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MALARIA: Obstacles and Opportunities
sonal and correlates with relatively predictable patterns of rainfall, although transmission may continue at lower levels during the dry season. Because of the extremely high inoculation rates, virtually all of those living in these areas become infected early in life. For children, treatment (until recently, chloroquine was very effective) may prevent death long enough for acquired immunity to establish itself, which can provide protection from malaria-related death or illness later in life. Young children who do not acquire this protective immunity, and whose infections are not treated adequately or promptly, are at particular risk of dying from the disease. These areas, which can be further subdivided into wet and dry savannah, are characterized by high-intensity transmission by efficient vectors in abundance, resulting in immunity in those who survive initial infections and an eventual ability to tolerate parasitemia without symptomatic or serious illness.
Forest Malaria Forest malaria, the result of human incursions into forests or jungles, occurs in several regions of the world, including the Amazon basin of South America, Southeast Asia (primarily Bangladesh, Myanmar, Thailand, and the Indochinese peninsula), Borneo, equatorial Africa, and certain of the Pacific Islands. The indigenous inhabitants of these forest and jungle areas, including Indians of the Amazon, the pygmies of central Africa, and the aborigines of the Malaysian jungle, traditionally have been relatively unaffected by malaria. Recently, however, economic pressures have brought large numbers of poor nonindigenous laborers into these regions for agriculture, timbering, gem and gold mining, and road construction. The results have been serious malaria-related morbidity and considerable mortality among the newcomers and the reintroduction or increase of malaria in indigenous populations.
Typically, the mosquitoes responsible for transmitting malaria parasites in forest areas are difficult or impossible to control by traditional means. For example, An. dirus, the primary malaria vector in Southeast Asian jungles, can develop in almost any shaded collection of water (even in animal hoofprints and the bracts of commensal plants); this mosquito feeds in the early evening, and rests outdoors after taking a blood meal. Such characteristics make the application of traditional larval control measures and other tactics aimed at reducing the adult mosquito population, such as use of residual insecticides, relatively fruitless.
Malaria Associated with Irrigated Agriculture Irrigation for agricultural purposes may both provide breeding sites for vector mosquitoes and allow concentrated parasite reservoirs in the form of large labor forces to become established, helping maintain a high level of malaria transmission. Examples of places where this type of malaria occurs include the cotton plantations of the Gezira in the Sudan and irrigated rice cultivation areas in Sri Lanka.