Babesia microti is an intraerythrocytic parasite transmitted by Ixodes scapularis, the same tick that causes Lyme disease and anaplasmosis. Babesia duncani causes babesiosis in California and Washington, but a tick has not been identified yet. A currently unnamed strain, designated MO-1, is found in Missouri. In Europe, Babesia divergens causes illness primarily in splenectomized persons.
The life cycle usually is between rodents and ticks, but humans can be bitten by ticks and become infected. Babesiosis can also be transmitted through blood transfusions and congenitally during fetal development in the uterus in rarely reported cases.
