An endemic transmission cycle of Babesia microti was discovered in Colorado in the foothills of the Rocky Mountains.
B. microti were found by PCR in 4 of 25 Ixodes spinipalpis tick pools tested (a 3·2% minimum infection rate) and in 87%
(13 of 15) of Microtus ochrogaster (the prairie vole) spleen and blood samples. Using naturally infected I. spinipalpis
collected from wild-caught M. ochrogaster as vectors, B. microti and Borrelia bissettii were successfully transmitted to
laboratory-born M. ochrogaster. Neither I. spinipalpis, nor M. ochrogaster (the prairie vole) have been previously reported
as a vector or a reservoir host of B. microti. Unlike the east coast of the United States where Peromyscus leucopus is an
important reservoir for B. microti, evidence for Peromyscus spp. (neither P. maniculatus nor P. difficilis) as B. microti
reservoirs was not found in this study.