January 13, 2017

Why Is Lyme Disease Less Common in the South?

More than 300,000 people are diagnosed with Lyme disease in the United States each year, most of whom live in the northern part of the country. Researchers set out to learn why.
By Kerry Lengyel
It seems that people and pets who live in the South may have escaped not only the cold weather but also the possibility of contracting Lyme disease.
 
Lyme disease in people is more common in the United States than any other illness transmitted by insects or arachnids, and it is the most commonly reported vector-borne illness. The Centers for Disease Control and Prevention estimates that 300,000 Americans are diagnosed with Lyme disease each year, but many believe this number is even higher due to the disease being easily misdiagnosed.
 
Most of those who contract Lyme disease live in the northeast and upper midwest areas of the country. In 2015, 95% of confirmed Lyme disease cases in humans were reported from the following states:
  • Connecticut
  • Delaware
  • Maine
  • Maryland
  • Massachusetts
  • Minnesota
  • New Hampshire
  • New Jersey
  • New York
  • Pennsylvania
  • Rhode Island
  • Vermont
  • Virginia
  • Wisconsin 

To understand why most cases are reported in northern and upper midwestern states, research ecologist Howard Ginsberg from the US Geological Survey and colleagues from the Universities of Rhode Island and Michigan studied the behavior of black-legged ticks (Ixodes scapularis).
 
In earlier research, the team discovered that tick larvae live longer in relatively cooler temperatures. They also discovered that northern ticks often climb plant stems while southern ticks usually like to stay hidden under a layer of leaves. "In the North, when you walk through the woods you're walking right through tick habitat," says Ginsberg. "In the South, you're walking on top of the habitat. … a crucial difference.” 
 
To understand exactly why this occurs, the team exposed immature black-legged ticks to temperatures mimicking a northern climate (72°F to 74°F) and a southern climate (90°F to 92°F) with varying levels of humidity (75%, 85%, and 95%). To avoid the effects of genetic differences between populations, the ticks used in the study included Rhode Island ticks and lab-raised hybrid ticks born of males from Wisconsin and females from South Carolina. Study results were published this week in PLOS One.
 
The effects of humidity quickly became clear. At the lowest humidity, the Rhode Island ticks typically perished within 2 to 4 days, but they lived for a month or longer at high humidity regardless of temperature. In the hybrid ticks, the combination of high temperature and lower humidity quickly proved fatal. Of those exposed to high temperature and high humidity, on the other hand, 80% survived 4 days or longer. At mid-range humidity and high temperatures, less than 30% of hybrid survived for that long.
 
Ginsberg and his colleagues concluded that southern ticks have adapted to survive in their environment by hiding in the moister areas under leaves. This makes it less likely for a human or animal to bump into a tick in the South and thus accounts for the lower prevalence of Lyme disease in the South.
 
As the climate continues to get warmer and drier, Lyme disease may become less common in some areas. Investigators even believe the disease could eventually fade out below the Mason-Dixon line.
 
 

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