Country Information

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Most cases of Babesiosis are reported in the U.S. A few cases have originated in Europe (mainly B. divergens), with sporadic incidents documented in France, the former Yugoslavia, United Kingdom, Ireland and the former Soviet Union. Less than a handful of cases have been reported in all of Asia (9).



From 1969 to 1993, about 450 cases of babesiosis have been diagnosed in the U.S., although there may be many more undiagnosed cases or asymptomatic carriers.  Most of these incidents appear in the northeast, particularly Nantucket Island, Cape Cod in Massachussetts, Block Island in Rhode Island, and Long Island in New York (9). Though the highest density of these cases has occurred in the Northeast, human babesiosis has also been reported in the Midwest and Northwest regions of the country. Epidemiologists have distinguished two main patterns: the first involving B. microti transmitted by blacklegged ticks in the Northeast and the second involving other unidentified etiologic agents in the Midwest and Northwest United States (10).

Overall, the risk of contracting babesiosis has been shown to be dependent on the number of Ixodes scapularis ticks in the region. A spatial study using bio-surveillance and sampling of ticks in different regions has identified the two highest density clusters of ticks in the Northeast and upper Midwest. The densities of the definitive host, Ixodes scapularis ticks, corresponds to the endemic regions of Babesia (4).

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Clinical presentations of babesiosis can vary from asymptomatic to fatally severe. Those at most risk for both being infected and developing severe symptoms include people over age 40 and immuno-compromised patients (10). Another factor that affects the distribution of babesiosis is time of year. Since human transmission depends on the tick vector, most cases are contracted during the summertime (7).

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            The risk factor for acquiring babesiosis through transfusion is low. In Connecticut, where the donor seropositivity ranged from 3.3 to 4.9 percent, the risk of being infected form a reb blood cell transfusion was .17%, while the platelet transfusion risk was even lower at .08% (2).