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Babesiosis: Clinical manifestations and diagnosis

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. It 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 [1-4].

Babesia microti is the primary agent of human babesiosis in the United States, particularly in the Northeast and upper Midwest where it is endemic. Nearly all cases in Europe have been attributed to Babesia divergens, but the infection is sporadic. Babesia venatorum is endemic in northeastern China.

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


Babesia infections range from asymptomatic to severe and sometimes are fatal. The severity of infection depends on the Babesia species and the immune status of the host.

Infection due to B. microti — B. microti is the predominant species that infects humans in the United States. B. microti also causes disease in Europe, Asia, and Australia. Clinical manifestations may be absent or range from mild to severe.

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