Emerging babesiosis in the mid-Atlantic: autochthonous human babesiosis cases and Babesia microti (Piroplasmida: Babesiidae) in Ixodes scapularis (Acari: Ixodidae) and Ixodes keiransi (Acari: Ixodidae) ticks from Delaware, Maryland, Virginia, West Virginia, and the District of Columbia, 2009 to 2024.

The range of Babesia microti (Franca, 1910)-infected ticks is expanding, resulting in locally acquired human babesiosis cases occurring in new areas: Maryland (2009), the District of Columbia (2013), Virginia (2016), and West Virginia (2017). We collected host-seeking ticks from old fields, ecotones, forested habitats and animal hosts in Delaware, Maryland, and Virginia, 2010 to 2024. Ixodes scapularis Say, the tick vector of babesiosis, was captured in all 3 states. PCR revealed B. microti in 2.7% (36/1310) of I. scapularis, with site prevalence ranging from <1% to 12.5% infected. The first B. microti-positive I. scapularis was collected in Northampton County, Virginia, 2012. Of the B. microti-infected ticks, 50% (18/36) were coinfected with Borrelia burgdorferi and one was triple-infected with B. microti, B. burgdorferi, and Anaplasma phagocytophilum. We collected Ixodes keiransi Beati, Nava, Venzal, and Guglielmone ticks from Delaware and Virginia. We found B. microti and B. burgdorferi in those from Virginia, and B. burgdorferi in ticks from a shrew in Delaware. To our knowledge, this is the first report of B. microti and B. burgdorferi-positive I. keiransi from Virginia, and the first report of B. burgdorferi-positive I. keiransi from Delaware. Ixodes keiransi ticks rarely bite humans but are involved in the maintenance and spread of pathogens when sympatric with I. scapularis. We tested a subset of both tick species for Babesia duncani; none were positive. Jurisdictions in the southern mid-Atlantic region should expect babesiosis cases, and Lyme disease and anaplasmosis coinfections, and healthcare providers should consider these tick-borne infections as part of the differential diagnosis.