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MALARIA: Obstacles and Opportunities
Type II farms are also distinguished by their large-scale use of water. Irrigation is common to almost all forms of type II farming systems. Studies of irrigation projects in India have linked increases in mosquito populations to higher subsoil water tables and to the creation of small side channels suitable for larval development when the canal is refilled (Covell, 1946; Hyma and Ramesh, 1980).
Large dams, which often accompany irrigation projects, create additional opportunities for malaria to gain a foothold. Mosquitoes can breed in the reservoir itself, especially if its edges are not kept clean of vegetation. Seepage from the dam and pooling also can occur, providing new larval development sites. Finally, all large dam projects require that considerable numbers of construction workers be brought to the dam site (Covell, 1946; Hyma and Ramesh, 1980). When nonimmune workers are exposed to malaria, or when malaria is introduced by workers from endemic regions, epidemic malaria may result.
Economic development programs that incorporate dam construction, irrigation, or “green revolution” technologies—also typical of type II farming—likely will change malaria epidemiology. Any development project that alters preexisting relationships between humans and their environment should be evaluated within an ecological framework (Hughes and Hunter, 1970). Careful planning before such projects are undertaken can reduce or eliminate the potential for malaria transmission.
Human Population Movements
The importance of human population movements to malaria control cannot be overemphasized. In fact, such shifts may contribute to the spread of chloroquine-resistant P. falciparum. In many African countries, for example, people move from permanent settlements to rural farms during the early months of the wet season, when cultivation, planting, and weeding are carried out (Prothero, 1961). A variation on this pattern is the seasonal movement of pastoralists tending their livestock between mountain and lowland pastures. Religious pilgrimage, need for employment, drought, famine, war, resettlement, and tourism all contribute to the ebb and flow of populations and the spread of malaria.
Population movement increases malaria transmission in four principal ways. First, such movement often introduces nonimmune people into endemic areas or infected people into malaria-free regions. For example, the higher incidence of malaria among Afghan refugees living in Pakistan than among the indigenous population reflects the Afghans' lower level of preexisting immunity (Suleman, 1988).
Second, the living conditions of migrants differ markedly from those of settled populations, resulting in greater exposure to infected mosquitoes.