The ticks often carry the bacterium Borrelia burgdorferi with them, the Detroit Free Press reported. The bacteria can transfer when they bite a human or animal and can cause Lyme disease, a serious infection that can be permanently debilitating when it's not treated early and well.
There were less than 30 human cases of Lyme disease reported in Michigan in every year between 2000 and 2004, according to a recent study by Jean Tsao, an associate professor in Michigan State University's departments of fisheries and wildlife and large animal clinical sciences. But the number had jumped to 90 reported cases by 2009, and by 2013, it was nearly 170 cases.
The Lyme disease spike in Michigan correlates with the spread of blacklegged ticks in the state.
In 1998, the ticks were established in only five counties and reported in more than 20 other counties. By 2016, the ticks were established in 24 Michigan counties and reported in 18 others.
Officials with the federal Centers for Disease Control and Prevention estimate the number of Lyme disease cases nationwide could be 10 times higher than what's reported.
Infected people and their doctors don't often test for the disease because it's relatively new in Michigan and its symptoms often mimic what feels like the flu. Erik Foster, an entomologist with the Michigan Department of Health and Human Services and co-author of the study, said doctors also don't always report finding Lyme disease to their local public health department.