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Babesiosis: Microbiology, epidemiology, and pathogenesis

Peter J Krause, MD
Edouard G Vannier, PhD
Section Editor
Johanna Daily, MD, MSc
Deputy Editor
Elinor L Baron, MD, DTMH


Babesiosis is an infectious disease caused by protozoa of the genus Babesia and is transmitted primarily by tick vectors. Transmission rarely occurs through blood transfusion, organ transplantation, or congenitally. Babesia protozoa infect mammals and cause lysis of host red blood cells [1-3].

The microbiology, epidemiology, and pathogenesis of babesiosis will be reviewed here. The clinical manifestations, diagnosis, treatment, and prevention of babesiosis are discussed separately. (See "Babesiosis: Clinical manifestations and diagnosis" and "Babesiosis: Treatment and prevention".)


Babesia species have been classified into four clades [4-6]:

Clade 1 contains Babesia microti organisms that cause human babesiosis in the United States and sporadically in Europe, Asia, and Australia. Initially considered as a single species, B. microti appears to be genetically diverse [6]. In fact, B. microti may be a genus on its own that ranks equally to the Babesia sensu stricto genus (clades 3 and 4, see below) [7].

Clade 2 contains Babesia duncani (WA1, WA2, CA5, CA6) and B. duncani–related organisms (CA1, CA3, CA4), the main etiologic agents of human babesiosis along the Pacific coast of the United States.

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Literature review current through: Nov 2017. | This topic last updated: Nov 13, 2017.
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