The team used surveillance methods similar to those used in 1998 so that they would be able to accurately judge the degree to which the distribution of these ticks had changed. Using the gathered data, they figured out which counties had established populations, which ones had one or more reports of a blacklegged ticks, and which ones had none.
They found that the blacklegged tick has been reported in more than 45% of U.S. counties, compared to 30% of counties in 1998. Even more alarming, the blacklegged tick is now considered established in twice the number of counties as in 1998.
Most of the geographic expansion of the blacklegged tick appears to be in the northern U.S., while populations in southern states have remained relatively stable. The range of the western blacklegged tick only increased from 3.4% to 3.6% of counties.
"This study shows that the distribution of Lyme disease vectors has changed substantially over the last nearly two decades and highlights areas where risk for human exposure to ticks has changed during that time," Dr. Eisen said. "The observed range expansion of the ticks highlights a need for continuing and enhancing vector surveillance efforts, particularly along the leading edges of range expansion."