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Babesiosis is a tick-borne infectious disease caused by parasites of the genus Babesia. These protozoans are obligate parasites of red blood cells (RBCs). Wild and domestic animals are the natural reservoirs of Babesia. Transmission to humans is incidental and was recognized only half a century ago. The vast majority of cases occur in the United States, where babesiosis has the status of an emerging infectious disease. Sporadic cases are reported in Europe and the rest of the world (see “Global Considerations,” below).

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Epidemiology

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Geographic Distribution

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Babesia microti, a parasite of small rodents, is the etiologic agent of babesiosis in the northeastern United States. Highly endemic areas include Nantucket Island, Martha's Vineyard, Block Island, Shelter Island, eastern Long Island, and Fire Island. On the mainland, babesiosis is endemic in southeastern Massachusetts, coastal Rhode Island and Connecticut, central New Jersey, Wisconsin, and Minnesota. On the West Coast, the etiologic agent is Babesia duncani, a species closely related to those found in wildlife. The index case of infection with this species was reported from Washington State. Several cases were identified in northern California, and one case may have been contracted in central Oregon. Three cases of babesiosis caused by Babesia divergens–like organisms—from Washington State, Missouri, and Kentucky, respectively—have been reported.

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Prevalence

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The number of cases of B. microti illness has increased steadily over the last decade. In 2009, more than 700 cases were reported to the public health departments in endemic states. The prevalence of babesiosis caused by B. microti is underestimated because young healthy individuals typically experience a mild and self-limiting illness and may not seek medical attention. Accordingly, seroprevalence is much higher than the prevalence of clinical babesiosis.

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Modes of Transmission

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The nymphal stage of the deer tick Ixodes scapularis is the primary vector for transmission of B. microti. Transmission occurs from May through September. The incubation period is 1–6 weeks long, with three-fourths of cases presenting in June and July. The vectors for transmission of B. duncani and B. divergens–like organisms remain unknown.

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Babesiosis is occasionally acquired by transfusion of blood products, primarily packed RBCs; more than 70 such cases have been caused by B. microti, and two cases caused by B. duncani have been transmitted by this route. The incubation period lasts 1–9 weeks. Most cases occur in endemic areas during fall and winter. Some cases are diagnosed in nonendemic areas to which blood products have been imported from endemic areas. Of the 11 transfusion-related babesiosis deaths reported to the U.S. Food and Drug Administration since 1998, 10 have occurred since 2005.

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Three cases of congenital babesiosis have been attributed to B. microti. Other cases of neonatal babesiosis have been acquired by transfusion or tick bite.

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Clinical Manifestations

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