Technical Abstract: The insidious emergence of tick borne Babesia (Theileria) equi infection and disease (piroplasmosis) recently in the U. S. is a poignant reminder of the vigilance required to remain free of the class of infectious diseases which include persistence in their pathogenesis. Pathogen persistence is the ability of an infectious organism to remain in the host long-term, even for life in the absence of easily detectable clinical disease. A critical outcome of persistence is infected populations which are clinically silent reservoirs for transmission. Three questions receiving increased consideration due to the recent re-emergence of B. equi infection in the U.S. are: (1) why was clinical disease not detected in the acute phase in the majority of infected horses; (2) how did infection enter the U. S., and (3) is it possible to treat and remove transmission risk from infected horses? Equine piroplasmosis is caused by either B. equi or B. caballi, due to its prevalence in recent diagnoses this discussion will focus on B. equi.