnesses are no longer a fringe concern for outdoor enthusiasts — they represent a quietly expanding public health burden across an entire Midwestern state, with every single county now reporting human cases. Understanding who gets sick, where, and when could meaningfully reshape how clinicians and public health officials think about risk in regions not traditionally flagged as high-alert tick zones.
This retrospective analysis drew on 6,423 human cases of reportable tick-borne diseases filed with the Illinois Department of Public Health between 2004 and 2022, cross-referenced against canine seroprevalence data from the Companion Animal Parasite Council spanning 2009–2022. The eight diseases under review — anaplasmosis, babesiosis, ehrlichiosis, Lyme disease, spotted fever group rickettsioses, Powassan virus disease, Heartland virus disease, and Bourbon virus disease — are each transmitted by one or more of four established tick species in the state: Ixodes scapularis, Dermacentor variabilis, Amblyomma americanum, and Amblyomma maculatum. Spatial and temporal variation was modeled using zero-inflated spatiotemporal approaches for the four most prevalent conditions in both humans and dogs, revealing that incidence patterns have been shifting over the study period in ways that county-level surveillance alone would miss.
The methodological choice to compare human incidence with canine seroprevalence is noteworthy. Dogs serve as sentinel species for tick exposure, and alignment — or divergence — between canine and human data can reveal whether underdiagnosis, reporting gaps, or genuine ecological differences are driving apparent patterns. This is not a causal study; it cannot establish what specific environmental or behavioral variables drive individual risk. Nonetheless, the 18-year timeframe and statewide coverage give it unusual breadth for a regional tick-borne disease analysis. The finding that no county was spared any human case signals a broader geographic footprint than public awareness campaigns typically convey, and should prompt reconsideration of which populations warrant clinical suspicion when presenting with febrile illness in summer months.