The 18 kDa antigen of Theileria equi is a specific but less abundant protein also expressed by parasites cultured in vitro.

A universally applicable test has not yet been developed for the reliable diagnosis of equine theileriosis, mainly because of the paucity of specific antigens and antigenic variation between different isolates. In this study, we used Theileria equi parasites cultured in vitro to identify potential diagnostic antigens. Using preparative isoelectric focusing to resolve the proteins in a lysate of infected erythrocytes, we identified an 18 kDa component of the parasite as a specific but poorly expressed antigen. This antigen also appears to have conserved epitope(s) between the isolates from the New and the Old World, as positive sera from both European and South American horses recognize it. The recombinant replica of this antigen might be a valuable tool for inclusion in the development strategy for a diagnostic test.