Ultrastructural characteristics of Babesia caballi in equine erythrocytes in vitro.

Babesia caballi cultured continuously in equine erythrocytes was examined by transmission electron microscopy. The use of cultured B. caballi permitted examination of a large number of parasitized cells with various stages of intra erythrocytic development. The piriform merozoites of B. caballi were composed of an outer membrane and an inner double-membrane complex. Numerous micronemes and three rhoptries were found in the pellicle of the merozoite, and a spherical body was seen in the anterior part of the merozoite which usually lay adjacent to the nucleus and the pellicle. These findings were similar to those for merozoites of bovine Babesia parasites such as B. bigemina. The trophozoites were surrounded by a single membrane, were continuously changing their body shape with extension and retraction of the pseudopod. A long pseudopod extended far into the host cell cytoplasm, and was finally completely enclosed in a cell, but did not have hemozoin pigment, the breakdown product of hemoglobin digestion.