Tick-borne disease on the rise in the Northeast, CDC reports

Cases of a tick-borne disease called babesiosis more than doubled in some Northeastern states between 2011 and 2019, according to researchers at the Centers for Disease Control and Prevention. reported on thursday.

Although many people with babesiosis are asymptomatic, others develop flu-like symptoms, including fevers, chills, sweats, and muscle aches. The disease can be serious or even fatal in people who have compromised immune systems or other risk factors.

The disease, which for decades was extremely rare in the United States, is now endemic in 10 states in the Northeast and Midwest, the agency said. The increase may have been driven by rising temperatures and a growing deer population, two factors that help ticks thrive, experts said.

“I think this is an unfortunate milestone,” said Dr. Peter Krause, a babesiosis expert at the Yale School of Public Health, who was not involved in the study.

Babesiosis is caused by parasites normally found at home in mice and other rodents. Blacklegged, poppy-seed-sized ticks, which are also known as deer ticks and can transmit Lyme disease, can transmit the disease to humans after feeding on infected mice.

The first person known to have been infected in the United States was reported in 1969 in Massachusetts. Today, most cases occur in the Northeast and Upper Midwest during the spring and summer. (The parasite can also be transmitted by blood transfusions, and the Food and Drug Administration recommends testing donated blood in certain states.)

In the new study, the researchers analyzed 16,174 cases of babesiosis reported in 10 states between 2011 and 2019. In 2019 alone, there were more than 2,300 cases, more than double the number in 2011. The disease was most common in New York, Massachusetts, and Connecticut, which normally had hundreds of cases a year.

But there were regional differences in trends. In two Midwestern states, Minnesota and Wisconsin, the number of annual cases remained relatively stable. By contrast, in eight Northeastern states, the number of cases rose significantly over that time period, with the steepest increases in Vermont, Maine, New Hampshire and Connecticut.

In three of those states, Maine, New Hampshire, and Vermont, babesiosis had not previously been considered endemic.

The increases in those states are particularly notable, said Edouard Vannier, a babesiosis expert at Tufts Medical Center in Boston, who was not involved in the new study.

“The disease is going north,” he said. “And it’s probably due to climate change.”

Ticks, which prefer warm, humid conditions, have expanded their range northward. The findings highlight the need for expanded surveillance and detection of the disease, Dr. Vannier said.

A growing deer population could also be driving the rise in babesiosis. Although deer do not carry the parasites that cause babesiosis, they are the preferred food source for adult ticks.

“That greatly amplifies the number of ticks,” said Dr. Krause. “Many more survive, many more females lay eggs.”

Building new homes in areas where there are ticks could also play a role, he added, as could increased awareness, which may be prompting more doctors to test patients for the disease.

It’s unclear why Midwestern states haven’t seen the same rise in cases. “I don’t have an explanation for it,” Dr. Vannier said. But the disease had typically been less common there than in the Northeast, he noted.

Babesiosis can be treated with antimicrobial medications. It can be prevented by avoiding tall grass and brush, and by wearing long pants and tick repellent, in areas where the disease is endemic. Daily tick checks can help people detect and remove ticks before the insects have a chance to transmit the parasite.