Risk

Babesia parasites are not transmitted from person-to-person like the flu or the common cold. People can get infected with Babesia parasites in several ways:

• The main way is through the bite of an infected tick—during outdoor activities in areas where babesiosis is found (see below).
• A less common way of becoming infected is by getting a transfusion from a blood donor who has a silent Babesia infection. (No tests have been licensed yet for screening blood donations for Babesia.)
• A few possible cases of congenital transmission—from an infected mother to her baby (during pregnancy or delivery)—have been reported.

Many different species (types) of Babesia parasites have been found in animals, only a few of which have been found in people. Babesia microti—which usually infects white-footed mice and other small mammals—is the main species that has been found in people in the United States. Occasional (sporadic) cases of babesiosis caused by other Babesia species have been detected.

Babesia microti is transmitted in nature by Ixodes scapularis ticks (also called blacklegged ticks or deer ticks). See Life Cycle.

• Tickborne transmission primarily occurs in the Northeast and upper Midwest, especially in parts of New England, New York State, New Jersey, Wisconsin, and Minnesota.
• The parasite typically is spread by the young nymph stage of the tick, which is most apt to be found (seeking or “questing” for a blood meal) during warm months, in areas with woods, brush, or grass.
• Infected people might not recall a tick bite because I. scapularis nymphs are very small (about the size of a poppy seed).