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article imageNew study sheds light on spread of ticks carrying Lyme disease

By Karen Graham     Jul 7, 2015 in Science
Lyme disease cases are on the rise in the U.S., and regions that have historically been free of Lyme disease are now seeing cases popping up. Scientists have discovered that black-legged ticks are on the move, migrating northward.
There are about 300,000 people affected by Lyme disease every year in the United States. Black-legged ticks, the disease's main vector, are now being seen to flourish in areas previously devoid of the arachnid.
The new study, conducted by biologists from the University of Pennsylvania, along with colleagues from the New York Department of Health and the State University of New York, found that newly detected tick populations likely arose from southern populations migrating to nearby northern locations.
To better understand the migrations, scientists used genetic and phylogeographic analyses to determine the origin and recent migratory history of black-legged ticks. The researchers discovered that migrations to new locations came from established populations, usually through short-distance local moves.
The discovery of how these short, local moves take place could have implications for strategies in controlling ticks in order to control tick-borne disease. The study was led by Dr. Camilo E. Khatchikian, a postdoctoral researcher in Penn's Department of Biology in the School of Arts & Sciences and a member of the lab of Dustin Brisson.
Dr. Khatchikian said, "The fine temporal and spatial scale of the samples analyzed allowed for precise estimates of the rate, timing, and direction of individual migratory events. Dr. Dustin Brisson, the senior author in the study added that "understanding the fine-scale migratory process is essential to interpret patterns of genetic variation across broad geographic regions."
Lyme disease spread in the U.S.
Since 1975, Lyme disease has been diagnosed in 49 of the U.S. states, with almost 300,000 cases a year, mostly in the Northeast and Midwest. With better diagnostic criteria, more cases are being diagnosed as physicians learn to interpret the symptoms.
In a 2012 study, Khatchikian, Brisson, and their colleagues were able to confirm the tight association between black-legged tick populations and the number of human cases of Lyme disease in areas of the Northeast previously thought to be tick-free.
"People didn't know where they were coming from," Khatchikian said. "From the point of view of human disease, it doesn't really matter where they come from; the point is that they're here. But when you start thinking about how to control and prevent infections, it matters to know how easily the vectors of the disease are moving."
What is interesting in this study is the discovery of short-term migrations. It is well known that birds can carry ticks over long distances, sometimes up to a hundred miles or more. But almost all the migrations were 62 miles or 100 kilometers or less. So it isn't really clear to the researchers how animals are moving the ticks.
Nothing in the study suggested there was any correlation with climate change and the black-legged tick migration moving further north. But on Feb. 22, 2015, Digital Journal reported on a study covering 19 years of data collection by the Cary Institute of Ecosystem Studies in Millbrook, N.Y.
That study showed that warming temperatures not only influenced the earlier emergence of ticks but influenced their spread to new regions. That study was published in the journal Philosophical Transactions of the Royal Society B: Biological Sciences, on February 16, 2015.
This study was published in the journal Evolution on July 6, 2015, and is entitled: Recent and rapid population growth and range expansion of the Lyme disease tick vector, Ixodes scapularis, in North America.
The study can also be found on the University of Pennsylvania's website.
More about blacklegged ticks, Migration, genetic variation, south to north patterns, expansions
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