Table 6-1 lists 12 epidemiological zones of malaria, and their principal and secondary vectors (Bruce Chwatt, 1985).

Anopheline Larval Habitats

The site chosen by mosquitoes for egg-laying and development of larvae is known as the larval habitat. Anophelines prefer relatively clean water as their larval habitat, although species vary in the amount of sun exposure, temperature, salinity, and organic content they prefer in their breeding sites.

Developmental Stages, Mating, Egg-Laying, and Adult Lifespan

The four developmental stages of an anopheline mosquito are egg, larva, pupa, and adult (or imago). Adult males copulate in flight, providing females with sufficient sperm for all subsequent egg-laying. Females need at least two blood meals for their first batch of eggs to develop, and one blood meal for each successive batch. Since egg development takes about 48 hours, blood-seeking is repeated every 2-3 nights.

Under optimal conditions, the average lifespan of an adult female anopheline is 3 weeks or longer (adult males, in contrast, usually live no more than a few days) unless external factors such as temperature, humidity, and natural enemies decrease longevity. When the mean ambient temperature exceeds 35oC, or humidity falls below 50 percent, longevity is significantly reduced, directly influencing the transmission of malaria.

Feeding and Resting Behavior

The male adult anopheline feeds on nectar, while the female adult feeds primarily upon blood. Females of most Anopheles species prefer warm-blooded animals, predominantly mammals. Some species prefer humans, and are termed anthropophagic or anthropophilic. Others prefer animals such as cattle, and are termed zoophagic or zoophilic. The distance over which a mosquito is attracted to its preferred blood source usually ranges between 7 and 20 meters.

The time of anopheline feeding is, almost without exception, between dusk and dawn. Some species have early peaks of biting (for instance, the Central American vector A. albimanus, which feeds between 1900 to 2100 hours). Anopheles gambiae, the leading malaria vector in Africa, can feed over a much wider period (2200 to 0600 hours). If biting precedes the local bedtime, insecticide-treated (bed) nets (ITNs) offer little protection against malaria infection.



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