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a genus of parasitic protozoa that has one identified species, Toxoplasma gondii. Studies of the life cycle and ultrastructure of T. gondii have shown that it belongs to the order Coccidia.
In the epithelium of the small intestines of cats and other Felidae, the parasites of Toxoplasma proceed through a complex life cycle, which is completed with the formation of minute egglike oocysts similar to those of coccidia of the genus Isospora. Each oocyst contains two spores, each of which develops into four single-celled sporozoites. The mature sporulating oocysts may reinfect felines, in which case the oocysts undergo both asexual and sexual developmental cycles. When the sporulating oocysts enter other mammals, such as mice, as well as birds and man, the oocysts undergo only asexual multiple reproduction.
After entering the macrophages and various organs and tissues, predominantly those of the reticuloendothelial, muscle, and nervous systems, the sporozoites develop into highly fecund mononuclear trophozoites (endozoites) that multiply through a type of internal budding known as endodiogeny. The trophozoites are crescent in shape and measure 4–7 micrometers in length. Their ultrastructure is identical to that of such merozoites as coccidia.
In the chronic form of toxoplasmosis, cysts are formed in a number of organs, including the brain. These cysts contain up to several hundred mononuclear organisms, called cystozoites. The endozoites and cystozoites may infest new hosts, primarily by means of the alimentary tract. Toxoplasma is the source of toxoplasmosis, a severe disease of animals and man.
IU. I. POLIANSKII