The incidence of Lyme disease in the United States has approximately doubled since 1991, from 3.74 reported cases per 100,000 people to 8.60 reported cases per 100,000 people in 2013. Among the states where Lyme disease is most common, New Hampshire and Vermont have experienced the largest increases in reported case rates since 1991, followed by Delaware, Maine, and Massachusetts. On average, these five states now report 50 to 100 more cases per 100,000 people than they did in 1991.
Climate is just one of many important factors that influence the transmission, distribution, and incidence of Lyme disease. However, studies provide evidence that climate change has contributed to the expanded range of ticks, increasing the potential risk of Lyme disease, such as in areas of Canada where the ticks were previously unable to survive.
The life cycle and prevalence of deer ticks are strongly influenced by temperature. For example, deer ticks are mostly active when temperatures are above 45˚F, and they thrive in areas with at least 85 percent humidity. Thus, warming temperatures associated with climate change could increase the range of suitable tick habitat, and are therefore one of multiple factors driving the observed spread of Lyme disease. Because tick activity depends on temperatures being above a certain minimum, shorter winters could also extend the period when ticks are active each year, increasing the time that humans could be exposed to Lyme disease. Unlike some other vector-borne diseases, tick-borne disease patterns are generally less influenced by short-term changes in weather (weeks to months) than by longer-term climate change.