Academic and health care investigators are learning more about the complex web of factors driving the expansion of tick habitat, including global warming.
By Joe Lawlor, Staff Writer
Portland Press Herald news story
The changing climate in Maine caused by global warming is potentially creating new tick habitats and accelerating the spread of Lyme disease, according to research being done in the state.
Maine Medical Center Research Institute in Scarborough is partnering with the University of Maine’s Climate Change Institute to study how climate change affects deer ticks and Lyme disease.
Susan Elias, a disease ecologist at the research institute, said persuasive research connects climate change with the increased range of the deer tick that carries Lyme disease, but there hasn’t been a comprehensive look at the many factors that are causing a surge in Lyme cases. Those factors include shorter winters, hotter summers and fewer days of extreme cold temperatures.
The number of confirmed Lyme cases in Maine reached an all-time high of 1,464 in 2016 and has exceeded more than 1,000 a year since 2011. In the early 2000s, fewer than 300 confirmed cases were recorded each year.
Anaplasmosis, another disease transmitted to humans by the deer tick, emerged as a threat in 2017, with a record 400 cases reported through October. Many Mainers are now being co-infected with Lyme and other tick-borne diseases like anaplasmosis, although there are no public health data tracking co-infections. Researchers, however, are finding more ticks that carry both Lyme and anaplasmosis.
Elias said weather conditions in 2016 led to erroneous expectations that showed scientists how much more they still have to learn about the ticks that carry Lyme.
Scientists hypothesized that a drought in the summer of 2016 might have killed ticks or made them so dormant that Lyme disease cases would decline. That proved to be true during the summer. But Lyme reports rebounded and soared in the fall, and 2016 turned out to be a record-setting year.
“We were so very wrong,” Elias said. “It looks like they buried themselves deep down in the leaf litter, almost to a hibernation-like state, and then came out very strong later that year.”
‘IT’S A COMPLICATED WEB’
Elias said she will be studying the many factors that may lead to ticks increasing their range and the prevalence of Lyme disease. Cases have now been reported in all 16 counties in Maine, compared to predominantly in the six southernmost and coastal counties.
Many things could be in play, Elias said, including not only temperature, but also humidity in the soil, snow, deer density near people, leaf cover, more people moving into tick habitats by building homes near wooded areas, summer precipitation, how cold the winters are and the expansion of the Japanese barberry invasive plant. The Japanese barberry is a prime habitat for ticks. Rodents can also contribute to the spread of ticks.
Even the re-emergence of deciduous trees growing on what used to be farmland may be a factor, Elias said. Ticks survive well under leaf litter, helping them to survive the winter.
“We’re looking at Lyme disease as a function of climate change,” Elias said. “We’re going to try to look at all of them and assign each of them a relative importance. It’s a complicated web of many different factors.”
What is undeniable, Elias said, is that deer ticks have increased their range in Maine over the past 20-30 years, and are appearing much farther north, especially along the coast.
Sean Birkel, assistant research professor at the University of Maine’s Climate Change Institute, said that decades ago, large areas of Maine were uninhabitable for the deer tick because larvae could not survive cool summer days.
“Most of Maine is now a likely tick habitat. Thirty years ago that definitely wasn’t the case,” Birkel said. “But it may still take awhile for significant populations to be established up in the north.”
Birkel said for tick larvae to survive a Maine summer, there must be a baseline temperature of 6 degrees Celsius (43 degrees Fahrenheit) and a total of 1,400 degrees Celsius over the course of an entire summer for the ticks to be able to survive. Even in the early 2000s much of northern Maine was not habitable for deer ticks, but that’s not the case now, Birkel said.
The warming winters also let adult ticks survive, especially if there are few days with temperatures below minus 4 degrees with little or no snow cover, Birkel said. Snow acts as an insulator and helps ticks survive.
WILL ‘KNOCKDOWN’ COLD WINTER HELP?
Maine Med Research Institute is entering its third winter of studying how well deer ticks survive at a site in Cape Elizabeth. Researchers place live ticks in vials in varying conditions – exposed to the elements, under snow, under leaves, and under leaves and snow. Devices monitor the ticks every 30 seconds to see whether they are still alive.
Chuck Lubelczyk, a Maine Med tick researcher, said over the past two years, about 60 percent of ticks covered by both snow and leaf litter survived, compared to 20 percent of exposed ticks. Temperatures were mild the past two winters.
“We’re doing this one more year to see if we get a knockdown cold winter, for comparison,” Lubelczyk said.
Elias said there are many preventive measures that can be taken to combat the spread of ticks.
Some communities, such as Falmouth and Portland’s Peaks Island, are removing Japanese barberry. Towns and cities could monitor deer populations and reduce them with altered hunting seasons or controlled kills, Elias said.
The Maine Center for Disease Control and Prevention cautions people to take tick-prevention measures, such as wearing long clothing and repellents when venturing into the woods, and doing “tick checks” after long stretches in tick habitats.
Ticks also thrive on dead wood, so people who have wood piles should always wear long sleeves and gloves when stacking or bringing firewood indoors. Even raking leaves can help prevent Lyme.
Outdoor pets can also be a problem, and limiting pets from sleeping on beds or nestling next to people on couches can help protect against tick bites.
Birkel said the studies are needed to prepare people for the continued spread of the deer tick and risk of Lyme disease.
“It’s important for us to recognize this is happening because our health and well-being are affected by vector-borne diseases. If we’re aware this is happening, we can be better prepared,” Birkel said.