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Giardia: Let's Talk SCIENCE
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Buck Nelson
(Colter) - MLife

Locale: Alaska
So, Hygiene vs Waterborne Giardiasis? on 09/19/2012 04:06:06 MDT Print View

A recurring theme for Welch and Rockwell is that it's hygiene, not the water, that's causing giardiasis for backpackers. What can be concluded from this study?

Bob Gross
(--B.G.--) - F

Locale: Silicon Valley
Re: Re: 5 x on 09/19/2012 04:34:34 MDT Print View

"But if you had only a 1 in 1000 chance of being infected in one year, it's hardly worth thinking about."

The problem there is that the disease presents itself with a multitude of symptoms of varying intensity.

Example 1. (about 1988) A backpacker friend of mine swam in the lake, then about ten days later there was a slow onset of standard symptoms of mild intensity. After a couple of days, the friend was at the doctor's office. After a couple of more days, the lab test confirmed it. A rather standard dose of medicine was prescribed, and the problem slowly went away.

Example 2. (1983) A Nepal trekker may have been infected by water vessels that were not properly washed. She got explosive diarrhea about eight days later. We are talking about ten or more trips to the bushes in one day. That same afternoon, a microscopic analysis was done, and it was overwhelmingly obvious that cysts were present by the billions. Good grief! She was given a four-day dose of Flagyl within 24 hours, and the problem started to clear. She was weak as a kitten for the next four or five days and then was not back to normal by the end of the trek another week after that. Incidentally, half of the other trekkers developed milder forms of it as well, so they knew where to find the doctor with the drugs.

Two extremely different examples. The first one was more tolerable. The second was not.


Buck Nelson
(Colter) - MLife

Locale: Alaska
Careful with the numbers though on 09/19/2012 04:54:12 MDT Print View

Jerry said "But if you had only a 1 in 1000 chance of being infected in one year, it's hardly worth thinking about."

When I read the Welch and Rockwell papers, I was getting ready for the PCT. My take away message was that the odds of waterborne transmission were near zero for backpackers. I got giardiasis.

While I think a solid finding from this study is waterborne transmission is a reality, it would be risky to depend on the numbers to calculate our personal odds. For example, some people rarely drink untreated water and are highly resistant to giardia so their risk of getting sick would be dramatically lower. I often drink backcountry water for months every year and am susceptible to giardiasis. My odds are likely orders of magnitude higher than normal. This study is decades old and for only one geographic area.

And, as I mentioned above, physicians often empirically treat (no lab tests,) fail to report (in Modoc County CA I found that no cases were reported over 3 years despite the doctor saying they treat several cases per week) and many people never go in for treatment at all, despite being sick.

An example of how this kind of under-reporting can mislead an odds calculation would be that in the United States there are about 2,000,000 plus people with Giardiasis. Of these cases, about 20,000 are reported per year. With 300,000,000 people, the odds of getting giardiasis are 1/150 in reality. The odds based on reported cases are 1/15,000. Big, big difference. And if the odds of getting giardiasis (very often asymptomatic) are 1/150 to begin with, the odds of getting it in a year if you drink untreated water too would have to be higher yet.

Another consideration is giardia is only one of the baddies that might be in backcountry drinking water.

jerry adams
(retiredjerry) - MLife

Locale: Oregon and Washington
Re: Re: Re: 5 x on 09/19/2012 08:55:51 MDT Print View

Not to be argumentative Bob, just trying to figure out how much to worry about treating my water

Example 1 lesson - don't drink lake water when swimming

Example 2 lesson - when in Nepal don't depend on a third party to treat your water

Bruce - I agree, if you've gotten Giardiosis before then it makes sense to religiously treat water

I treat my water about half the time. I don't bother if the water is coming out of a spring, or from a snow-field I can see (although there could be a dead critter underneath), or if up-stream is wilderness and there's good stream flow to dilute anything and it looks clear. I haven't (knock on wood) gotten sick and have done this at least 100 days.

I have this stuborness about not doing things that aren't necesary, but the Sawyer Squeeze is so convenient and lightweight - if it only didn't have this weakness about freezing because I go when it's below freezing a lot.

Bill Segraves
(sbill9000) - F - M
water quality comparisons on 09/19/2012 16:27:50 MDT Print View

Catching up on an older part of the thread, and not disagreeing, just adding my $.02.

IMO, the water quality comparisons are relevant, and should not be dismissed without a careful analysis of their strengths and weaknesses. That they may predate the current water treatment methods does not diminish their potential value - if the water was OK for 5 million people in 2000 and it was more highly contaminated than Sierra water, then that tells us something important.

