Cases of Lyme disease are rebounding in Maine, with the number of cases so far this year on pace to exceed the totals for 2020 and 2021.

Maine recorded 1,433 Lyme cases through Aug. 14 this year, compared with 1,127 in all of 2020 and 1,510 in all of 2021, according to the Maine Center for Disease Control and Prevention. And the deer ticks that cause Lyme will actively search for hosts well into the autumn before becoming less active and burrowing beneath leaf litter for the winter.

The increase in cases this year comes despite the dry summer in much of the state. Warm, humid and rainy weather brings out ticks, while dry weather is not as favorable for the arachnids.

Griffin Dill, integrated pest management professional for the University of Maine Cooperative Extension’s tick lab, said it’s difficult to determine what is causing the increase in Lyme cases this year, but several factors may be at work.

A rainy late spring and early summer may have contributed. Dill said even with mostly dry conditions in July and August, the ticks may have been better poised to survive a dry spell after the near-ideal weather conditions earlier this year.

“The adult ticks in spring and early summer were highly active,” Dill said. “It started to tail off at the end of June, early July.”

Dill said another factor may be that a higher percentage of deer ticks are carrying the bacteria that causes Lyme disease. The lab accepts tick samples from all over the state to test for infectious diseases, and the percentage of ticks found to be carrying the bacteria increased from 38 percent in 2019 to 45 percent so far this year.

“We have more ticks testing positive for the pathogens than in previous years,” Dill said.

The range of deer ticks also keeps expanding into new parts of the state, he said.

“We are starting to see more of an increase in Down East, in Washington County, as far east as Calais,” Dill said. “The habitat along the coast is more conducive to ticks.”

Cases of another tick-borne disease called anaplasmosis also are tracking higher this year, with 610 cases through Aug. 14, compared to 841 for all of 2021. Maine has reported 97 cases of babesiosis, the other disease most commonly associated with deer ticks, so far this year.

Lyme disease peaked in Maine in 2019, with 2,167 total cases, before falling off to 1,127 cases in 2020. Dill said it’s difficult to tell why cases were so much lower in 2020, but it may be that there were many undiagnosed cases during the first year of the COVID-19 pandemic.

“Many people were not seeking medical attention in 2020,” he said.

VACCINE IN THE WORKS

Meanwhile, scientists, including some in Maine, are testing a Lyme vaccine that soon could be approved. Pfizer is currently conducting human clinical trials for a Lyme vaccine, and Brewer-based Northern Light Health in Maine is the site of one of the trials.

Kris Currier, a Northern Light Health spokeswoman, said the health care system “will soon share more information about a Lyme disease vaccine trial that was recently addressed in the news. We anticipate approval to begin enrollment in September and will send out information including dates and the process for enrollment when details are confirmed.”

Lyme can be treated with a course of antibiotics. But many cases go undetected, especially if people are bitten by ticks in the nymphal stage, which are so tiny they can be hard to spot. Symptoms of Lyme disease can include fatigue, joint pain, headache and fever. If untreated, Lyme disease can cause heart and nervous system conditions.

Dill said the tick lab is studying the role of the white-footed mouse as a reservoir for Lyme disease. The white-footed mouse is more prevalent in southern Maine but may be expanding its habitat. Dill said it’s unknown if the deer mouse, a more common mouse in northern Maine, is less likely to be a reservoir for Lyme than the white-footed mouse. He said it’s also unclear if the white-footed mouse is expanding its habitat farther north and displacing the deer mouse.

Meanwhile, the Maine Medical Center Research Institute is conducting several research projects, including “investigating ways to eliminate Lyme disease at its source by preventing ticks from spreading the Lyme bacteria in the wild” and also studying how temperature and humidity impact the survivability of deer ticks.