AGENT: 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
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.
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