Protective effects of an experimental sporozoite-based immunization strategy against Theileria equi
Ochoa, Jennine Nicole
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Theileria equi is a tick-transmitted apicomplexan hemoparasite parasite of equids with a worldwide distribution and a cause of equine piroplasmosis, a World Organization of Animal Health (OIE) reportable disease. Although a protective vaccine would improve current T. equi control strategies, no vaccine is available. In natural transmission the infective life stage is the sporozoite, which is inoculated by ticks; therefore specific immune responses directed against sporozoites could prevent the establishment of infection. The purpose of the present study was to evaluate the protective effects of an experimental radiation-attenuated whole sporozoite immunization strategy against T. equi sporozoite challenge. Three horses were immunized with irradiated sporozoites three times at six week intervals, while three additional horses were inoculated with non-irradiated infectious sporozoites at the same interval, and cleared of infection three weeks after each inoculation by treatment with imidocarb dipropionate. All horses were subsequently challenged with non-irradiated infectious T. equi sporozoites. Repeated immunization with irradiated sporozoites prevented infection in one horse, while partial protection (mitigation of clinical signs) was observed in the two other horses. Prior to challenge, antibodies against equi merozoite antigen (EMA)-1 and 2 were identified in both groups but did not correlate with complete protection. Our results suggest that a protective sporozoite-based vaccine is feasible against T. equi, but correlates of protection are yet undefined. In addition, this study describes a method for efficiently generating a T. equi sporozoite preparation with enhanced purity.