(HealthDay News) — From 1993–2012 there has been geographic expansion of high-risk areas for Lyme disease, according to a study published online July 15 in the U.S. Centers for Disease Control and Prevention’s Emerging Infectious Diseases.

Kiersten J. Kugeler, MPH, from the CDC in Fort Collins, CO, and colleagues present a method for defining high-risk counties based on the number of observed vs. expected reported human Lyme disease cases. The number of cases were aggregated into five-year intervals from 1993–2012.

The researchers note that two major foci of largely contiguous counties met the high-incidence county designation in each five-year period. Sixty-nine counties were characterized as having high incidence of Lyme disease in the first five-year period (1993–1997). In 1998–2002, 2003–2007, and 2008–2012, 130, 197, and 260 counties, respectively, were characterized as having high incidence. Over time, there was a more than 320% increase in the number of counties identified as having high incidence in the northeastern states; in north-central states, the increase was about 250% for the same periods.

“Systematic application of this method to four consecutive periods showed geographic expansion of high-risk areas,” the authors write. “Despite the substantial increase in the number of counties with high incidence, the limited movement of the geographic centers suggests relatively constant rates of geographic expansion in all accessible directions.”

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