Housing, if available, is crowded, of poor quality, and often located adjacent to Anopheles breeding sites.

Third, the type of work performed by migrants and the conditions under which they work often result in greater exposure to malaria-infected mosquitoes. Much of the work, especially if it is illegal (for example, gem mining, gold prospecting, and drug smuggling), is performed at night, when Anopheles mosquitoes are most likely to be biting. In addition, since employment for migrants is usually seasonal or temporary, there are periods when no funds are available for malaria treatment.

Finally, migrant populations rarely are served by government malaria control programs. Even if they are served, most control methods, such as residual application of insecticides to walls of houses, chloroquine prophylaxis for pregnant women, and screening of houses, may be difficult or impossible to implement. For example, gold miners who work along the Madeira River in the Brazilian Amazon live in temporary shelters without walls. The absence of walls makes insecticide spraying impractical and allows mosquitoes to enter freely at night (Coimbra, 1988).

The illegal status of some migrants further complicates efforts at malaria control. During the 1960s, for example, Mozambican laborers were hired to work on sugar estates in Swaziland. When the use of Mozambican laborers instead of indigenous Swazi workers became a political issue, the government of Swaziland imposed restrictions on the use of foreign labor, but Mozambicans continued to migrate and work illegally in the country. Swaziland health authorities had difficulty detecting cases of malaria among these migrants because of their illegal status and the sugar industry's lack of cooperation (Packard, 1986).

Most studies of the dynamics of malaria transmission treat human populations as though they are static. Even research that examines the effects of population movement, such as the Garki project (Molineaux and Gramiccia, 1980), fails to assess such shifts or to adequately distinguish between them.

Urbanization

Diseases transmitted by insect vectors continue to be thought of primarily as illnesses of rural areas. While the insect vectors for some diseases, such as onchocerciasis (river blindness) and African trypanosomiasis (sleeping sickness), are exclusively rural, some of the mosquito vectors that carry malaria, Bancroftian filariasis (elephantiasis), Japanese encephalitis, and dengue hemorrhagic fever have become well adapted to city life (Bang and Shah, 1988; Dunn, 1988). In general, urban areas have higher population densities, which allow for increased rates of disease transmission, large numbers of larval development sites due to water storage practices, and limited methods for disposal of wastewater and refuse.



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