Control of , the primary cause of equine theileriosis, is largely reliant on acaracide use and chemosterilization with imidocarb dipropionate (ID). However, it is currently unknown if ID is effective against , the recently identified second causative agent of equine theileriosis, or if the drug maintains effectiveness against in the presence of co-infection. The purpose of this study was to address these questions using ID treatment of the following three groups of horses: (1) five infected horses; (2) three - infected horses; and (3) three - infected horses. Clearance was first evaluated using nPCR for each sp. on peripheral blood samples. ID failed to clear in all three groups of horses, and failed to clear in two of three horses in group two. For definitive confirmation of infection status, horses in groups two and three underwent splenectomy post-treatment. The -nPCR-positive horses in group two developed severe clinical signs and were euthanized. Remaining horses exhibited moderate signs consistent with . Our results demonstrate that ID therapy lacks efficacy against , and - co-infection may interfere with ID clearance of .