Unit 8: Water Resources // Section 9: Water-Related Diseases
More than 2 million people die each year from diseases such as cholera, typhoid, and dysentery that are spread by contaminated water or by a lack of water for hygiene. These illnesses have largely been eradicated in developed nations, although outbreaks can still occur. In 1993 an infestation of cryptosporidium, a protozoan that causes gastrointestinal illness, killed 110 people and sickened an estimated 400,000 in Milwaukee, Wisconsin. The city's water treatment system was in compliance with federal and state regulations at the time, but after the outbreak federal regulators increased testing requirements for turbidity (cloudiness) in drinking water, an indicator of possible contamination.
Water-related illnesses fall into four major categories:
- Waterborne diseases, including cholera, typhoid, and dysentery, are caused by drinking water containing infectious viruses or bacteria, which often come from human or animal waste.
- Water-washed diseases, such as skin and eye infections, are caused by lack of clean water for washing.
- Water-based diseases, such as schistosomiasis, are spread by organisms that develop in water and then become human parasites. They are spread by contaminated water and by eating insufficiently cooked fish.
- Water-related insect vectors, such as mosquitoes, breed in or near water and spread diseases, including dengue and malaria. This category is not directly related to water supply or quality.
As noted above, more than 1 billion people worldwide lack safe drinking water, mainly in developing countries. Conventional large-scale engineering projects that pipe water from central distribution systems can provide safe water at a cost of approximately $500 per person. Small-scale approaches, such as drilling wells and chlorination, can reduce this cost to less than $50 (Fig. 16).
Figure 16. Sodium hypochlorite solution for disinfecting water
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Source: © U.S. Department of Health and Human Services, Centers for Disease Control and Prevention, National Center for Infectious Diseases.
Scientists are still learning how many water-related diseases spread and how infectious agents behave. For example, until the 1970s cryptosporidium was not believed to infect humans, although it was recognized as a threat to animals. A 2003 World Health Organization report on water-related infectious diseases warned that "the spectrum of disease is altering and the incidence of many water-related microbial diseases is increasing." Processes such as urbanization and dam construction can spread water-related diseases by creating new environments for infectious agents, and global climate change is expanding the range of mosquitoes and other disease vectors. However, advances in microbiology are enabling researchers to detect pathogens in water more quickly and to identify and characterize new infectious agents (footnote 15).