What to Know About Babesiosis

After Lyme, babesiosis or a bacterial disease called anaplasmosis tends to be the next most common tick-borne illness, depending on the state, Diuk-Wasser says. 

But while babesiosis is on the rise, documented cases are still not common. Between 2011 and 2019, 16,456 cases were reported to the CDC from 37 states, according to the 2023 CDC study, with the vast majority of cases coming from 10 states. New York reported the most cases, about 526 per year, followed by Massachusetts and Connecticut. As with most tick-borne illnesses, many cases go unreported, Auwaerter says. But reports can show trends, including when a disease is becoming more common, he says. 

CDC surveillance data from 2011 to 2019 indicates that babesiosis is now endemic in 10 states: Connecticut, Maine, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Rhode Island, Vermont, and Wisconsin. That’s up from seven states in previous years. To prevent the illness from spreading through the blood supply, the Food and Drug Administration recommends that donated blood be screened for the parasites in those 10 states plus Delaware, Maryland, Pennsylvania, Virginia, and Washington, D.C.

Babesiosis is likely spreading for many of the same reasons that Lyme spreads, Auwaerter says, including the large populations of deer and mice that help spread blacklegged ticks across neighborhoods and state lines. 

Infected mice can transmit the parasites to their pups, so pups can be born infected, which provides opportunities for the illness to spread, especially because more mice may be surviving warmer winters, Diuk-Wasser says. Plus, while more research is needed to understand this interaction, co-infection with Lyme disease seems to increase the likelihood that babesiosis spreads. One potential explanation for this could be that the immune system may be less able to fight off a babesia infection if it’s simultaneously trying to respond to the borrelia bacteria that transmit Lyme, she says.