Cell culture-derived Babesia bovis vaccine: sequential challenge exposure of protective immunity during a 6-month postvaccination period.

Cell culture-derived soluble Babesia bovis vaccine admixed with a saponin adjuvant was administered in 2 doses, 3 weeks between doses, to 18 yearling heifers. An additional 9 heifers served as nonvaccinated controls. Comparable groups of these animals were challenge exposed at 131 and 178 days after vaccination, using 1 X 10(8) B bovis organisms contained in freshly collected blood from a splenectomized calf with ascending parasitemia. Challenge exposure led to no deaths of vaccinated animals, whereas 4 of the control (nonvaccinated) animals died of disease. All clinical and hematologic variables examined indicated that the vaccinated animals had gained immunologic protection in comparison with the control group. An immune recognition as manifested by an anamnestic humoral response to challenge exposure was in evidence in the vaccinated animals.