Lisa Jordan, a patient advocate who lives in Brewer, just southeast of Bangor, says she’s inundated with phone calls from people stricken by Lyme. On her cul-de-sac, she counts 15 out of 20 households touched by the disease. Three of her family members, herself included, are among them. “It’s a huge epidemic,” she says.
'Disease emergency' in Canada
The link between Lyme disease and climate change isn’t as direct as with other vector-borne diseases. Unlike mosquitoes, which live for a season and fly everywhere, deer ticks have a two-year life cycle and rely on animals for transport. That makes their hosts key drivers of disease. Young ticks feed on mice, squirrels and birds, yet adults need deer — some suggest 12 per square mile — to sustain a population.
Rebecca Eisen, a federal CDC biologist who has studied climate’s influence on Lyme, notes that deer ticks dominated the East Coast until the 1800s, when forests gave way to fields. The transition nearly wiped out the tick, which thrives in the leaf litter of oaks and maples. The spread of the deer tick since federal Lyme data collection began in the 1990s can be traced in part to a decline in agriculture that has brought back forests while suburbia has sprawled to the woods’ edges, creating the perfect habitat for tick hosts.
Eisen suspects this changing land-use pattern is behind Lyme’s spread in mid-Atlantic states like Pennsylvania, where the incidence rate has more than tripled since 2010. “It hasn’t gotten much warmer there,” she says.
But climate is playing a role. Ben Beard, deputy director of the federal CDC’s climate and health program, says warming is the prime culprit in Lyme’s movement north. The CDC’s research suggests the deer tick, sensitive to temperature and humidity, is moving farther into arctic latitudes as warm months grow hotter and longer. Rising temperatures affect tick activity, pushing the Lyme season beyond its summer onset.
Canada epitomizes these changes. Over the past 20 years, Dr. Nicholas Ogden, a senior scientist at the country’s Public Health Agency, has watched the tick population in Canada spread from two isolated pockets near the north shore of Lake Erie into Nova Scotia, Quebec and Ontario, the front lines of what he calls “a vector-borne disease emergency.”
Scientists say ticks can use snow as a blanket to survive cold temperatures, but long winters will limit the deer tick, preventing it from feeding on hosts and developing into adults. In the 2000s, Ogden and colleagues calculated a threshold temperature at which it could withstand Canada’s winter. They surmised that every day above freezing — measured in “degree days,” a tally of cumulative heat — would speed up its life cycle, allowing it to reproduce and survive. They mapped their theory: As temperatures rose, deer ticks moved in.
By 2014, the researchers had published a study examining projected climate change and tick reproduction. It shows higher temperatures correlating with higher tick breeding as much as five times in Canada and two times in the northern U.S.; in both places, the study shows, a Lyme invasion has followed.
The Canadian health agency reports a seven-fold spike in Lyme cases since 2009. “We know it’s associated with a warming climate,” Ogden says.
The U.S. Environmental Protection Agency concluded as much in 2014, when it named Lyme disease an official “indicator” of climate change — one of two vector infections to receive the distinction. In its description, the EPA singles out the caseloads of four northern states, including Maine, where Lyme has become most common.
Maine researchers have found a strong correlation between tick activity and milder winters. According to their projections, warming in Maine’s six northernmost counties — which collectively could gain up to 650 more days above freezing each year by 2050 — will make them just as hospitable to deer ticks as the rest of the state.