It’s an accident of weather that makes Lyme disease such a scourge in northern states, but rare in southern ones, according to a study by URI and the U.S. Geological Survey.

The ticks that transmit Lyme disease to people die of dehydration when exposed to a combination of high temperature and lowered humidity, the study found. In an earlier related study, researchers found that southern black-legged ticks, unlike northern ones, usually stay hidden under a layer of leaves, where they are less likely to encounter people.

The research group, whose findings were published January 11 in the journal PLOS ONE, hypothesizes that southern ticks typically shelter under leaves to retain moisture, and that this behavior is a key reason why Lyme disease is uncommon in the South.

Lyme disease sickens an estimated 300,000 Americans each year, according to the U.S. Centers for Disease Control and Prevention, making it more common in this country than West Nile virus or any other illness transmitted by insects or arachnids. Black-legged ticks pick up the disease-causing bacteria, Borrelia burgdorferi, by biting infected animals, and can then transmit Lyme disease to people. The disease causes fever, headache, fatigue, and sometimes a rash. If not treated promptly, Lyme disease can damage the heart, joints and nervous system.

Just 14 states in the Northeast, Mid-Atlantic and northern Midwest typically account for 95 percent of all reported Lyme disease cases.

USGS research ecologist Howard Ginsberg and URI professor Roger Lebrun collected tick larvae from different parts of the eastern U.S. and found that no matter where they came from, the larvae all live longer in relatively cool temperatures. Longer life spans increase the odds that the ticks will live long enough to transmit Lyme.

In a related 2015 study, Michigan State University researchers found that northern ticks often climb plant stems, where a passerby may brush against them, but southern ticks usually stay hidden under a layer of leaves.

“In the North, when you walk through the woods you’re walking right through tick habitat,” said Ginsberg, leader of the USGS Patuxent Wildlife Research Center’s field station at URI. “In the South, you’re walking on top of the habitat. We think that is a crucial difference.”