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Giardia: Water
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Giardia can ... cause diarrhea in humans, and children are especially susceptible. Therefore, environmental disinfection is important. The use of chlorine bleach, one cup in a gallon (500mL in 4 liters) of water, is effective if the surfaces and premises can be safely treated with it.
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If you really want to eliminate the possibility of Giardia, the only certain ways are by boiling and filtration. Boil water for a minimum of 10 minutes; 15 minutes if you’re in the mountains.
In water, Giardia exists as a hardy, highly infectious cyst, which can survive for months, even in fresh water devoid of all nutrients. In the gut, Giardia exists in a swimming and feeding form known as a trophozoite.
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Outside of the Sierra, Giardia cysts in concentrations “as high as four per gallon [ยง]” have been detected in untreated water in northeastern and western states. [39] But even with this concentration, one would have to consume over nine liters of water to have a 50 percent chance of ingesting 10 or more cysts.
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Giardia infection can occur through ingestion of dormant cysts in contaminated water, or by the faecal-oral route (through poor hygiene practices). The Giardia cyst can survive for weeks to months in cold water[3], and therefore can be present in contaminated wells and water systems, and even clean-looking mountain streams, as well as city reservoirs, as the Giardia cysts are resistant to conventional water treatment methods, such as chlorination and ozonolysis.[3] Zoonotic transmission is ... possible, and therefore Giardia infection is a concern for people camping in the wilderness or swimming in contaminated streams or lakes, especially the artificial lakes formed by beaver dams (hence the popular name for giardiasis, "Beaver Fever").
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Giardia can be found in well water, particularly if the well is located on a farm. But the most common sources of Giardia are surface water sources, like rivers, streams, lakes, and reservoirs.
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