Fine structure of Babesia equi Laveran, 1901 within lymphocytes and erythrocytes of horses: an in vivo and in vitro study.

The development of Babesia equi (Piroplasmia) in the vertebrate host was studied by electron microscopy. The tick-borne sporozoites initiated an exoerythrocytic schizogony in lymphocytes in vivo and in vitro. In lymphocyte cultures the schizonts of B. equi lay as fissured bodies directly within the host cell forming numerous cytomeres. These cytomeres were bordered by a single membrane and contained polymorphous nuclei. Merozoite differentiation was initiated at several places by protrusions appearing at the periphery of the cytomeres. The cytoplasm of the schizont merged progressively into the young merozoites. The mature merozoites were pear-shaped (2.0 X 1.5 microns), bounded by a pellicle and had an apical complex without conoid. Numerous merozoites were observed entering erythrocytes in vitro. In vivo, lymphocytes containing B. equi schizonts were seen in capillaries within lymph nodes from the 12th day onwards after attachment of infected ticks. At the same time some erythrocytes were found that contained B. equi merozoites. Development of B. equi in the vertebrate host shows many similarities to the corresponding development of Theileria parasites. This makes a new discussion of the taxonomic position of B. equi necessary.