Development of tick gut forms of Babesia bigemina in vitro

Development of a laboratory cultured tick-transmissible strain of Babesia bigemina was followed in vitro after addition of gut material from engorged female Boophilus microplus ticks and incubation at 37 degrees C. Sequential development of stages, from intraerythrocytic strahlenkörper through multiplication to the fusion of what is assumed to be two gametes, is described. A change in physical environment (temperature, gas composition) experienced during passage of Babesia stages into the in vitro culture tubes possibly mimics the changes experienced in passage from host blood to the midgut of the tick vector. The effect in vitro was to induce the erythrocytic parasites to remain inactive at a trophozoite-like stage. Addition of factor(s) within midgut initiated further development of strahlenkörper. Two populations of strahlenkörper were recognized; an elongated form which did not appear to develop further, and a polymorphic population which underwent further multiplication initiated while the parasites were still within the erythrocyte, and continuing after they had emerged. These strahlenkörper increased in size as multiple division of nuclei occurred. with cell division being completed more slowly. Large aggregations of multinucleated strahlenkorper formed, but once division was complete, single-nucleated strahlenkörper emerged from the aggregates. Two individuals of post-aggregation strahlenkörper, assumed to be gametes, fused together. The morphology and ultrastructure of all stages of development are described and compared with forms already described from the tick midgut.