But was it? The trail of information is sketchy, as far as I can tell, but it appears that the cyst concentrations in LA and SF Bay area water were in the same general range as tested Sierra sources. What isn't at all clear is what proportion of those cysts may have been viable. As far as I can tell, those water supplies were being chlorinated, and various authors may have extrapolated from older studies that appear to have exaggerated giardial cyst chlorine resistance to conclude that chlorination was not substantially reducing potential giardia infection. My reading of a key Jarroll et al 1981 paper, "Effect of Chlorine on Giardia lamblia Cyst Viability," suggests the contrary: even at 5 degrees C and pH 7, 1 mg/L is sufficient to achieve 98% killing in an hour.


Bill S.

Bill Segraves
(sbill9000) - F - M
Re: Re: 5 x on 09/19/2012 17:41:44 MDT Print View

"But if you had only a 1 in 1000 chance of being infected in one year, it's hardly worth thinking about."

I agree, but while 1/1000 is the approximate chance of winding up in the CO case group, it seems clear that this would have been a substantial underestimate of the likelihood of contracting giardiasis at that time in CO, let alone any water-borne disease.

Whether it's closer to 1/20 or 1/200 per year for someone who backpacks extensively and routinely drinks unpurified water, I don't think anyone knows.

Perhaps some day, one of the physicians in Mammoth Lakes or a similarly ideal location will lead a systematic study.


Bill S.

Katy Anderson
(KatyAnderson) - F
re water quality comparisons on 09/19/2012 22:49:42 MDT Print View

Thanks Bill, this explains something I could not quite wrap my head around.

Hetch Hetchy water comes from the Tuolomne river, pretty typical High sierra water.
That water is NOT FILTERED, but it is treated and that treatment in 2003 was chlorine, and then the water was piped to the residents of San Francisco.

If chlorination as you say leaves the Giardia cysts in place but renders them not viable, then a cyst count of the treated water would yield a fairly accurate count of the average Giardia cyst count in High Sierra water. As luck has it that treated Hetch Hetchy water heading for the residents of San Francisco is carefully analyzed and the results published in yearly reports for all to see.

According to the 2003 Rockwell study in 2001 there were 0.12 cysts / liter in San Francisco water. This pretty close to the highest level recorded in the Sierra in 1984 at Susie Lake south of Lake Tahoe, which measured at 0.108 cysts per liter.

But that never made sense to me, why would the San Francisco water, mainly coming out of High Sierra Hetch Hetchy water, be ever so slightly dirtier than the most polluted High Sierra Lake?

The answer became obvious when I read the City of San Francisco Water Quality Data for the Year 2011 report.

This shows the range of levels detected are from ND (none detected) to 0.07 cysts per liter. Max detection values where 0.07 Giardia cysts per liter for 2011.

So when the Rockwell study shows 0.12 cysts per liter for San Francisco water that is a MAX of 0.12 cysts per liter recorded in 2001. The number is almost identical to the MAX recorded cysts count per liter in the Sierra at Susie lake at 0.108 in 1984.

Now it makes more sense. The MAX Giardia cyst count measured in the High Sierra is pretty close to 0.1 cysts per liter whether measured at a lake or at the Hetch Hetchy reservoir.

So this still leaves open the question: "Is 0.1 Giardia cysts per liter something we really ought to filter out or not?"

PS. A slight wrinkle to this is that sometime between 2001 and today the SF Public Utilities Commission switched purification methods from Chlorine to Chloramine.
If Chloramine also leaves the Giardia cysts in place but yields them not viable then argument still stands that the SFPUC annual reporting of maximum Giardia cysts measured in Hetch Hetchy water is a good measure of Giardia contamination levels in the High Sierra.

Bill Segraves
(sbill9000) - F - M
Re: re water quality comparisons on 09/20/2012 04:45:40 MDT Print View

Good research, Katy - thanks!



Sean Heenan

Locale: Southeast mountains
hetch hetchy water on 09/20/2012 07:02:40 MDT Print View

Seems they just started to use UV also.

Tom Kirchner
(ouzel) - MLife

Locale: Pacific Northwest/Sierra
Re: re water quality comparisons on 09/20/2012 17:13:15 MDT Print View

"then argument still stands that the SFPUC annual reporting of maximum Giardia cysts measured in Hetch Hetchy water is a good measure of Giardia contamination levels in the High Sierra."

This seems a bit of a stretch to me. For one thing, Hetch Hetchy is at a much lower altitude than much of the Sierra, which makes it a much less harsh environment for just about any aquatic organism, in terms of temperature range and UV exposure. Also, the same environmental considerations plus more vegetation would likely increase the density of critters serving as vectors for giardia. I am always leery of drawing broad conclusions from limited data, especially when conditions vary so widely in the Sierra. For instance, the giardia concentration in the LA aqueduct, which sources water from the eastern slopes of the Sierra, measures only .03 cysts/liter.

