Pathobiology professor Becky Smith, graduate student Abrar Hussain and their colleagues analyzed state data on tickborne diseases in humans and dogs. The study reveals hotspots of disease in various parts of the state. Credit: Fred Zwicky
A nearly two-decade analysis of Illinois health data shows that tickborne illnesses have spread across every county in the state, with more than 6,400 residents diagnosed between 2004 and 2022 and case numbers climbing steadily over that period.
The research, conducted at the University of Illinois Urbana-Champaign, examined state health department records to map where and when residents contracted tick-borne infections. Lyme disease emerged as the most common threat, responsible for 4,297 of the reported cases and appearing most frequently in the northern part of the state. Anaplasmosis showed up most often in both northern and southern counties, while ehrlichiosis and spotted-fever group rickettsioses—a category of infections that can turn deadly—were concentrated in southern Illinois.
For the study, published in Ticks and Tickborne Diseases, the researchers also reviewed veterinary testing records from 2009 to 2022 that tracked tick exposure in dogs through antibody screening. Although the animal data was more limited, it largely mirrored the patterns seen in human cases, reinforcing the geographic trends the researchers identified.
The numbers point to a disease burden that has grown substantially over time. The team calculated that human cases increased by an average of 23 per year across the study period, though the totals swung widely from one year to the next—dropping to 343 cases in 2022 after reaching 645 the year before.
Researchers were able to connect these disease patterns to an earlier study from the same lab that tracked the spread of three medically significant tick species statewide. That work found the lone star tick concentrated in southern Illinois, the black-legged or deer tick more common in the northern and central regions, and the dog tick widespread across central and southern areas. A fourth species, the Gulf Coast tick, was once limited to coastal Gulf states but has now taken hold across the southern half of Illinois, where it can spread a bacterium that causes rickettsiosis.
Each tick species carries its own set of risks. The deer tick can transmit Lyme disease, anaplasmosis, babesiosis and Powassan virus disease. The lone star tick has been linked to Heartland virus disease, Bourbon virus disease and alpha-gal syndrome, an allergy to mammalian meat. The dog tick can cause Rocky Mountain spotted fever and tularemia.
“Rickettsiosis is one of the deadliest of the tick-borne diseases,” said Becky Smith, an epidemiologist and professor of pathobiology at the University of Illinois Urbana-Champaign who led the research. “We have had children dying in Illinois of rickettsiosis.”
Smith noted that there are currently no government programs designed to control tick populations, which makes public awareness the primary defense against infection.
“My big takeaway for the public is that we see tickborne diseases 12 months out of the year and in every county in the state of Illinois,” said Smith. “We have different tickborne diseases in different places and different risks in different places, but every county, every month, there is a risk.”