Why Lyme disease is common in the North, rare
in the South
The ticks that transmit Lyme disease to
people die of dehydration when exposed to a combination of high temperature and
lowered humidity, a new study by scientists at the U.S. Geological Survey and
the University of Rhode Island 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 Jan. 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 very 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 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 big 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. Just 14 states in the Northeast,
Mid-Atlantic and northern Midwest typically account for 95 percent of all
reported Lyme disease cases.
Seeking an explanation for this phenomenon,
USGS Research Ecologist Howard Ginsberg and URI Professor Roger Lebrun have
been studying the metabolism, life cycle and behavior of black-legged ticks.
In
2014 they 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 first bite an animal carrying Borrelia
burgdorferi, and then bite a human.
In a related 2015 study, colleagues on the
research team from Michigan State University 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. So
the next step was to find out why it occurs.”
To find out what role regional temperature
and humidity conditions play in tick survival, Ginsberg and his colleagues
recently conducted the first study ever to look at both factors.
In the
laboratory, they exposed immature black-legged ticks to a simulated northern
climate with temperatures between 72 and 74 degrees Fahrenheit, and a southern
climate of 90 to 92 degrees Fahrenheit. Humidity was set to 75, 85 or 95
percent.
To rule out the effects of genetic differences between northern and
southern populations, the researchers tested ticks from Rhode Island and
lab-raised hybrids, with one parent from Wisconsin and the other from South
Carolina.
The researchers found the combination of high
temperature and lower humidity was lethal to the hybrid ticks. At high
humidity, about four-fifths of the hybrid ticks survived temperatures in the
90s for four days or more.
But at the mid-range humidity, less than a third of
the ticks survived the high temperatures for that long. The effects of humidity
were also clear in the Rhode Island ticks, which typically died within two to
four days at the lowest humidity, but lived for a month or more at high
humidity regardless of how hot it was.
The researchers hypothesize that over time,
southern ticks have evolved to stay in the moist environment under leaves,
where they are less likely to encounter humans.
“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,” said LeBrun. “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.”
If the climate gets warmer and drier in
border zones like the Mid-Atlantic, Lyme disease may eventually become less
common there, Ginsberg said. “For example, in the Chesapeake Bay region, we
might see natural selection pressure on northern ticks to behave more like
southern ticks and stay under cover, so we could get less Lyme disease.”
The study is available
at http://dx.plos.org/10.1371/journal.pone.0168723
