PLASMODIUM, a genus of sporozoan protozoa that contains the malarial organisms. The genus belongs to the order Hemosporidia which contains highly specialized parasites adapted for fife in the blood stream. There are included three parasites of human malaria and also a species which produces avian malaria. The young Plasmodium (see Fig. A) which is active and amoeboid occurs in the red blood corpuscles and after a period of growth, dur ing which it attains rapidly the size of the corpuscle, the organism divides into numerous spores (merosoites) which scatter and attack new blood cells. The process of schizogony just described is repeated until the infection has be come general. When a corpuscle breaks up and the merozoites are scattered a central residual mass of protoplasm loaded with black pigment (melanin) is dispersed in the blood plasma.
At times the merozoites in the corpuscles de velop into two new types, the gametes, one of which is finely granular and opaque whereas the other is hyaline. These do not undergo further change until the blood is withdrawn into the stomach of a mosquito, though the same changes follow its experimental removal to a culture tube. In the mosquito's stomach (see Fig. B) the motile microgametes are formed and one such fuses with a non-motile macrogamete after the latter has extruded a portion of its nuclear substance. There is formed by the union a motile ookinet which penetrates the stomach wall of the mosquito (see Fig. C), encysts and undergoes multiple division to form sporoblasts within which arise also by division sporozoites. The latter are very
minute. Set free by rupture of the cystwall they wander to the proboscis and enter the salivary glands (see Fig. E) whence they are injected into a new human host when the mosquito bites. They attack the red cells at once and the life cycle begins anew.
Laveran in 1880 was the first to attach significance to the structures found in red blood cells in cases of malaria though the malarial organism had actually been seen and figured in 1843. Manson set forth distinctly in 1896 the part of the mosquito in transmitting the disease and a year later Ross followed in detail for the first time the fate of the parasites in the body of the mosquito. Malarial parasites have been cultivated in vitro by Bass and Johns.
The three species parasitic in man give rise to different and generally recognized types of malaria. Plasmodium malarice, which extends farthest north and is not found in the tropics, the cause of quartan fever. Plasmodium gives rise to the tertian fever; it is widely distributed and not very dangerous. Plasmodium falciporunt, the smallest of all three, produces the dangerous fevers known as the pernicious, tropical, quotidian or estivo autumnal type; this type is epidemic only in the tropics and subtropics. Other varieties have been described though their independent char acter is not fully recognized.