Ultrastructure of the intra-erythrocytic stage of Theileria species from cattle and waterbuck.

A transmission electron microscopic study of the intra-erythrocytic stages of a pathogenic Theileria parva from cattle and a previously uncharacterized Theileria sp. from waterbuck (Kobus defassa) in Kenya revealed several novel ultrastructural features, associated with feeding and multiplication, in these parasites. In trophozoites a connecting channel was observed between the parasite's cytostome and its intracytoplasmic food vacuole. In some cases the limiting membrane of the food vacuole was seen to be continuous with a close-meshed network of membrane-bounded, anastomosing tubules. This labyrinthine structure, which has not been described previously, may function as a digestive organelle in theilerial trophozoites. Electron micrographs also revealed the mode of intra-erythrocytic multiplication of these parasites in vivo. Prior to division, electron-dense cisternae and rhoptries appeared beneath the parasite's plasmalemmal membrane, marking the sites of merozoite formation. From a single parasite, a maximum of four merozoites were formed by schizogonous division and subsequently separated from a residual body by constriction at the base of each merozoite. In addition, observations on two double-membraned organelles seen in trophozoites and the intra-erythrocytic crystalline structures associated with Theileria sp. in waterbuck are reported.