Arthropod-borne apicomplexan pathogens that cause asymptomatic persistent infections present a significant challenge
due to their life-long transmission potential. Although anti-microbials have been used to ameliorate acute disease in
animals and humans, chemotherapeutic efficacy for apicomplexan pathogen elimination from a persistently infected host
and removal of transmission risk is largely unconfirmed. The recent re-emergence of the apicomplexan Theileria equi in U.S.
horses prompted testing whether imidocarb dipropionate was able to eliminate T. equi from naturally infected horses and
remove transmission risk. Following imidocarb treatment, levels of T. equi declined from a mean of 104.9 organisms/ml of
blood to undetectable by nested PCR in 24 of 25 naturally infected horses. Further, blood transfer from treated horses that
became nested PCR negative failed to transmit to naı¨ve splenectomized horses. Although these results were consistent with
elimination of infection in 24 of 25 horses, T. equi-specific antibodies persisted in the majority of imidocarb treated horses.
Imidocarb treatment was unsuccessful in one horse which remained infected as measured by nested PCR and retained the
ability to infect a naı¨ve recipient via intravenous blood transfer. However, a second round of treatment eliminated T. equi
infection. These results support the utility of imidocarb chemotherapy for assistance in the control and eradication of this
tick-borne pathogen. Successful imidocarb dipropionate treatment of persistently infected horses provides a tool to aid the
global equine industry by removing transmission risk associated with infection and facilitating international movement of
equids between endemic and non-endemic regions.