Sugarcane cultivation in dry areas that require irrigation has also resulted in an increase in malaria transmission where it had not been a serious problem in the past.

Highland Fringe Malaria Although malaria has been transmitted at altitudes of up to 2,800 meters, the disease does not generally occur above about 1,500 meters (Bruce-Chwatt, 1985). Because of fluctuations in climate, and perhaps even global warming, vector anophelines may begin to flourish at higher and higher altitudes. The altitudes subject to this type of malaria problem may vary greatly according to the local geography. Populations, in particular settled populations, living at higher, normally malaria-free altitudes may have little acquired immunity to malaria and so may suffer devastating epidemics. Such a situation recently occurred in the highland plateaus of Madagascar, where malaria had previously been eradicated or nearly so. Because of unusual climatic conditions, or perhaps as a result of a documented gradual increase in the mean temperature, the malaria vector An. funestus recolonized the high plateau, causing many hundreds of thousands of cases of the disease and significant mortality in the nonimmune population. Similar, less devastating epidemics have been seen in Papua New Guinea, in Ethiopia, and on the mountain slopes of Kenya. Even without an expansion in the range of vector anophelines, however, economic conditions may force nonimmune highland populations to search for work in lower highland fringe areas, where they may be exposed to intense malaria transmission.

Desert Fringe and Oasis Malaria This paradigm shares many characteristics with highland fringe malaria, including the occurrence of periodic and serious epidemic outbreaks of disease in nonimmune populations. Since there is normally no transmission in these arid grassland areas, the populations living in or at the edge of deserts generally do not have protective immunity against malaria. When climatic conditions change—when, for example, unusually heavy rains occur that allow vector mosquitoes to develop—nonimmune populations may be at risk of infection and of severe and often fatal malaria. Such conditions have prevailed in recent years in areas of the Sahel, along the southern edge of the Sahara, and on the fringes of the Kalahari desert, where increasingly serious epidemics have occurred in northern Namibia over the past four years. Ethiopia regularly experiences such epidemics. Both settled populations and nomadic groups may be at risk near oases that support malaria transmission, such as in the western desert of Egypt.

Urban Malaria Urban malaria can be of two basic types. Malaria can be transmitted by vectors well adapted to city conditions (e.g., An. stephensi,



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