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MALARIA: Obstacles and Opportunities
The link between farming and malaria has a long history. In West Africa some 2,000 years ago, the advent of iron tools and higher-yield crops made the rain forest worth exploiting for agriculture (Livingstone, 1958). The Bantus of the central Benue River valley (what is now Nigeria) were perhaps the first to use iron tools to clear forests for farming. Their “slash-and-burn” and “slash-and-mulch” agricultural techniques may have set the stage for much of the current experience with malaria on the African continent.
Anopheles gambiae mosquitoes account for most of the malaria transmission in Africa south of the Sahara. These insects do not breed in full shade, and it was only after tropical rain forests began to be cleared for crop production that sunlight-exposed pools of standing water, ideal sites for larval development for An. gambiae, were created (Coluzzi et al., 1985). Forest clearing also causes soil erosion, which in river valleys can attract mosquitoes to newly formed swamps (Laderman, 1975).
The formation of small towns facilitates malaria transmission as a consequence of farming, which concentrates populations in relatively confined areas. Towns are often located near water supplies or, through change and disruption of the local environment, promote other potential larval development sites. A burrow pit dug at the edge of a village to collect material for construction often fills with water and serves as an ideal haven for mosquito larvae (Bruce-Chwatt and de Zulueta, 1980).
It is no accident that most malaria occurs in rural agricultural communities. Agricultural settlement often brings changes in water use, alters the concentration of domesticated and wild animals, and causes deforestation, all of which can boost the number of larval development sites and increase human-mosquito contact.
Irrigation practices that use canals or pumps to periodically flood fields for crops such as rice have a great impact on mosquito breeding. One type of livestock may attract a particular species of Anopheles but not another. The form of ownership of agricultural land and the type of labor utilized, such as sharecropping, also can affect the spread of and exposure to malaria parasites.
Finally, the use of certain chemicals to increase crop production may have an indirect impact on the spread of malaria. High-yield varieties of crops require significant quantities of pesticides and fertilizers. Several studies have cited the use of pesticides in agriculture as a principal contributor to mosquito resistance to DDT in Central America, India, and other regions (Chapin and Wasserstrom, 1981, 1983). Other evidence suggests that DDT resistance can also be caused by antimosquito spraying (Curtis, 1981; Chapin and Wasserstrom, 1983).