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I often use CLO2 (for example this product) to treat drinking water when I’m backpacking and boiling is not convenient. The instructions for the product say that the dose should be tripled if the presence of cryptosporidium is suspected.

What are the warning signs that could indicate that a source of water is contaminated with cryptosporidium?

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I'm not sure enough to post as an answer, but I suspect the only way is if there is a warning about it, usually from some government agency. –  Kevin Feb 13 '12 at 5:01

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As Cryptosporidium is passed in the faeces of animals, the more heavily the land is used by animals, the greater the risk of Cryptosporidium contamination in water sourced from that land.

This article suggests some ways (quoted below) to avoid Giardia (which has a similar lifecycle to Cryptosporidium), conversely, if you are unable to source water following these guidlines the greater risk of Cryptosporidium contamination.

  • Drink from large streams whenever possible, preferably those entering from the side rather than those paralleling the trail.
  • Water in fast-flowing streams is safer because any contaminants present at any location are swept downstream, being quickly displaced by presumably clean water from above.
  • Water at higher elevations is safer, partly because of reduced human and animal presence, and partly because water flowing to lower elevations has a chance to pick up more contaminants the farther it travels.
  • Taking water from a lake is best advised at the inlet, with the next best place at the outlet. Inlet water has a tendency to flow somewhat directly to the outlet, undergoing little mixing with the lake water as a whole.
  • Few Giardia cysts survive harsh Sierra winters. Contamination begins essentially anew each year, so springtime water is safer than summer or fall.
  • The colder the water is, the more likely it is freshly melted, meaning less opportunity for contamination.
  • Because filtration of water through soil removes Giardia cysts, deep well water is considered safe. By implication, springs in the wilderness should be, too.
  • One would think that, after a heavy snow year when streams run full and long, some kind of "flushing out" of lakes and streams must be occurring. Conversely, it makes sense to be more cautious in dry years.
  • Avoid water that likely could have passed through an area subject to heavy human or animal use.
  • If it doesn’t look good—it's cloudy or has surface foam—treat it or don't drink it.
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thanks, that was an interesting article. One significant difference between giardia and cryptosporidium to keep in mind though is that while giardia cannot survive for a long time in cold water (less than a day in freezing water according to your source), cryptosporidium can (for over a year). Therefore, it seems that for cryptosporidium, a very cold water source is not quite as advantageous. –  Big General Feb 13 '12 at 21:15
    
This answer gives the false impression that Giardia is a problem in the Sierra. It's not. See Robert L. Rockwell, Sierra Nature Notes, Volume 2, January 2002, web.archive.org/web/20051026030831/www.yosemite.org/naturenotes/… . More info at lightandmatter.com/article/hiking_water.html –  Ben Crowell Jul 9 at 1:02

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