Cases of Lyme disease in Michigan have risen dramatically in recent years, and a new study links that trend to larger and more widespread tick populations.

Researchers collected data from the Michigan Department of Health and Human Services on 1,057 Lyme disease cases diagnosed between 2000 and 2014, and aligned them with a new analysis of tick distribution across the state. Results showed that not only did the number of yearly infections in the state increase significantly over the 15-year period, but so did the number of counties where ticks had been seen, or found to be established.

And the number of infected people may be much higher than the records indicate, the researchers said. Because Lyme disease is frequently misdiagnosed as other illnesses, reported cases likely represent only a fraction of true Lyme disease infections 

Lyme disease is caused by the bacterium Borrelia burgdorferi, and can be transmitted to humans only through the bite of a tick carrying that bacterium. Black-legged ticks (Ixodes scapularis), also known as deer ticks, are the most common vectors for Lyme disease in the northeastern, north-central and mid-Atlantic United States, according to the Centers for Disease Control and Prevention (CDC).