Scientists have not definitively traced the virus to tick bites, although it is highly likely because of the patients’ histories of tick bites and the virus’ similarity to the Heartland virus, discovered in Missouri in 2009.

The two viruses are extremely rare and do not constitute a public health threat, Lawrence said. While there is a possibility that more cases of Bourbon virus are going undetected, “we know it would have to be a situation where most cases are very mild because we don’t have a lot of people sick in the hospital,” he said.

However, two more common tick-borne bacterial diseases that can cause serious illness have spiked in 2017 after a mild winter.

There have been 229 cases of Rocky Mountain spotted fever reported in Missouri this year, up 100 percent over the five-year median. Ehrlichiosis is up slightly, with 149 reported cases. Tick-related bacterial illnesses in Illinois have grown from about 40 confirmed cases in 2000 to nearly 350 cases last year. The diseases are treatable with antibiotics if caught early.

It will take several months to complete the investigation into tick-borne diseases at Meramec State Park, according to a state health official.