Water Testing
There is no need to test for specific pathogens (for each disease).
Not only would that be slow and very expensive, but it would actually tell us
less than we need to know. What we really want to know is, not only whether
the water could make someone sick at present, but whether it is inclined to.
In other words, if we did test for a specific disease, and found it was
not there,
shortly thereafter, if the disease appeared in the community, our test result
would not have told us
whether the water point could become a point of transfer.
What WatSan workers actually test for (what they use as an indicator) is
coliforms. Because they are the indicator of choice, quick and inexpensive
tests have been developed for them. When there are coliforms in the water,
we know the water is subject to contamination, and that when disease does occur
in the population, the water will carry, along with the usual coliforms, the
pathogens being expelled by the sick.
In the developed world, the practice is usually to test for "total coliforms",
some of which are fecal coliforms, and some of which are not. That is because
water systems in the developed world can expect to keep out not only fecal waste,
but soil. In the developing world, where soil is usually unavoidable, a total
coliform test would be over "sensitive", and under "specific". In other words,
it would tell us all water has some turbidity, which is not what we need to know.
Instead, it is better to use a test for fecal coliforms, or a test for E.coli.
Either would tell us (with acceptable accuracy) whether or not a water point is
being exposed to fecal waste.
The DelAgua
Advantages
- portable (although it requires a battery, and a still setting)
- tests for fecal or total coliforms
- quantitative, gives a count
Disadvantages
- the kit itself costs 2-3000$US (although materials for each test cost only 1-2$US)
Colilert
Advantages
- can be carried out without an incubator
- each test costs only 1-2$US
Disadvantages
- qualitative, detects only the presense or absence of E.coli (There is a
quantitative alternative which is relatively costly.)
- I am unsure of the reliability of the non incubated test.
Pathoscreen Pillows
Advantages
- can be carried out without an incubator
- each test costs only 1-2$US
Disadvantages
- qualitative, detects only the presence or absence of hydrogen-sulfide
producing bacteria, but gives no count (a quantitative version is available,
but I know little about it).
- hydrogen-sulfide producing bacteria do not coincide as predictably with fecal
waste as do E.coli, making this test both under sensitive, and under specific.
Burning questions
I would appreciate any comments on the tests mentioned above, or on other tests
that I may not be aware of. I'm especially interested in comments about the
possibility of using them, or "petri films", without an incubator.
moirawalsh@tulanealumni.net
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