Imidocarb Dipropionate Lacks Efficacy against Theileria haneyi and Fails to Consistently Clear Theileria equi in Horses Co-Infected with T. haneyi

Abstract

Control of <i>Theileria equi</i>, 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 <i>Theileria haneyi</i>, the recently identified second causative agent of equine theileriosis, or if the drug maintains effectiveness against <i>T. equi</i> in the presence of <i>T. haneyi</i> co-infection. The purpose of this study was to address these questions using ID treatment of the following three groups of horses: (1) five <i>T. haneyi</i> infected horses; (2) three <i>T. haneyi</i>-<i>T. equi</i> infected horses; and (3) three <i>T. equi</i>-<i>T. haneyi</i> infected horses. Clearance was first evaluated using nPCR for each <i>Theileria</i> sp. on peripheral blood samples. ID failed to clear <i>T. haneyi</i> in all three groups of horses, and failed to clear <i>T. equi</i> 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 <i>T. equi</i>-nPCR-positive horses in group two developed severe clinical signs and were euthanized. Remaining horses exhibited moderate signs consistent with <i>T. haneyi</i>. Our results demonstrate that ID therapy lacks efficacy against <i>T. haneyi</i>, and <i>T. haneyi</i>-<i>T. equi</i> co-infection may interfere with ID clearance of <i>T. equi</i>.