Enhanced parasitization of platelets by Plasmodium berghei yoelii

In previous studies plasmodia have been found by electron microscopy within human platelets naturally infected with Plasmodium vivax and within platelets of mice infected intraperitoneally with P. berghei. In both situations the number of parasitized platelets was low. An enhancement of platelet parasitization was attempted in order to study in greater detail the mechanisms and implications of such a phenomenon. Various in vitro incubation mixtures of normal mouse platelets and free merozoites of the 17X strain of P. berghei yoelii failed to produce any recognizable parasitization of platelets. In vivo, however, large numbers of invaded platelets were obtained by the use of massive intraperitoneal inocula of plasmodia (5 X 10(8) infected erythrocytes). By the 5th day of infection the proportion of parasitized platelets was 13.2 times higher in the animals receiving the large dose than in those receiving the regular passage inoculum (6 X 10(6) infected erythrocytes). Ultrastructural study of 266 intrathrombocytic parasites over eight days of infection failed to show schizogonic maturation beyond the trophozoite state.