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A genus of the protozoan family Plasmodidae (suborder Haemosporina, subclass Coccidia), blood parasites of vertebrates, characterized by separate microgametes and macrogametes, a motile ookinete, sporogony in the invertebrate host, and merogony (schizogony) in the vertebrate host; includes the causal agents of malaria in humans and other animals, with an asexual cycle occurring in liver and red blood cells of vertebrates and a sexual cycle in mosquitoes, the latter cycle resulting in the production of large numbers of infective sporozoites in the salivary glands of the vector, which are transmitted when the mosquito bites and draws blood. Primate malaria is transmitted by various species of Anopheles mosquitoes, bird malaria by species of Aedes, Culex, Anopheles, and Culiseta. [Mod. L. from G. plasma, something formed, + eidos, appearance]

- P. aethiopicum SYN: P. falciparum.

- P. berghei a species of protozoan that is the etiologic agent of rodent malaria from central Africa; an important source of experimental nonprimate mammal malaria.

- P. brazilianum a protozoan species found in New World monkeys of the family Cebidae in northern South America and Panama, which can cause mild malaria in humans.

- P. cynomolgi a protozoan species similar to P. vivax occurring naturally in the macaque, but infecting humans both accidentally and experimentally; it produces a P.-vivax type of malaria.

- P. falciparum Laverania falciparum, a species that is the causal agent of falciparum (malignant tertian) malaria; a young trophozoite is about one-fifth the size of an erythrocyte, but developing erythrocytic stages are rarely seen in circulating blood, as they render infected cells sticky and cause them to concentrate in capillaries in the vital organs, particularly the brain and the heart; a schizont occupies about one-half to two-thirds of the red blood cell and has fine, sparse granules (observed in peripheral blood only from moribund patients); infected erythrocytes are normal or contracted in size and are likely to contain basophilic granules and red dots (Maurer clefts or dots); multiple infection is extremely frequent and causes bouts of fever somewhat irregularly, since the parasite's cycles of multiplication are usually asynchronous. SYN: malignant tertian malarial parasite, P. aethiopicum.

- P. knowlesi a species of protozoan from Southeast Asia that causes monkey malaria with a quotidian fever cycle; highly fatal in rhesus monkeys; naturally acquired by a human in Malaysia, and also transmitted to humans experimentally.

- P. kochi a P. species now recognized as Hepatocystis kochi.

- P. malariae a protozoan species that is the causal agent of quartan malaria; a ring-stage trophozoite is triangular, ovoid, or slightly bean shaped, with fine or coarse black granules, approximately one-third the size of an eythrocyte; the schizont is oval or rounded and nearly fills the red blood cell; infected erythrocytes are normal or slightly contracted in size, usually with no stippling (the two most important characteristics that distinguish it from P. vivax), although extremely fine Ziemann dots may be observed; multiple infection is extremely rare; thus, bouts of fever occur fairly regularly at 72-hour intervals; prolonged asymptomatic parasitemia is characteristic of the species, and recrudescence of fever may occur 10 years or more after the initial episode. SYN: quartan parasite.

- P. ovale a protozoan species that is the agent of the least common form of human malaria; resembles P. vivax in its earlier stages but often modifies the cell membrane, causing it to form a fimbriated outline and, often assume an oval shape; Schüffner dots are abundant and appear early, host cells are normal or only slightly enlarged, and only about 8–10 grapelike merozoites are produced; fever is tertian (every 48 hours), and relapses are infrequent.

- P. vivax a protozoan species that is the most common malarial parasite of humans (except in West Africa, where the form of the Duffy antigen (FyFy) protects most of the resident populations, which has permitted P. ovale to replace P. vivax); the early trophozoite is irregular and ameboid in shape, one-fourth to one-third the size of a red blood cell, and contains several fine granules; the schizont is irregular in shape, fills the enlarged erythrocyte, and contains numerous yellow-brown pigment granules; affected red blood cells are pale, enlarged, and contain Schüffner dots in the later stages of growth; characteristically causes bouts of fever fairly regularly at 48-hour intervals, but multiple infection, causing irregular fever patterns, is common. SYN: tertian parasite.

plas·mo·di·um plaz-'mōd-ē-əm n

1) pl -dia -dē-ə

a) a motile multinucleate mass of protoplasm resulting from fusion of uninucleate ameboid cells also an organism (as a stage of a slime mold) that consists of such a structure

b) SYNCYTIUM (1)

2 a) cap the type genus of the family Plasmodiidae of sporozoans that includes all the malaria parasites affecting humans

b) pl -dia any individual malaria parasite

a genus of protozoans (see Sporozoa) that live as parasites within human red blood cells and liver cells. The parasite undergoes its asexual development (see schizogony) in humans and completes the sexual phase of its development (see sporogony) in the stomach and digestive glands of a bloodsucking Anopheles mosquito. Four species cause malaria: P. vivax, P. ovale, P. falciparum, and P. malariae.