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Cases of Lyme disease have risen substantially over the last two decades, and a new report may partially explain why — the tick species that carries it has spread across even more of the United States.

That’s according to a new study that is the first since 1998 to examine where these ticks live. Researchers searched the scientific literature to update the map on a county-by-county basis. They specifically were interested in tick species Ixodes pacificus and Ixodes scapularis, the main carriers of the bacterium that causes Lyme.


They found that these ticks live in 44 percent more counties than in 1996, spread over 43 states. The most dramatic changes were seen in the northeast and northern US, where climate change is predicted to make habitats more amenable to their spread. The results were published Monday in the Journal of Medical Entomology.

Regions where ticks that carry Lyme are established (red) or reported (blue). Alex Hogan/STAT

Ticks spread to take over large swaths of Minnesota, Wisconsin, Pennsylvania, New York, and Vermont.

But the expansion wasn’t limited to the north. Virginia, Florida, Ohio, and Georgia saw spreading populations.


Tennessee went from one county with established tick populations in 1996 to 16 counties by 2015. Illinois went from four to 35. In Minnesota, there were nine counties in 1996, and 45 by last year. In 2015, Ohio had 33 counties with ticks, way up from zero in 1996.