| Summary: | The current vaccines to control bovine <i>Babesia bigemina</i> (<i>B. bigemina</i>) infection are not fully protective and vaccination failures incur heavy losses to the cattle industry around the world. Using modified micro-aerophilous stationary phase, we developed a culture-derived attenuated live vaccine against <i>B. bigemina</i> and tested a single subcutaneous inoculation of 2 × 10<sup>8</sup> infected erythrocytes in calves. The protection was measured after a lethal intravenous challenge with 5 × 10<sup>8</sup> virulent calf-derived <i>B. bigemina</i>. Our results demonstrated that a single shot of attenuated vaccine was capable of inducing robust humoral and cell-mediated immune responses in calves. We found a significant increase in the IgG antibody titers post-challenge and a strong proliferation of both CD4+ and CD8+ T cells contributing towards the protection. Our vaccine provided complete protection and parasitic clearance, which was followed for more than 100 days post-challenge. This immunity against babesiosis was directly linked to strong humoral responses; however, the parasitic clearance was attributed to significant T cells effector responses in vaccinated calves as compared to the infected control calves. We anticipate that these results will be helpful in the development of more efficient culture-derived vaccines against <i>Babesia</i> infections, thus reducing significant global economic losses to farmers and the cattle industry.
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