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MALARIA: Obstacles and Opportunities
into a schizont containing 8 to 32 new merozoites. The red blood cell eventually ruptures and releases the merozoites, which are then free to invade additional red blood cells. The rupturing of red blood cells is associated with fever and signals the clinical onset of malaria.
Some merozoites differentiate into sexual forms, gametocytes, which are ingested by a mosquito during its next blood meal. Once in the mosquito, the sexual forms leave the blood cells, and male and female gametes fuse to form a zygote. Over the next 12 to 48 hours, the zygote elongates to form an ookinete. The ookinete penetrates the wall of the insect's stomach and becomes an oocyst. Over the next week or more, depending on the parasite species and ambient temperature, the oocyst enlarges, forming more than 10,000 sporozoites. When the oocyst ruptures, the sporozoites migrate to the mosquito salivary glands, from where they may be injected back into the human host, thus completing the cycle.
Relapse and Recrudescence
Malarial illness may recur months to years after apparently successful treatment. In patients infected with P. vivax and P. ovale, this phenomenon is known as relapse. Relapse is caused by dormant liver-stage forms of the parasite that resume their developmental cycle and release merozoites into the bloodstream. The recurrence of malaria caused by P. falciparum and P. malariae is due to recrudescence, which is caused by surviving blood-stage parasites from an earlier infection.
Of the more than 2,500 known species of mosquitoes worldwide, only a sub-group of 50 to 60 species belonging to the genus Anopheles are capable of transmitting malaria. Female anophelines require blood meals to reproduce (Figure 2-4). Some anopheline species are indiscriminant feeders; others prefer to feed on animals (zoophilic) or humans (anthropophilic).
Anopheline mosquitoes breed in relatively clean water, with certain species having very specific preferences as to their aquatic environment (Figure 2-5). Understanding these preferences is crucial for targeting effective malaria control interventions. For example, An. stephensi can breed in tin cans and water cisterns, while Anopheles gambiae, the most important malaria vector in Africa south of the Sahara, prefers small, sunlit pools.
The mosquito undergoes four stages of growth: egg, larva, pupa, and adult (imago). Adult females mate once and store the sperm. The female