The apicomplexan parasite is one of two known causative agents of equine theileriosis. It causes milder clinical disease than its more virulent counterpart, , in experimentally infected horses, and can superinfect -positive horses. The current equi merozoite antigen 1 (EMA1)-based competitive enzyme-linked immunosorbent assay (ELISA)used in the U.S. to detect equine theileriosis detects but not , and the complexity of molecular assays precludes widespread use for epidemiologic studies. In order to facilitate urgently needed studies on the prevalence of , the goal of this study was to develop a sensitive and specific serologic assay for the diagnosis of based on the equi merozoite antigen 11 (EMA11). To achieve this objective, EMA11 was recombinantly expressed in eukaryotic cells and its antigenicity assessed using sera from -experimentally infected horses. Confirmation of sera reactivity enabled design and optimization of an indirect ELISA. Specificity of the ELISA for was assessed using a cohort of sera from horses experimentally infected and confirmed PCR-positive for either or . Data from field samples further demonstrate that the EMA11 ELISA is capable of identifying antibodies in horses from multiple continents around the world.