Why Lyme Disease Is Common in the North, Rare in the South; Heat & Humidity Are Key

The ticks that transmit Lyme disease to people die of dehydration when exposed to a combination of high temperature and lowered humidity, a new United States Geological Survey (USGS)-led study has found. In an earlier related study, the 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 online on January 11, 2017 in the open-access 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 very uncommon in the South of the United States. The article is titled “Environmental Factors Affecting Survival of Immature Ixodes scapularis and Implications for Geographical Distribution of Lyme Disease: The Climate/Behavior Hypothesis.” Lyme disease sickens an estimated 300,000 Americans a 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 in a subsequent bite. The disease causes fever, headache, fatigue, and sometimes a rash. If not treated promptly, Lyme disease can damage the heart, joints, and nervous system. There are significant regional differences in Lyme disease prevalence. In 2015, Alabama reported 11 confirmed cases to the CDC from a population of about 5 million people. Vermont, with fewer than 700,000 residents, had 491 confirmed cases.
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