Water is like a two-edged sword. Without water we cannot live; but by drinking water we can run the risk of disease and death.
Why is this? Because untreated water has a high probability of carrying pathogenic protozoa, bacteria and viruses, many of which can survive in water for days. Drinking such water can lead to infection and disease, primarily diarrheal, especially in infants.
Control of the quality of potable water is thus a major means of controlling disease, one that is almost taken for granted in developed countries.
Testing of water for contamination with pathogens is an essential step in the control of water quality, ensuring that community water systems continue to supply uncontaminated water. When tests show contamination Health Promoters can work with the community to find and repair the source of contamination.
Testing is also used to identify clean water sources for new systems.
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It looks clean, but is it fit to drink?
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Indicator tests for water-borne pathogens
Tests do exist for each of the many water-borne pathogens, but it would be impractical and prohibitively expensive to test each water sample for each pathogen listed in the table to the right.
Instead a single test is used which measures an indirect but reliable indicator of contamination with pathogens.
Almost all pathogens that enter water do so via contamination with human or animal feces; thus tests for the presence of fecal contamination of water are the accepted indicators of contamination with pathogens.
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Common waterborne pathogens
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Protozoa
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Bacteria |
Viruses |
| Giardia |
Vibrio cholerae |
Hepatitis A |
| Amoeba |
Campylobacter |
Rotavirus |
| Cryptosporidium |
Salmonella |
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Shigella |
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Escherichia coli |
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Leptospira |
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Fecal Coliform Bacteria
The standard measure of fecal contamination is the presence of fecal coliforms, bacteria which are universally present in human and animal feces. Standard methods for the measurement of fecal coliforms are the Most Probable Number method and the more modern and commonly used Membrane Filtration (M-FC) method.
We currently use this method primarily in the Departmental laboratories; its use in the rural laboratories has been largely supplanted by the simple field test.
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| Testing water samples by the M-FC method, León |
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Bacteria Producing Hydrogen Sulfide
Another measure of fecal contamination is the presence of bacteria producing hydrogen sulfide. Several studies, including one carried out in Nicaragua by TASCA and ENACAL/DAR have shown that in potable water samples there is a high correlation between the presence of fecal coliforms and of bacteria producing hydrogen sulfide. Testing for the presence of these bacteria is the basis of the Simple Field Test.
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| Training for the Simple Field Test, Ayapal, 2008 |
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