Cases of a tick-borne disease called babesiosis have significantly increased in three New England states, where the illness is now considered endemic, according to a new report from the Centers for Disease Control and Prevention.
The CDC concluded that babesiosis is now endemic in New Hampshire, Maine, and Vermont based on case numbers documented between 2011 and 2019.
The disease has already been considered endemic in Massachusetts. The state, which reported 4,136 cases, had the second-largest number of cases reported to the CDC over the same eight-year period. New York had the highest number of cases during that time frame, a total of 4,738,, the report said.
The first human case of babesiosis detected in the United States was identified on Nantucket in 1969, according to the CDC report, which was published Thursday.
Most cases are transmitted to humans by bites from blacklegged ticks also known as deer ticks, the same insects that carry Lyme disease.
Because babesiosis and Lyme disease are carried by the same ticks, researchers predicted babesiosis would take root in places where Lyme disease is prevalent, said Edouard Vannier, an infectious disease researcher and babesiosis expert at Tufts Medical Center.
“We know that Lyme disease was already there in New Hampshire, Maine, and Vermont. It was only a matter of time until babesiosis would get there,” Vannier said Saturday in a phone interview.
The CDC had previously said babesiosis was endemic in six states — Connecticut, Minnesota, New Jersey, New York, Rhode Island, and Wisconsin — in addition to Massachusetts.
Climate change is probably a factor, as the milder winters much of New England is experiencing makes it easier for ticks to survive, Vannier said.
Dr. Peter J. Krause, a senior researcher at the Yale School of Public Health and Yale School of Medicine, called the new CDC report a “milestone” that confirmed the growing prevalence of babesiosis in the Northeast and found that the disease now poses an ongoing threat in the three additional New England states.
“Global warming is one of several reasons for the increase,” Krause said Saturday in a phone interview. “The disease is moving northward.”
CDC researchers examined babesiosis data from 10 states that accounted for 16,174 of cases reported to the agency between 2011 and 2019.
The 10 states include the six New England states, plus New York, New Jersey, Minnesota, and Wisconsin. Cases in those states represent more than 98 percent of all babesiosis infections reported to the CDC during the eight-year period, the report said.
Ticks the size of poppy seeds pick up the parasites that cause babesiosis by feeding on white-footed mice, according to the TickEncounter Resource Center. Once the ticks are infected with the parasite, they can pass the disease to humans.
People can also become infected from blood transfusions, and the US Food and Drug Administration recommends blood donations be screened for infection in 14 states and Washington, D.C., according to the CDC.
Vermont, Maine, and New Hampshire, the three states just recently put on the endemic list, experienced the largest swing in case numbers during the study period, the report said.
In Vermont, the number of cases increased from two in 2011 to 34 cases in 2019. Maine reported nine cases in 2011, and then 138 cases in 2019. In New Hampshire, the number of cases grew from 13 in 2011 to 63 in 2019, the CDC said.
The CDC and state public health agencies in those three states didn’t respond to an inquiry from the Globe.
Babesiosis is a parasitic infection of the red blood cells and can cause illness ranging from asymptomatic or mild to severe, even fatal disease, the CDC said. People who are 50 or older, have a compromised immune system, or take immunosuppressants face greater risk of developing serious illness from babesiosis, Krause said.
Also at risk are patients who don’t have a spleen, researchers said, because the organ removes red blood cells carrying the Babesia parasite from blood.
Symptoms of babesiosis include fever, chills, headache, and fatigue, Krause said. The disease can be diagnosed by examining a blood sample under a microscope. Babesiosis is treated with prescription medications, experts said.
The CDC recommends people who spend time outdoors in spots where babesiosis is prevalent wear long pants, avoid underbrush and long grass, and use tick repellents to lower their risk of infection.
There is no vaccine for the disease.
“Researchers like me are working on it,” Vannier said.