THE ENTRY OF THEILERIA-PARVA MEROZOITES INTO BOVINE ERYTHROCYTES OCCURS BY A PROCESS SIMILAR TO SPOROZOITE INVASION OF LYMPHOCYTES

The entry of Theileria parva merozoites into bovine erythrocytes In vi vo is described and compared to sporozoite invasion of lymphocytes. Me rozoites make initial contact with erythrocytes with any part of their surface and invasion of the host cell does not require the re-orienta tion of the apical end of the merozoite towards the surface of the ery throcyte. After the initial attachment the merozoite and host cell mem branes form a continual close junction with the two apposed membranes separated by a 6-8 nm gap containing moderately dense material. The pr ogressive circumferential 'zippering' of these closely apposed membran es leads to the movement of the parasite into the erythrocyte. The new ly internalized merozoite which is completely surrounded by the erythr ocyte plasma membrane escapes from this enclosing membrane by a proces s involving the discharge of at least the rhoptries; whether the meroz oite also contain other types of secretory organelles (e.g. micronemes , microspheres or dense bodies) remains to be determined. Morphologica lly, the events involved in merozoite invasion of erythrocytes are alm ost identical to the process of sporozoite invasion of lymphocytes but differ significantly from the entry process of the invasive stages of other Apicomplexan parasites.