The global importance of the hemoparasite to equine health was recently shown by its resistance to imidocarb dipropionate (ID) and its interference with clearance by ID in some co-infected horses. Genetic characterization of revealed marked genomic reduction compared to , and initial experiments demonstrated reduced clinical severity in spleen-intact horses. Furthermore, in early experiments, splenectomized horses survived infection and progressed to an asymptomatic carrier state, in stark contrast to the high fatality rate of in splenectomized horses. Thus, we hypothesized that is less virulent than . To objectively assess virulence, clinical data from nine splenectomized, -infected horses were evaluated and compared to published data on -infected, splenectomized horses. Seven of eight splenectomized, -infected horses survived. Further, in six horses co-infected with and , only horses cleared of by ID survived splenectomy and became asymptomatic carriers. The reduced virulence of in splenectomized horses instructs why was, until recently, undetected. This naturally occurring comparative reduction in virulence in a natural host provides a foundation for defining virulence mechanisms of theileriosis and Apicomplexa in general.