About UNEP UNEP Offices News Centre Publications Events Awards Milestones UNEP Store
GEO Year Book 2004/5  
UNEP Website GEO Home Page
Water

Traditionally, concern about water and human health has focused on the diseases that result from inadequate or unsafe water supplies or sanitation. For example, the presence of human and animal wastes in surface waters has resulted in devastating outbreaks of cryptosporidiosis in North America and in cholera in many parts of the world (Colwell 1996, Rodo and others 2002).

However, there are many other ways in which environment-related changes in human use and management of, and contacts with, water can affect disease incidence and transmission, at every scale from the puddle in the yard to a major irrigation system. Dam construction is a driving force in infectious disease because it alters the nature of aquatic habitats and affects species survival (Patz and others 2004). The construction of large dams has caused an increased incidence of schistosomiasis (Box 5). By providing habitats for infectious disease vectors, irrigation has resulted in dramatic increases in morbidity and mortality due to malaria in Africa and to Japanese encephalitis in Asia.

Box 5: Irrigation, schistosomiasis and West Nile Virus

Schistosome snails, Biomphalaria glabrata, shedding schistosome larvae which burrow into people and cause schistosomiasis.
Source: Darlyne A. Murawski/Still Pictures

Snails serve as an intermediate reservoir host for schistosomiasis, and irrigation canals can provide an ideal habitat. Increasing fecundity and growth of freshwater snails are related to decreased water salinity and increased alkalinity following irrigation development along the Senegal River, and to water flow changes associated with the Aswan Dam in Egypt (Abdel-Wahab and others 1979).

Irrigation ponds, canals, and ditches can also provide larval habitat for vector mosquito species such as Culex tarsalis. As it bites both animals and humans, Culex tarsalis is a major bridge vector for enzootic diseases (diseases constantly present in animal populations) such as St. Louis encephalitis in the western United States (Mahmood and others 2004). As West Nile virus has moved into the region in the past three years, this species has emerged as the principal mosquito vector, resulting in a major epidemic in humans, and in birds and horses (Reisen and others 2004).

Schistosome snails, Biomphalaria glabrata, shedding schistosome larvae which burrow into people and cause schistosomiasis.

Source: Darlyne A. Murawski/Still Pictures


Earthprint.com Order the Book