Biology Reference
In-Depth Information
2.5. SUMMARY
This chapter has presented an overview of the different types of
waterborne pathogens, concentrating mainly on those that cause infection
through ingestion of drinking water.
In summary, viruses are the smallest of these pathogens, typically around
20-300 nm in diameter, making them difficult both to detect and to remove.
Many viruses exhibit considerable resistance to chlorination as well as lon-
gevity, and therefore their presence in water does not correlate well with
that of indicator bacteria, one of the monitoring methods described in the
next chapter. Often highly infectious, viruses are in general more species-
specific than the other pathogens, with less known examples of zoonoses.
In general, bacteria are the next smallest, with sizes on the order of a few
micrometers, followed by the slightly larger protozoa and finally the hel-
minths. Bacteria are generally less infectious than the other types of patho-
gen, with a few notable exceptions (e.g. E. coli O157:H7 and Shigella ) and
more susceptible to chlorine disinfection. However, some of them are able
to enter a spore-like state. Some waterborne bacteria are also capable of
multiplication in the water environment, enhancing their numbers.
Protozoa have a low infectious dose and many are zoonotic. Like viruses,
these organisms are unable to reproduce outside of a host. Additionally,
protozoa are similar to viruses in that there is a poor indicator bacteria cor-
relation, due to chlorine resistance and robustness. Finally, the incidence of
disease caused by the guinea worm helminth has been significantly reduced
over the past few decades, whereas liver fluke cases are not well character-
ized. Both helminths rely upon a life cycle involving a water host, and the
causative agents are easily removed by filtration.
All of these pathogens contribute to the large disease burden impacting
both health and economic productivity worldwide. The effects of water-
borne pathogens are greater in developing countries where poor water
treatment and sanitation coverage prevents the delivery of safe drinking
water. However, even in developed countries these pathogens have a signifi-
cant impact. The following chapter will describe the guidelines and regu-
lations developed to ensure drinking water safety as well as the existing
methods of monitoring to detect these pathogens.
ACKNOWLEDGMENT
We would like to acknowledge the WHO Guidelines and Microbial Fact Sheets, which have
provided an extremely useful source of information for this chapter: Ref. 4 .
 
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