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Babesiosis is caused by unicellular apicomplexan parasites in the genus Babesia. Although more than 100 Babesia species have been described, only a few have been identified as causing human infections, including B. divergens, B. microti, B. duncani, and several other forms that have not yet been formally named. Human Babesia infections occur worldwide, but little is known about the prevalence of Babesia infection in malaria-endemic countries, where misidentification as Plasmodium (the apicomplexan parasite causing malaria) probably occurs. In Europe, most reported cases have generally been attributed to B. divergens and believed to occur only in splenectomized patients. In the United States, reported babesiosis cases have been concentrated in the northeast and midwest, have been generally attributed to B. microti, and have clearly not been limited to splenectomized individuals. The recently described B. duncani (Conrad et al. 2006) has been isolated from patients in Washington and California (U.S.A.). (Centers for Disease Control Parasites and Health website) Although this presents the picture of babesiosis that has prevailed for decades, beginning in the 1990s it has become increasingly clear that the diversity of Babesia is much more complex than previously realized, with undescribed species and broader host and geographic ranges than recognized. Babesia microti, for example, is now recognized to actually represent a complex of distinct species. (Gray 2006; Hunfeld et al. 2008; Leiby 2011).
Tthe definitive host for B. divergens is cattle. The reservoir host for B. divergens sensu stricto, which is implicated in most cases of human babesiosis in Europe, is cattle and the vector for this parasite is the hard tick Ixodes ricinus (the European Sheep Tick). However, B. divergens-like parasites have also been described from deer and from I. ricinus and these parasites may have caused some infections attributed to B. divergens sensu stricto. In the U.S., the reservoir hosts for the B. divergens-like isolates from Missouri (MO1) and from Kentucky are probably species of cottontail rabbits. (Hunfeld et al. 2008 and references therein)
Hunfeld et al. (2008) and Leiby (2011) provides wide-ranging reviews of biological and medical aspects of babesiosis. Hunfeld et al. (2008) review what is known about reservoir hosts and tick vectors with respect to babesiosis.