If you are gathering water that has flowed through areas populated by humans or domesticated animals, then it seems these nasties could be a threat.

If the water is comes from a relatively pristene area, then my take is that the threat is pretty minimal.

In a place like the Boundary Waters, where there are fair numbers of human (and domesticated animal) visitors, I'd be cautious of untreated water. In "real" back areas, I wouldn't be too concerned.

Here are a few articles I've found on the backcountry threat:

http://wms.org/pubs/Giardiasis.htm

Here is another very good article that suggests food contamination from "dirty" hands may be potential source:

http://www.yosemite.org/naturenotes/Giardia.htm
and (same article & author, slightly modified)
http://lomaprieta.sierraclub.org/pcs/articles/giardia.asp

Here's a kind of funny (?) article about New Zealand water that mentions that Germans are often blamed from bringing nasties to N.Z.

http://www.wildernessmag.com/features_cool.htm

It doesn't mean much.

This reference is in conflict with the vast majority of studies which have shown that iodine-based water purification tablets reliably kill giardia, and that chlorine dioxide takes care of both giardia and beaver fever.

If chlorine dioxide didn't work, everyone who uses municipal water that includes treated and recycled water would be dead, and bleach wouldn't work in the manner that it does. Ditto iodine as biocide for medical work.

Remember, it was once "proven" that bees couldn't fly.

One problem with some protozoan parasites like Giardia is the difficulty diagnosing it. Physicians and vets alike will both admit that it's a tough bugger to pin down. It can be there, merrily causing trouble, but it's just hard to find in fecal specimens. And if it's difficult to find where it is specifically suspected, how accurate are those random water tests done on backcountry water sources? Was it not there, or was it there and they just didn't find it?

I don't know about Crystosporidium, but Giardia can infect every kind of mammal there is, it isn't host-specific at all. To me, that means it's endemic in the wildlife, and if there's wildlife where you're hiking, it's there. Maybe not in the particular quart of water you just gathered, but maybe in the next one.

"Better to be safe than sorry" is a very old saying, and there's a reason that it's so old and so well-used.


Sue

The article made it sound like switching over would be a prudent move. I carry iodine as a backup to a filter, but have never used it. As for how big a threat, well, giardia CAN be asymptomatic (my girlfriend had it for a while while in Africa and didn't realize it). I think cryptosporidium is more "potent" in terms of the diarrhea and other "fun" effects.

Once again, the original article was NOT talking about ClO2, merely regular cholorine/bleach. I agree that ClO2 is better. But... don't forget that there are occasional outbreaks of parasites in municipal water. The wonderful little town I'm confined to had a boil order in certain neighborhoods last year.

Back in the early 90s I was working on the Houston sewers, and got really sick. I was like that for a couple weeks, and finally went to the doctor. When I told him the symptoms and the exposure, he prescribed flagyll, which is a potent antibiotic that will wipe out just about every wee beastie in your digestive system. I took that, ate a couple gallons of yogurt, and all better. The doc said I most likely had caught giardia.

I saw a poster a while ago of a coyote [censored] urinating in a "pristine" stream, the kind you see up in them thar hills... below it was the caption "Do you really know how clean your water is?" or something to that effect. Yep, distillation and/or reverse osmosis is my hands down winner. I've seen migrant farm workers bathing and washing their clothes in the Columbia, Snake and Yakima Rivers in the summer, by the dozens, while the kids were swimming around downstream, while upstream the cattle feed lots were being washed down and the runoff was going straight into the river. This was Washington State, not some Amazonian tributary. Makes you think, don't it?

Yes, it makes me think, that if it was as easy to catch these things as some people think, that ALL those people would be sick...
Do not get me wrong, I am a belt AND suspenders sort of guy, and I do not believe in taking chances that you do not have to. But in a short term survival situation, where you are sure of being back in civilization within a couple of days, If I could not purify the water, I think the prudent thing would be to go ahead and drink, and see a doctor when you get back...
Does anyone have any experience with field expedient filters made from natural or readily available materials???

Well, hmmm, "field expedient"... you may laugh, but, as a young and stupid youth, my buddies and I once filtered water from the Penobscott river with clean socks from my rucksack, and then boiled it. The socks just got all the big chunks out! We would dip the socks in the river to fill them up (they were pretty stretched out so they probably held about a quart each), and then we'd hold them over the pot, and let them drip into it. It was already on the fire, which was good, because it warmed our hands while we held the dripping socks! (The water in the Penobscott in the spring is basically just recently melted snow and ice, so it's a little chilly.)

(Hey, at least I used my clean socks!)

Odds are, a great many of them are sick. Waterborne illness is still one of the leading causes of death on this planet. And some people just build up a tolerance or are lucky- I can think of a couple of places I've been where people had wells or springs and they were contaminated. People got sick, got better and thought it was the flu, or they toughed it out for life. Either way, they became carriers of giardia.

Other things, like shistosomiasis and crypto, really don't have symptoms until you are boned. The first doesn't happen in the US, but if you are traveling or someone upstream has been traveling and you've got the worst luck in the world, it probably won't kill you, but you aren't going to be going anywhere for a while. If you need to MOVE NOW, that isn't something you want to have to deal with. Giardia and crypto can kill you by making you spend too much time squatting over a latrine, the same way cholera and dysentery kill. Again, if you need to evacuate on foot, passing 8+ quarts of water a day (crypto and giardia can both do that) is going to at least slow you down. Assuming you can rehydrate.

Cholera is almost unheard of in the US; the last significant outbreak was in the early nineteen-teens. But common in many parts of the world, and once you are a carrier, you are a carrier for life. Last major outbreak I know of in this hemisphere was 95-97 in Central America. Mexico STILL has pockets of it breaking out on a somewhat regular basis. We lucked out in NOLA- this nasty little thing is one of disaster's two ugly step sisters (along with typhoid) in a global sense. This can kill in a matter of hours as you try move more water out of your backside than is in your body.

Sometimes, mobility is life. Bad water usually gives you the runs. If you can't get it under control, all you are doing is running to a grave. I'm saying this because sometimes, you don't get a chance to see a doctor a few days later. I don't know how many times I've read people say things like "I'll risk it and see a doc later", but there is almost always a way to purify or at least filter water.

Actually, diarrhea is the NUMBER ONE cause of death for children, world wide. Almost always due to compromised water.

Cholera, just as a tidbit of info, can cause you to lose up to 30 liters (might be pints, I'm pulling from memory) of water daily. Given the average adult male is only about 60L of water in them, you can see why it'll kill quick.

Regarding field expedient filters, I don't think ANY made up filter can get down to the microscopic level. I did read somewhere, maybe Discover magazine, that most of the water in the Sierra Nevadas is clean enough to not need filters, sort of. Some college professor had sampled dozens of lakes, and in the first few inches deep, enough UV light from the sun did a good job of keeping it relatively clean. I think if worst came to worst, a very gentle slurping of the top of a good size lake would be a safer bet than most other options. Or collecting rain, but we all know how often CA sees THAT.