Plasmodium, Haemoproteus and Leucocytozoon

These genera belong to the suborder Haemospororina, which is differentiated from the Eimeriorina and Adeleorina by the facts that the microgametocyte produces a moderate number of microgametes and the sporozoites are naked. The gamonts are similar and develop independently. The zygote is motile (i.e., it is an ookinete). All species are heteroxenous; schizogony takes place in a vertebrate host, and sporogony in an invertebrate. If the erythrocytes are invaded, pigment (hemozoin) is formed from the host cell hemoglobin.

This suborder was customarily divided into 2 families, the Plasmodiidae containing the genus Plasmodium, and the Haemoproteidae containing the genera Haemoproteus and Leucocytozoon. The principal difference was that in the Plasmodiidae schizogony was thought to take place only in the erythrocytes, while in the Haemoproteidae it takes place in the lungs, liver, spleen, kidneys and other internal organs. However, when the complete life cycles of several species of avian and human Plasmodium were worked out (Huff and Coulston, 1944, 1946; Shortt and Garnham, 1948; Short et al., 1951; Garnham, 1954; Bray, 1957), it was realized that schizogony may occur both within the erythrocytes and exoerythrocytically. The distinction between the two families is thus an artificial one, and there is no point in retaining more than a single family in the suborder.

It is likely, as Manwell (1955) has suggested, that the Haemospororina may well have arisen from the coccidia of vertebrates rather than from those of insects, as had been more commonly supposed. Genera like Lankesterella and Schellackia, in which schizogony, gametogony and sporogony all take place in the vertebrate host and in which the sporozoites invade the blood cells and are transmitted by mites or other blood-suckers, could well be the starting-point for the transition from the Eimeriorina to the Haemospororina.