to cause waterborne disease. Reclaimed water used for drinking is more of a concern because of residual contamination.

Not unexpectedly, the higher the quality of source water entering water treatment facilities, the easier it is to produce safe water for drinking. Unfortunately, high-quality sources of potable water are increasingly difficult to locate and maintain. This is especially true for areas that depend on large watersheds, such as the Mississippi and Ohio rivers, which are often polluted with chemicals and biological wastes. To treat such polluted water adequately puts a heavy burden on water treatment plants. In many urban areas, the designed capacities of municipal water treatment facilities are being challenged by the growth in population; in addition, equipment or procedural breakdowns may allow inadequately treated water to reach the consumer.

Outbreaks of waterborne disease in the United States are uncommon because of the nation's extensive public health infrastructure. Public health authorities, however, are especially mindful of potential outbreaks following natural disasters, such as earthquakes and hurricanes, that can lead to contamination of municipal water supplies.

ECONOMIC DEVELOPMENT AND LAND USE

Dam Building and Rift Valley Fever

Until relatively recently, Rift Valley fever, which is caused by a mosquito-borne virus, occurred only in Africa south of the Sahara and was primarily a disease of sheep and cattle. Periodic outbreaks were prevalent in South Africa, Tanzania, Kenya, and, during the mid-1970s, in the Sudan.

The first major outbreak of human disease occurred in Egypt in 1977. An estimated 200,000 people became sick, and 598 died (Meegan and Shope, 1981). Death was usually associated with acute hemorrhagic fever and hepatitis. The outbreak also caused abortions in sheep and cattle, which resulted in a drastic shortage of red meat in the Cairo marketplace.

The Egyptian epidemic has been linked by some to the construction of the Aswan Dam. Completion of the dam, in 1970, required that 800,000 hectares of reclaimed land be flooded. The dam stabilized the water table in the Nile Valley, which caused water to puddle and to serve as breeding sites for mosquitoes. The mosquitoes, in turn, may have offered a conduit for the virus to enter Egypt from Sub-Saharan Africa, although this has never been proven.

Awareness of the possible association of Rift Valley fever with the ecologic change following completion of the Aswan Dam led scientists to examine the potential for outbreaks of the disease in other areas of dam construction. One such effort, started in 1977 by the U.S. Agency for International Development (USAID), focused on the Diama Dam, then under



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