In 2023, experts at the U.S. Centers for Disease Control and Prevention reported a dramatic rise in babesiosis, a historically rare tick-borne disease, throughout New England.

Here in Maine, where babesiosis was not previously considered endemic, incidence of the illness soared a whopping 1,422% in less than a decade. Reported cases in Maine jumped from 9 in 2011 to 138 in 2019. Last year there were more than 300 cases, according to preliminary data. 

It is a troubling trend that a team of Massachusetts doctors now warns is likely linked to climate change-induced warming across the region. 

In an article published Aug. 1 in the scientific journal Open Forum Infectious Diseases, doctors at Brigham and Women’s Hospital and Massachusetts General Hospital looked at more than 1,000 cases of babesiosis at Boston hospitals between 1993 and 2024 and observed what they describe as a “striking expansion of both the annual caseload and the active season” for Babesia microti, the blood-borne parasite that causes babesiosis and is transmitted to humans by infected deer ticks.

The ticks, also known as blacklegged ticks, can carry and transmit multiple diseases at once, including babesiosis, Lyme and anaplasmosis. 

Prior to 2000, the active season for the disease — the average number of months when positive cases were documented — was 2.2 months, with most cases occurring in peak summer. Less than two decades later, that active season had grown to 9.2 months. And in 2023, positive cases of babesiosis occurred 11 months out of the year. 

“It’s going from something that we just see a case in July and August here and there to something that we’re essentially seeing year round — more so in the summer months, but basically on a year round basis,” Dr. John Ross, the lead author of the paper and an assistant professor of medicine at Harvard Medical School, told The Maine Monitor. 

Ross’s research centered on babesiosis cases in Massachusetts, but he expects the same trends would hold true in Maine, perhaps with some lag time. 

Babesiosis is a parasitic infection that attacks red blood cells. Many infected people never know they have it, whereas others can experience a variety of flu-like symptoms, including fever, fatigue, nausea and body aches, which typically develop within a few weeks. Unlike Lyme disease, infected persons do not usually develop a rash. In severe cases, babesiosis can lead to life-threatening complications. 

The paper stops short of attributing the expansion of New England’s Babesia season to any one thing, but details several possible explanations, including “diminished die-offs of blacklegged ticks during warmer winters, more rapid tick maturation in warm conditions, greater activity of adult ticks during winter warm spells, expansion of the active season for nymph forms into early spring and late fall, expanding populations and improving winter survival of tick hosts,” as well as non-climate related factors such as a growing public awareness of the disease and the increased use of immunosuppressive agents, which may “unmask” latent parasites in the blood. 

“I think, honestly, the major driver is probably climate change,” Ross said of the surge in winter cases of babesiosis. 

Research shows New England is warming faster than the rest of the world, which has correlated with a marked uptick in cases of tick-borne diseases, including Lyme, Anaplasmosis and babesiosis in Maine and throughout the Northeast.

“That warming is most marked in the winter months,” Ross said. “In terms of what that means for ticks, it doesn’t get cold enough anymore to kill off ticks.

Griffin Dill, the director of the University of Maine Cooperative Extension Tick Lab, told The Maine Monitor that the new research further confirms and corroborates what his lab is seeing both in terms of Maine’s expanding tick season and the increasing prevalence of Babesia in deer ticks. The lab’s surveillance data shows that the number of tested ticks carrying the parasite jumped from around 6 percent in 2019 to 14 percent today. 

“The take home message for me is that we are seeing an expansion of quote unquote ‘tick season,’” Dill said. “We are seeing expanding tick numbers. We are seeing higher human incidence rates of tick-borne disease and higher rates of pathogen prevalence within the tick populations. That said, there are a number of personal protection measures that can be taken — the use of protective clothing and gear, repellents, getting into the habit of conducting frequent tick checks — and in doing all those things you can really minimize your exposure to babesia and other tick-borne illnesses.” 

“We really want to stress that while these risks are there and we want people to be aware of them, we don’t want people to be afraid of going outdoors and enjoying everything that Maine and New England and the Northeast has to offer.”

More than anything, Ross hopes his research will put clinicians and other healthcare professionals on alert that babesiosis is no longer just a summer illness. 

“In the climate change era, babesiosis should be considered in the differential diagnosis of patients presenting with fever and anemia outside of peak summer months,” he and his coauthors write.

This story was originally published by The Maine Monitor, a nonprofit civic news organization. To get regular coverage from The Monitor, sign up for a free Monitor newsletter here.