LIFE CYCLE
The life cycle of the malarial parasite (Plate 1) may be divided into 3 stages: one is sexual (sporogonic cycle) and takes place inside the mosquito (A) while the other two are asexual and occur inside the human host: the erythrocytic cycle (B) (in red blood cells) and the exoerythrocytic cycle (C) (in liver cells). When an infected mosquito bites a person, it injects at most about a hundred sporozoites (1), which, upon entering the bloodstream, will reach the liver (hepatocytes) within 30-40 minutes. Here they grow and multiply and after a period ranging from 6 to 15 days, according to the species of plasmodium, thousands of merozoites are released (2) into the bloodstream, where they will penetrate and invade red blood cells rapidly (3). In P. vivax and P. ovale, some sporozoites enter into a dormant stage, becoming “liver hypnozoites”; these forms will remain latent for weeks, months or even years until reactivated (“awakened”), at which point they will complete the hepatic cycle, giving rise to the relapses characteristic of these two species. The merozoites that have penetrated red blood cells transform into trophozoites, which, after completing the vegetative growth stage, develop into schizonts (4). Once mature (5), the schizonts cause the infected red blood cell to burst, setting free other merozoites (from 6 to 24 for each erythrocytic schizont) that go on to invade other red blood cells. The destruction of red blood cells also coincides with the release of pyrogenic substances, which trigger the mechanisms producing fever and other clinical symptoms. The morbidity and mortality associated with malaria derive solely from the erythrocytic stages. The asexual erythrocytic cycle lasts 48 or 72 hours, depending on the species of plasmodium, and continues until brought under control either by the body’s immune response or chemotherapy. After invading a red blood cell (6), some merozoites do not develop into trophozoites but rather, for reasons as yet unclear, differentiate into sexual forms, male gametocytes (microgametocytes) and female gametocytes (macrogametocytes) (7). At this point, if the appropriate female anopheline mosquito bites a human for her blood meal, inside the mosquito’s stomach the micro gametocytes will initiate a process of exflagellation (8) resulting in the extrusion of 4 to 8 slender forms (each of which has a nucleus) called microgametes [48]. Simultaneously, each macrogametocyte transforms into a macrogamete; fusion of a microgamete with a macrogamete results in a zygote (sexual or sporogonous cycle). The zygote elongates and is motile, becoming known as an ookinete (9) which penetrates the stomach wall and develops into a spherical oocyst (10). The outer wall of the stomach of infected moquitoes may exhibit few to many oocysts. Within the oocyst repeated division gives rise to the formation of numerous sporozoites, which eventually cause the oocyst to rupture (11). The sporozoites are released into the hemocoel, from where they subsequently migrate into the salivary glands. The time it takes for sporozoites to mature will vary according to temperature and, to a lesser extent, humidity, but will in any case fall within an interval of 7 to 18 days. As noted earlier, the female Anopheles mosquito requires a blood meal to mature her eggs and as she will lay eggs several times in the course of her life, she will need blood meals each time. She will therefore have several occasions for either acquiring or transmitting malaria parasites.