Evidence for transmission of the zoonotic apicomplexan parasite Babesia duncani by the tick Dermacentor albipictus.

•Babesiosis in the western US is caused by Babesia duncani whose enzootic cycle has not been previously described.•We provide molecular evidence that the vector of B. duncani may be the winter tick Dermacentor albipictus.•We detected high parasite prevalence in mule deer, Odocoileus hemionus, suggesting this is an important reservoir host.•For the first time since B. duncani was identified, we propose an enzootic transmission cycle of this zoonotic parasite.