WATERBORNE DISEASES

An adult human needs to drink at least 1.5 liters of water a day to replace fluid lost in urine, sweat, and respired air and to perform essential biochemical functions. Moreover, almost 90 percent of body mass is water. Water, however, can also carry dangerous pathogens and toxic chemicals into the body. The catalogue of waterborne pathogens is long, and it includes many that are well-known as well as far larger numbers of more obscure organisms. Waterborne pathogens include viruses (e.g., hepatitis A, poliomyelitis); bacteria (e.g., cholera, typhoid, coliform organisms); protozoa (e.g., cryptosporidiosum, amebae, giardia ); worms (e.g., schistosomia, guinea worm); and toxins (e.g., arsenic, cadmium, numerous organic chemicals).

Water also harbors the intermediate stages of many parasites, either as free-living larvae or in some other form, and it is the vehicle for essential stages in the life cycle of many dangerous insect vectors, notably mosquitoes and blackflies.

Chemical contamination or pollution of drinking water is another serious problem—one that has become a great deal worse in the modern industrial era, due to the widespread, and often unregulated, discharge of toxic substances into rivers, lakes, and oceans.

For practical purposes, this discussion of waterborne diseases and their control focuses mainly on the pathogenic organisms for which water is a common vehicle. It is important to note that not only drinking water, but also water used for cleaning fruit, vegetables, and cooking utensils, and for washing, can convey disease. Indeed, salads that have been washed in polluted water are a frequently overlooked and rather common source of waterborne disease, responsible for an occasional outbreak of cholera or typhoid.

Water sources (springs, rivers, lakes, ponds, streams, wells, reservoirs, and rainwater runoff into tanks and cisterns) can all be contaminated by fecal matter of human or animal origin. Organic matter of other origin (dead animals, decaying vegetation) can contaminate drinking water too, in ways that range from very dangerous to merely unpleasant.

Water from suspect sources usually can be made safe to drink by boiling. Ancient empirical observation of this fact in India and China may have led to the popularity in those countries of drinking tea and other infusions made with boiling water. However, boiling is neither practical nor sensible for the treatment of large municipal water supplies. These must be protected by appropriate treatment measures—filtration and purification (generally through chlorination) that were developed mainly in the nineteenth century in the industrial nations. Provision of safe drinking water supplies has been among the most effective and important measures ever taken to advance the public's health.

The other essential components in the prevention of waterborne diseases are the sanitary disposal of sewage and the environmental control of toxic chemicals. Sanitary services are based on sewage disposal systems in most organized urban communities. Some rapidly growing suburban developments may lack adequate sanitation during their early stages, but local regulations usually prohibit occupancy until sanitation is installed and working. In rural regions and other sparsely settled localities, including campgrounds, human waste is often disposed of in septic tanks or pit privies. The combination of sanitary disposal of human sewage and the provision of safe water supplies has virtually eliminated many of the serious waterborne epidemic diseases that took such a heavy toll of life until the early years of the twentieth century. However, sanitary services break down when floods, earthquakes, and other disasters occur, and at such times it is essential to boil water to ensure that pathogens are killed. Other methods, such as the use of iodine or chloramine in tablet or powder form are sometimes used, both under emergency conditions and by backpackers and the like, but these methods are less effective than boiling.

Even with the best protective measures, however, there are occasional serious large waterborne epidemics, and innumerable small ones. Recent large epidemics include the 1993 outbreak of cryptosporidiosis in Wisconsin, which affected about 400,000 people, and several lethal outbreaks of E. coli 0157:H7 infection, which is very dangerous because it causes kidney damage that can be fatal. Both these and other waterborne diseases are often due to pollution of public drinking water supplies by animal waste. Modern factory-farming methods generate enormous quantities of manure, and after heavy rains it is easy for runoff contaminated with animal manure to enter the water supply. Animal manure can contain the dangerous E. coli 0157:H7 strain. Even frequent testing can fail to detect evidence of pollution in time to prevent serious waterborne outbreaks. When testing laboratories have suffered budget cuts, the staff is often downsized, making waterborne disease outbreaks more likely. Several recent outbreaks in the United States and Canada are directly attributable to this sequence of events.

Chemical pollution of water supplies presents problems of a different kind. Chemical contamination can cause acute illnesses, but more often the toxic contaminants are slow poisons, such as carcinogens, and the effects may be manifest in only a small proportion of all those who are exposed. The pollution can come to light when a cluster of cases of leukemia or some unusual variety of cancer or other illness is detected in a community, as in Woburn, Massachusetts, where ethylene chloride that had leeched into the soil contaminated ground-water that fed several wells.

Municipal water supplies should be routinely monitored by frequent bacteriological and chemical testing. Bacteriological testing focuses on coliform organisms that, if present, are not only harmful in themselves but also are a marker for other varieties of fecal contamination. Chemical pollution presents a more difficult problem because of the wide variety of chemicals that can pollute a water supply.

John M. Last

(see also: Ambient Water Quality; Cholera; Clean Water Act; Cryptosporidiosis; Typhoid; Water Treatment )