Diseases – A Tropical Malady

The large reservoirs impounded by dams provide breeding grounds for some water-borne diseases and parasites, especially in tropical climates. Among the most prevalent of these is schistosomiasis, a disease caused by parasitic worms. The parasite is spread by freshwater snails, and has come to be known as the "disease of hydroelectric dams". It infects an estimated 200 million people per year (with 200,000 fatalities), primarily in Asia, Africa, and South America. Through expansion of habitat for the disease vector by large slackwater reservoirs, incidence of this and other diseases is greatly increased. For example, in the Yangtze River Basin, the incidence of schistosomiasis is near 5%, versus less than 1% in less or undeveloped areas.

Dams and major irrigation projects also provide expanded breeding habitat for insects (mosquitos) that serve as vectors of Dengue fever, malaria, and West Nile virus, among others. The World Health Organization (WHO) estimates that malaria cases in villages near the Bargi reservoir in India increased more than twofold following the dam's construction, and up to four-fold in villages closest to the dam itself. Likewise, malaria incidence increased by seven times in proximal Ethiopian villages following construction of small dams on the Tigray River. A similar increased incidence of West Nile virus has been documented as a result of increased mosquito breeding area in many parts of the world, including the Midwestern U.S., California, and Oregon.