Life cycle of babesiosis

The Babesia microti life cycle involves two hosts, which include a rodent, primarily the white-footed mouse, Peromyscus leucopus, and a tick in the genus Ixodes. Human infection is accidental; humans are not definitive reservoir hosts. The dashed blue arrows convey the parts of the life cycle that occur in humans; the solid red arrows convey the parts of the life cycle that occur outside of humans. During a blood meal, a Babesia-infected tick introduces sporozoites into the mouse host (1). Sporozoites enter erythrocytes and undergo asexual reproduction (budding) (2). In the blood, some parasites differentiate into male and female gametes, although these cannot be distinguished by light microscopy (3). The definitive host is the tick. Once ingested by an appropriate tick (4), gametes unite and undergo a sporogonic cycle resulting in sporozoites (5). Transovarial transmission (also known as vertical, or hereditary, transmission) has been documented for "large" Babesia species but not for the "small" Babesia, such as B. microti (A).
Humans enter the cycle when bitten by infected ticks. During a blood meal, a Babesia-infected tick introduces sporozoites into the human host (6). Sporozoites enter erythrocytes (B) and undergo asexual replication (budding) (7). Multiplication of the blood-stage parasites is responsible for the clinical manifestations of the disease. Humans usually are dead-end hosts. However, human-to-human transmission is well recognized to occur via contaminated blood transfusions (8).