Life Cycle

The agents of babesiosis include members of the Babesia genus within the Protozoa phylum. Of the more than 100 species of Babesia, B. microti, a rodent species, and B. divergens, a cattle species, are the two that cause the most human infections.
Back to the top


Ixodes dammini.
Ixodid (hard-bodied ticks), particularly I. scapularis, I. dammini and I. ricinus serve as babesiosis vectors.
Back to the top


White-footed mouse.
White-footed mouse, Peromyscus leucopus, and other rodents, as well as deer, bovines, and equines serve as reservoirs for babesiosis.
Back to the top


The Babesia microti life cycle involves two hosts, the first of which is often a rodent, primarily the white-footed mouse. During a blood meal, a Babesia-infected tick introduces sporozoites into the mouse host (#1). Sporozoites enter erythrocytes and undergo asexual reproduction (budding) (#2). Some parasites differentiate into male and female gametes in the blood, but these cannot be distinguished with light microscopes (#3). The definitive host is a tick, in this case the deer tick (A). Once ingested (#4), gametes unite and undergo a sporogonic cycle resulting in sporozoites (#5).

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 are largely considered dead-end hosts and there is probably little, if any, subsequent transmission that occurs from ticks feeding on infected persons. However, human to human transmission is well recognized to occur through blood transfusions (#8).

Note: Deer are the hosts upon which the adult ticks feed and are indirectly part of the Babesia cycle as they influence the tick population. When deer populations increase, the tick population also increases, thus heightening the potential for transmission.

Back to the top

The Ixodes dammini image is from nantucket3.html
The white-footed mouse is from
The Life Cycle image is from

Questions? Please e-mail me at