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eFigure 35–15. Life cycle of Plasmodium vivax, the agent of vivax (tertian) malaria. The female Anopheles mosquito, shown in the typical biting angle, injects sporozoites into the human bloodstream. 1–3: Exoerythrocytic phase of asexual division (schizogony). Within an hour, the sporozoites enter liver cells to initiate the exoerythrocytic phase. Penetration of a hepatocyte is followed by growth and multiplication within a few days (2a), or the parasite may become a hypnozoite, with development delayed for as long as 1–2 years. Asexual multiplication fills the enlarging infected liver cell with merozoites (2a). The cell then ruptures, releasing thousands of parasites (3) to initiate either a second hepatic generation or the erythrocytic phase. 4–9: Erythrocytic phase of schizogony. Invasion of the bloodstream and penetration of red blood cells initiate the blood phase of schizogony. Penetration of the red blood cell may begin as a surface or appliqué (accolé) form (4), after which it enters the cell, rounds up, and ingests hemoglobin to form the ring stage (5), which enlarges and becomes an ameboid trophozoite, with numerous cytoplasmic granules (Schüffner dots) (6) produced as a by-product of hemoglobin breakdown. Nuclear division follows (7), eventually filling the infected cell as the parasites' cytoplasm surrounds each nucleus (8), rupturing it to release more merozoites (9) about 48 hours after onset of the red cell infection (tertian cycle). This asexual erythrocytic schizogony repeats itself with a 48-hour synchronous periodicity. 10a, 10b: Gametogony. Some sporozoites give rise to sexual parasites, gametocyte-producing merozoites that form male (microgametocyte) (10a) or female (macrogametocyte) stages (10b). When taken up in another Anopheles mosquito blood meal, the asexual forms are killed but the sexual stages quickly complete gamete formation. 11–13: Fertilization. "Exflagellation" is the release of male microgametes, which scatter, find a female macrogamete, and fuse. The zygote elongates, becomes a motile ookinete (13), and travels to the mosquito's stomach wall. 14–16: Sporogony. In the stomach wall of the mosquito, the ookinete becomes an oocyst (14). Masses of these oocysts may cover the mosquito's stomach. The oocyst nucleus divides innumerable times (15), and the progeny develop into elongated sporozoites, a process termed sporogony. The oocysts rupture (16), passing sporozoites throughout the female body. Most eventually concentrate in the salivary glands, after which the mosquito becomes infective and can initiate a new malaria cycle. (Reproduced, with permission, from Goldsmith R, Heyneman D [editors]. Tropical Medicine and Parasitology. Originally published by Appleton & Lange. Copyright © 1989 by The McGraw-Hill Companies, Inc.)

Current Medical Diagnosis & Treatment 2018 > Protozoal & Helminthic Infections

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