The areas with the highest risk for Lyme disease are expanding faster than expected, according to a report from the U.S. Centers for Disease Control and Prevention. Lyme disease was first detected and diagnosed in Connecticut and has grown to become a problem in much of the Northeast and upper Midwest United States. But the affected areas are expanding dramatically.
There are counties that are considered to be high risk areas for Lyme disease in 17 states. It commonly occurs in wooded suburban areas, where deer and the ticks that carry the Lyme bacteria are common. Ten years ago, there were 130 counties where the number of cases of Lyme disease was more than twice what was expected based on the size of their population. Now there are 260 counties where this is true.
All of Connecticut has been a high-risk area for a long time. Now, most of Massachusetts and New Hampshire are also high risk, along with more than half of Maine and Vermont. High risk areas have expanded in Virginia, Pennsylvania, eastern New York, Iowa, Michigan, and Minnesota.
This expansion in the number of counties at high risk for Lyme disease may be because favorable conditions for the ticks have expanded or because the disease-carrying ticks are becoming more widely dispersed in areas that already had favorable conditions.
However, a few counties have been removed from the CDC's high risk list. These are in Virginia, Georgia, Missouri, and North Carolina, where there had been clusters of cases of Lyme disease in the 1990s. It is probable that those cases were a different tick-borne illness (southern tick-associated rash illness, also known as STARI) or that they were detected using tests for Lyme that are not as accurate as those used now.
All states and territories must report cases of Lyme disease to the CDC as part of the National Notifiable Diseases Surveillance System.
You can read the CDC's report at http://wwwnc.cdc.gov/eid/article/21/8/14-1878_article, at the agency's journal, Emerging Infectious Diseases.