Drinking Water Hazards
Large rivers may be polluted with sewage effluent, surface runoff, or industrial pollutants from sources far upstream. However, even small streams, springs and wells may be contaminated by animal waste and pathogens. The presence of dead animals upstream is not uncommon. In most parts of the world, water may be contaminated by bacteria, protozoa or parasitic worms from human and animal waste or pathogens which use other organisms as an intermediate host. Pathogenic strains of E coli bacteria survive briefly outside the body, to infect new hosts.
Giardia lamblia and Cryptosporidium spp., both of which cause diarrhea (see giardiasis and cryptosporidiosis) are common pathogens. In backcountry areas of the United States and Canada they are sometimes present in sufficient quantity that water treatment is justified for backpackers, although this has created some controversy. (See wilderness acquired diarrhea.) In Hawaii and other tropical areas, Leptospira spp. are another possible problem.
Less commonly seen in developed countries are organisms such as Vibrio cholerae which causes cholera and various strains of Salmonella which cause typhoid and para-typhoid diseases. Pathogenic viruses may also be found in water. The larvae of flukes are particularly dangerous in area frequented by sheep, deer, or cattle. If such microscopic larvae are ingested, they can form potentially life threatening cysts in the brain or liver. This risk extends to plants grown in or near water including the commonly eaten watercress.
Read more about this topic: Point-of-use Water Treatment
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