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Babesiosis is a worldwide (Fig. 220-1) emerging tick-borne infectious disease caused by protozoan parasites of the genus Babesia that invade and eventually lyse red blood cells (RBCs). More than 100 Babesia species infect a broad array of wild and domestic animals, but only a few of these species have been identified as etiologic agents of human babesiosis. Most cases are due to Babesia microti and occur in the United States. The infection typically is mild or asymptomatic in young and otherwise healthy individuals but can be severe and sometimes fatal in the elderly and the immunocompromised.

FIGURE 220-1

Worldwide distribution of human babesiosis. The geographic distribution of human babesiosis and its tick vectors is shown. Dark colors indicate areas where human babesiosis is endemic or sporadic (defined by ≥5 cases), whereas light colors indicate areas where tick vectors are present but human babesiosis is rare (<5 cases), undocumented, or absent. Circles depict single cases except in two locations (Montenegro and eastern Poland), where ≥5 cases occurred but all patients were diagnosed at the same hospital. Colors distinguish the etiologic agents: red for B. microti, orange for B. duncani, blue for B. divergens, green for B. venatorum, brown for B. bovis, black for KO-1, and yellow for Babesia species XXB. White circles depict cases caused by uncharacterized Babesia species. Asymptomatic infections are omitted. (Adapted from E Vannier, PJ Krause: N Engl J Med 366:2397, 2012.)


United States


In the United States, human babesiosis caused by B. microti is endemic in the Northeast and upper Midwest; seven states in these two regions (Connecticut, Massachusetts, Minnesota, New Jersey, New York, Rhode Island, and Wisconsin) account for more than 90% of reported cases. Whole-genome analysis shows that isolates in the continental United States began to diverge from those in Asia between 1400 and 14,000 years ago and are polyphyletic. Isolates in New England appear to have separated from those in the Midwest some 600 years ago; those on Nantucket Island form a separate subgroup. Other Babesia species causing sporadic disease in the United States include B. duncani and B. duncani–type organisms along the Pacific Coast and B. divergens–like organisms in Arkansas, Kentucky, Missouri, and Washington State.


National surveillance for human babesiosis was begun in the United States in January 2011. More than 1600 cases were reported in 2016—up from ~100 cases in 1996 and ~500 cases in 2006. The steady increase in the number of reported cases is due to the geographic expansion of Babesia-infected ticks and reservoir hosts as well as to a greater awareness of the disease among health care workers and improved reporting to state health departments and the Centers for Disease Control and ...

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