CHAMPAIGN, Ill. — Scientists analyzed the distribution of three potentially harmful tick species in Illinois, identifying regions of the state with higher numbers of these ticks and, therefore, at greater risk of infection with multiple tick-borne diseases.
The study found that, of the three species tracked, the lone star tick, Amblyomma americanum, is most prevalent in southern Illinois; the black-legged tick or deer tick, Ixodes scapularis, is more common in northern and central Illinois; and the dog tick, Dermacentor variabilis, dominates the central and southern parts of the state. The findings, reported in the journal Ticks and Tick-borne Diseases, include clusters of counties with the highest number of ticks of each species.
All three tick species are likely present in every Illinois county, said Rebecca Smith , a pathobiology professor at the University of Illinois Urbana-Champaign who led the new research with graduate student Abrar Hussain .
"There are some counties where we've looked and we haven't found them, but most counties that have looked for ticks have found all three species," Smith said. "It's just that some ticks are more common in the south, some are more common in the north, and the dog tick does better in central Illinois, where there is a lot of grassland and open habitat."
Each of these ticks can be infected with one or more of several pathogens, and a tick bite can pass along those infections to humans or other animals. Many of these tick-borne diseases undermine human health and some, like the Heartland virus, can be life-threatening.
The lone star tick is a disease vector for ehrlichiosis, tularemia and the Heartland virus, the latter "a condition with low incidence, thankfully, but high mortality," Smith said. Its bite also can trigger Alpha-gal syndrome, an allergy to consuming mammalian meat.
"It's not a pathogen at all. It's just a reaction to a sugar molecule present in the saliva of the tick," Smith said. "The response can be anything from discomfort to anaphylactic shock."
Dog ticks can transmit ehrlichiosis, Rocky Mountain spotted fever, tularemia and Powassan virus disease, which in rare cases causes encephalitis. The deer tick can transmit Lyme disease, anaplasmosis, babesiosis and POWV.
For the new study, the researchers gathered tick-occurrence and tick-borne-disease data from several sources, including the Illinois Department of Public Health, the Illinois Natural History Survey Insect Collection, the Global Biodiversity Information Facility, several museum and academy collections, and iNaturalist, a platform that records citizen-scientist field observations. The team focused on five years of recorded observations ending in 2023.
"We knew that tick-borne diseases from the most common, medically important tick species are here in Illinois, so we wanted to see if the hotspots for ticks match with the incidence of diseases transmitted by those ticks," Hussain said.
Analyzing the data was tricky, however. Only 80 of 102 Illinois counties collected tick occurrence data during the study timeframe, and some counties had more aggressive tick-sampling initiatives than others, the researchers said.
The data included 1,414 ticks collected through active surveillance in 80 Illinois counties from 2018-2022. The team built spatial statistical models to identify county-level clusters with higher-than-expected tick distributions.
"Hamilton, Pope and Macon counties had the highest tick-collection numbers, each reporting 100 or more ticks," the researchers wrote. Hamilton, Jackson and Williamson had the highest numbers of lone star ticks. Hamilton, Macon and Pope counties in southern and central Illinois had the most dog ticks; and Macon, Piatt and Kane counties in central and northern Illinois had the highest numbers of deer ticks.
The scientists used two spatial analyses to identify multicounty clusters with high numbers of ticks of a particular species.
The lone star tick was the most prevalent tick species in southern Illinois.
"Ten counties — Union, Johnson, Hardin, Jackson, Williamson, Saline, Franklin, Hamilton, Jefferson and Perry — were identified as spatial clusters" for the lone star tick, they report. "The Illinois Department of Public Health has also classified all ten of these counties as having a high incidence of Ehrlichia chaffeensis," a pathogen transmitted by this tick. There were 219 reported human cases of ehrlichiosis in Illinois from 2018-2022.
Illinois also "ranks among the top 13 states with increasing incidence of Alpha-gal syndrome," caused by the bite of the lone star tick, they wrote.
"With 350 cases, southern Illinois has a high incidence of Rocky Mountain spotted fever group rickettsiosis," which is transmitted by the dog tick, the team reports. Pope and Hardin counties in southern Illinois, and Piatt and Moultrie in central Illinois were identified as hotspots for the dog tick.
Macon, Piatt, Champaign and Douglas counties in central Illinois, and Cook, DuPage and Kendall in northeastern Illinois were identified as clusters for the deer tick.
Between 2018 and 2022, the state recorded "1,728 cases of Lyme disease, 81 cases of anaplasmosis, and 23 cases of babesiosis," all of which are transmitted by the deer tick, the researchers report.
Illinoisans who don't live near regional hotspots of ticks are still at risk of tick bites from each of these species, Smith said.
"Just because there isn't a county-level hotspot near you doesn't mean that there's no ticks," she said. "You can have tick hotspots within counties, too. We just don't have that level of specification for where the ticks are within a county."
Smith urges people to protect themselves from ticks whenever they venture out into wild areas, road edges, parks, woodlands or prairies. This includes wearing long pants and light-colored clothing, tucking pants into socks, wearing close-toed shoes, spraying clothes with insecticides and scouring one's body for ticks at the end of a hike. The insecticide permethrin can be applied to clothing or individuals can buy permethrin-treated clothing, she said.
The new analysis offers insight into some of the areas that are most at risk of tick-borne disease, Smith said, and will allow health and safety officials to concentrate their resources accordingly. The lack of data from 22 Illinois counties may undermine these public health efforts in those parts of the state, however.
Smith also is a professor in the Carl R. Woese Institute for Genomic Biology and the National Center for Supercomputing Applications at the U. of I.
