What causes babesiosis?

Updated: Apr 01, 2021
  • Author: Rachel E Strength, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Babesiosis is an infection caused by parasites of the Babesia genus. It is a zoonosis that is transmitted from vertebrates to humans through the bite of a tick from the Ixodidae family (most commonly I. scapularis in the United States, I. ricinus in Europe). Ixodes ticks are small and differ from the large Dermacentor ticks that transmit Rocky Mountain spotted fever (RMSF) and ehrlichiosis.

More than 100 species of Babesia exist, but only a small number of them are known to be responsible for the majority of symptomatic disease. The causative agent of babesiosis varies according to geographic region.

In the United States, human infection with Babesia is primarily due to the rodent strain B. microti, found mostly in northeastern and north midwestern states. A few cases have been reported in Missouri, California, and Washington. These are caused by Babesia-like agents named after their geographic location: MO-1 (Missouri, closely related to B. divergens), CA-1 (California), and WA-1 (Washington, also known as CA5 and B. duncani).

In Europe, the causative agent of babesiosis is typically the cattle strain B. divergens, though B. microti and B. microti-like agents have been identified. Another cattle strain found in Europe, B. bovis, also causes disease in humans on occasion. China and some European countries have also reported B. venatorum as a cause of babesiosis. There are multiple other species that are under investigation. [3]

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