Tick-borne diseases reported every month of the year and in every Illinois county

A new analysis of state health department data reveals that more than 6,400 Illinois residents were diagnosed with tickborne diseases from 2004-2022.

The most commonly reported infections in the state are Lyme disease, occurring across Illinois but at higher rates in northern counties; anaplasmosis, reported most often in northern and southern counties; ehrlichiosis, seen most often in southern Illinois; and spotted-fever group rickettsioses, serious and sometimes-deadly infections that also are more prevalent in southern Illinois.

Reported in the journal Ticks and Tickborne Diseases, the new study also looked at tick exposures in dogs from 2009-2022 as measured by veterinarians testing for antibodies to the bacterial and viral vectors of these diseases. That data, though limited, tends to parallel the findings in humans, the researchers report.

“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 Becky Smith, an epidemiologist and professor of pathobiology at the University of Illinois Urbana-Champaign who led the research. “We have different tickborne diseases in different places and different risks in different places, but every county, every month, there is a risk.”

Dramatic growth

Smith and her colleagues report “dramatic growth” in the number of human cases of tickborne disease between 2004 and 2022.

“On average, the annual number of human cases increased by 23 cases every year, despite large fluctuations, with 343 in 2022 and 645 in 2021,” the researchers wrote.

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While some Illinois counties have higher rates of tickborne disease, all counties are at risk, Smith said. Lyme disease is the most common tickborne malady in humans in the state, accounting for 4,297 infections from 2004 to 2022.

Medically important tick species

A previous study from Smith’s lab reported on the prevalence of three medically important tick species across Illinois. That study found that, of the species tracked, the lone star tick, Amblyomma americanum, was most prevalent in southern Illinois; the black-legged tick or deer tick, Ixodes scapularis, was more common in northern and central Illinois; and the dog tick, Dermacentor variabilis, dominated the central and southern parts of the state. When compared with the current human data, Smith said, “We see clusters of disease where we see clusters of ticks.”

The dog tick can transmit Rocky Mountain spotted fever and tularemia. The lone star tick may cause Heartland virus disease, Bourbon virus disease and/or alpha-gal syndrome, the latter an allergy to mammalian meat. A deer tick bite can bring on Lyme disease, anaplasmosis, babesiosis and Powassan virus disease.

The new study also focused on Amblyomma maculatum, the Gulf Coast tick, which can transmit the bacterium Rickettsia parkeri, causing rickettsiosis. Once restricted primarily to the Gulf coast states, A. maculatum is now established in the southern half of Illinois.

Deadly disease            

“Rickettsiosis is one of the deadliest of the tick-borne diseases,” Smith said. “We have had children dying in Illinois of rickettsiosis.” Infection with the Heartland virus can also be life-threatening, but so far in Illinois those infections are rare, she said. There have been only three confirmed cases in the state.

Smith said getting the word out about the distribution of ticks and tickborne disease in Illinois is important because there are no government initiatives or public policies that can control the distribution of ticks.

“The best thing that we can do to control tickborne disease is for people to protect themselves,” she said. Guidance on this topic is available here.

SIgns and symptoms

Awareness of ticks in the environment and the signs and symptoms of tickborne disease can prevent or reduce the likelihood of severe disease, Smith said.

“If you find a tick on yourself before it bites you, you’re not going to get a disease,” she said. “If you find it before it’s been on you too long, you’re less likely to get a disease. If you find it and get treated early, you’re less likely to get sick, and if you get treated as soon as you get sick, you’re less likely to have long-term impacts.”

The U.S. Department of Defense grant TB180052 and the Centers for Disease Control and Prevention Cooperative Agreement #U01CK000651 supported this research.