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Babesiosis is an emerging tick-borne infectious disease caused by protozoan parasites of the genus Babesia that invade and eventually lyse red blood cells (RBCs). Most cases are due to Babesia microti and occur in the United States, particularly in the Northeast and upper Midwest. The infection typically is mild in young and otherwise healthy individuals but can be severe and sometimes fatal in persons >50 years of age and in immunocompromised patients. Sporadic cases have been reported in Europe and the rest of the world.


Geographic distribution

image More than 100 Babesia species are found in wild and domestic animals; a few of these species cause infection in humans (Fig. 124-1). B. microti, a parasite of small rodents, is the most common etiologic agent of human babesiosis and is endemic in the northeastern and upper midwestern United States. Seven states in these two regions (Connecticut, Massachusetts, Minnesota, New Jersey, New York, Rhode Island, and Wisconsin) account for >95% of the reported cases. Other etiologic agents include Babesia duncani and B. duncani–type organisms on the West Coast and Babesia divergens–like organisms in Kentucky, Missouri, and Washington State.

FIGURE 124-1

Worldwide distribution of human babesiosis. Dark colors designate areas where human babesiosis is either endemic or sporadic (as defined by more than three tick-borne cases reported in a country or state). Isolated cases of babesiosis are denoted by circles. Colors designate causative Babesia species: Babesia microti and B. microti–like organisms in red, Babesia duncani and B. duncani–type organisms in orange, Babesia divergens and B. divergens–like organisms in blue, Babesia venatorum in purple, KO1 in black, and unspeciated Babesia organisms in white. Due to space constraints, the 10 cases reported from Montenegro are denoted by a single white circle, and those from Australia, Mozambique, and South Africa are not shown. Light colors denote areas that are enzootic for Ixodes tick species known to transmit one or several Babesia species but where human babesiosis has yet to be documented. (Adapted from E Vannier, PJ Krause: N Engl J Med 366:2397, 2012.)

The primary causative agent of human babesiosis in Europe is B. divergens, but Babesia venatorum and B. microti also have been reported. In Asia, cases due to B. microti–like organisms have been documented in Japan, Taiwan, and the People’s Republic of China. A case caused by B. venatorum also has been reported from the People’s Republic of China. A case of B. microti infection was described in Australia. Sporadic cases due to uncharacterized species have been reported in Colombia, Egypt, India, Mozambique, and South Africa.


More than 1100 cases were reported in the United States in 2011, the ...

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