Also found in: Medical.
A relatively small and generally rather compact group of protozoans in the subphylum Sporozoa. Authorities differ as to the group's taxonomic status; that assigned it by the Committee on Taxonomy and Taxonomic Problems of the Society of Protozoologists is followed here: a suborder of the order Eucoccida, subclass Coccidia, class Telosporea, subphylum Sporozoa. The Haemosporina are common protozoan parasites of vertebrates, and some of them are important as causes of illness and death. The best known of the group are the four species (genus Plasmodium) of malarial parasites of humans.
Transmission of these parasites is probably always effected in nature by the bite of some bloodsucking invertebrate. In the vertebrate host they reproduce asexually; sometimes this occurs in the tissues of certain internal organs, such as the lungs, liver, spleen, and brain; sometimes in the red blood cells (erythrocytes), or even in other types of blood and blood-forming cells; and often in both tissues and blood cells. The immature sex cells, gametocytes, always occur in erythrocytes or leukocytes (white blood cells). Gametocytes mature into gametes after ingestion by an intermediate host (arthropod). Fertilization ensues, with a subsequent period of development culminating in the production of numerous sporozoites. These tiny filamentous forms are infective for the vertebrate host. Since they can develop no further in the arthropod, infection in this host is self-limited, in the sense that no further buildup is possible; the insect is seldom harmed by the parasite. However, sporozoites may remain infective for a long time in the invertebrate host, perhaps as long as the insect lives. See Sporozoa