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Causal Agents:

Babesiosis is caused by apicomplexan parasites of the genus Babesia. Although more than 100 Babesia species have been reported, relatively few have caused documented cases of human infection; these include (but are not limited to) B. microti, B. divergens, B. duncani, and a currently unnamed agent designated MO1.

Life Cycle:

Babesia microti life cycle

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. During a blood meal, a Babesia-infected tick introduces sporozoites into the mouse host The number 1 . Sporozoites enter erythrocytes and undergo asexual reproduction (budding) The number 2 . In the blood, some parasites differentiate into male and female gametes, although these cannot be distinguished by light microscopy The number 3 . The definitive host is the tick. Once ingested by an appropriate tick The number 4 , gametes unite and undergo a sporogonic cycle resulting in sporozoites The number 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 The letter A .

Humans enter the cycle when bitten by infected ticks. During a blood meal, a Babesia-infected tick introduces sporozoites into the human host The number 6 . Sporozoites enter erythrocytes The letter B and undergo asexual replication (budding) The number 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 The number 8 .

Life cycle image and information courtesy of DPDx.