Babesia microti is an emerging tick-borne pathogen and the causative agent of human babesiosis. Mathematical modeling of the reproductive rate of B. microti indicates that it cannot persist in nature by horizontal tick-host transmission alone. In this study, Danielle and Maria report for the first time, the occurrence of transplacental transmission of B. microti occurring in its natural reservoir host, P. leucopus in the United States. Peromyscus leucopus collected from Connecticut and Block Island, Rhode Island showed a 100% (n = 103) infection prevalence with B. microti and transplacental transmission was observed in 74.2% of embryos (n = 89). Transplacental transmission of B. microti is thus a viable and potentially important infectious pathway in naturally infected rodent species. This vector-independent pathway could contribute to the increased geographic range of B. microti or increase its abundance in endemic areas.
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