A retrospective analysis of all human tick-borne disease (TBD) cases reported to the Illinois Department of Public Health (2004–2022; n=6,423) and county-level canine seroprevalence data (2009–2022), examining spatial, temporal, and demographic factors associated with eight reportable TBDs across four tick vector species.
Every Illinois county reported at least one human TBD case; annual cases rose by an average of 23/year (95% CI: 15–31), reaching 343 in 2022 (peak 645 in 2021). Most cases occurred in males (61%), white (71%), non-Hispanic (64%) individuals over 40 years (56%). Rural counties, higher deer harvests, and lower median household income were independently associated with higher incidence.
Retrospective reporting data likely underestimates true incidence due to underdiagnosis and underreporting. Rare diseases (Powassan, Heartland, Bourbon virus) lacked sufficient case counts for spatial modeling. Canine seroprevalence is a proxy for human exposure and may not directly reflect human risk.
Clinicians in Illinois—especially in rural southern counties (for *Amblyomma*-associated diseases) and northern counties (for *Ixodes*-associated Lyme/anaplasmosis)—should maintain a high index of suspicion for TBDs in patients over 40, particularly males in lower-income rural areas. Public health messaging should prioritize tick-bite prevention in these high-risk geographic and demographic groups.
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