Ebselen, a seleno-organic compound, has antioxidant, anti-inflammatory, and antiparasitic effects. In this study, the
inhibitory effects of ebselen were evaluated against three Babesia species and Theileria equi in vitro cultures. The
drug showed significant growth inhibition in serum-free GIT medium from an initial parasitemia of 1% for Babesia
bovis and Babesia caballi with IC50 values of 12.73 and 19.55 μM, respectively. Complete inhibition was observed
at 100 and 50 μM of ebselen for the Babesia bovis and Babesia caballi, respectively. Ebselen-treated parasites
were swollen compared with parasites in the control culture. On the other hand, there was no growth inhibitory
effects for any of the used concentrations of ebselen on the parasites in cultures used media supplemented with
either bovine or equine sera. The present study shown that serum factors may inactivate ebselen; therefore, it could
not be used for treatment of Babesia and Theileria parasites.