Climate called culprit in Lyme disease's prevalence in US North

Lyme disease is less common in the southern United States because the ticks that transmit it tend to die from dehydration in areas with high heat and lower humidity, researchers from the University of Rhode Island and the Geological Survey (USGS) said recently.

The researchers followed up on an earlier study that showed that black-legged ticks, which carry the disease, tend to hide under leaves, something their northern counterparts don't do. They said this keeps them out of the way of most people.

The Rhode Island and USGS researchers go a step further, contending in their study published in the Jan. 11 issue of PLOS ONE that the ticks hide under the leaves to retain moisture and stay alive. The northern insects don't have this issue. In fact, a lack of moisture kills off many ticks that could otherwise bite humans, the researchers argue.

"There has been a lot of research aimed at finding out what makes black-legged ticks more efficient hosts for Lyme disease in the North than in the South,” URI Professor Roger LeBrun said. “People have looked at everything from the effects of temperature on tick life cycles to the types of animals the ticks feed on. Probably all of these play a role. But our results suggest that evolutionary pressure to conserve moisture by staying under the leaf litter surface is a critical factor.”
 
Approximately 300,000 Americans get sick from Lyme disease each year, with a disproportionate amount in the northern part of the country. For example, there were 11 confirmed cases in Alabama in 2015, but Vermont, which has 700,000 fewer residents, had 491.
 
“In the North, when you walk through the woods you’re walking right through tick habitat,” Howard Ginsberg, a USGS research ecologist who leads the USGS Patuxent Wildlife Research Center’s field station at URI, said. “In the South, you’re walking on top of the habitat. We think that is a crucial difference.”