If this link has not already been cited in this thread, I would highly recommend reading the report. It makes far more sense to me than anything I have ever come across on the subject.

Bill Law
(williamlaw) - M

Locale: SF Bay Area
giardia concentrations in Sierra H2O on 09/22/2012 19:53:34 MDT Print View

I recalled a local (SJMN?) news story a few years back about a perfessor who was measuring pathogens in Sierra water, going back yearly at numerous locations.

I didn't find the story, but did find the guy who I think was mentioned in the article: Dr. Robert Derlet, PhD.

There's some interesting stuff on, of all places (the number one profiteer on water purification systems, I'll wager):

Also, another article he authored (not a research paper, by any means):

That article mentions that they couldn't find giardia cysts in the manure of Sierra pack animals, which I found surprising. In the REI page, Derlet says he would treat water downstream from cattle grazing. But perhaps that's due to concerns over coliform or other bacteria.

I haven't actually found any quantitative reports from his giardia-counting efforts, save for the "you'd have to drink 250 liters to get sick" statement, which maybe implies one cyst per 25 liters, or .04 cysts/liter, assuming he's using the "10 cysts minimum myth. I'll concede that the 10-cyst thing is a bit of a myth. Note that that does not invalidate the hard number contained therein: .04 cysts per liter.

There seems to have been much discussion here, in the past week, of the question of how to interpret the words of the one Colorado study. It has been claimed that Rockwell's paper has been "debunked." Has anybody tried to debunk that Colorado study?

It seems easy to pick nits with details in any study or article. I will go back and try to re-read the debunkification of the Rockwell paper. But has his basic thesis been debunked? That might be summed up as:
1. Giardia is everywhere. Has that been debunked?
2. There isn't (much) giardia in Sierra streams. That seems bunked, still, so far as I can tell. See Derlet, above.
3. You aren't likely to get giardia from drinking the water. Has that been debunked? How do you reconcile that with the absence of giardia in the water (assuming that's not debunked). See also articles to the contrary (e.g., "Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe").
4. If you get giardia, you likely won't even know it. I've cited one study already that supports that claim.
5. If you get sick, it is most likely something else. See the paper cited under 3.

While one can pick holes in specific statements in the midst of this argument, that doesn't mean that the argument doesn't still hold up. The flip side of "even a stopped clock is right twice a day" is that "even an argument with a couple of flaws can still lead to a valid conclusion."

Perhaps the OP's biggest mistake was the admonition "Let's talk SCIENCE." It's not really a scientific question. It's a question about the level of risk one is willing to accept.

BTW, somebody above said: Still unanswered, my original question that started my readings: how long does a giardia cyst live in the air, out of water or outside of fecal matter?

That very question is answered plainly enough in the very report that has been roundly criticized (and allegedly "debunked") here. Most loudly, ironically, by the guy who asked the question. I wonder if he even read that report.

Here are the citations for that particular detail (I won't repeat the "debunked" answer to the question; look it up yourself).

DeReigner D. P., et al: Viability of Giardia Cysts Suspended in Lake, River, and Tap Water. Applied Environmental Microbiology, Vol. 55 no. 5, 1989
Olson, M. E: Human and Animal Pathogens in Manure. Conference on Livestock Options for the Future. Winnepeg, Manitoba, June 2001

Bill Segraves
(sbill9000) - F - M
Re: giardia concentrations in Sierra H2O on 09/23/2012 09:31:55 MDT Print View

"It's not really a scientific question. It's a question about the level of risk one is willing to accept."

I agree wholeheartedly with the second sentence, but whether I’m willing to accept a 5% risk/year or a .05% risk/year, I have to have some way to make a meaningful estimate of the risk in order to make a rational decision. The only way I know to do that is to look at the science.

A casual reading of Rockwell’s analysis might lead one to believe that the risk of contracting giardia from drinking untreated Sierra water during an extended trip would be on the order of 1 in a quadrillion. Depending on how one interprets the data, that may be off by around a factor of ten trillion. In my acceptable risk range, this turns out to make a difference. (N.b., it didn't have to turn out that way - if that 1 in a quadrillion had only turned into 1 in a million on further analysis, I wouldn't have to care.)


Bill S.

Edited by sbill9000 on 09/24/2012 18:08:31 MDT.

Buck Nelson
(Colter) - MLife

Locale: Alaska
The debunking of the giardiasis "science" on 09/27/2012 20:44:53 MDT Print View

Hi Bill,

Only by establishing some baseline scientific facts can a rational risk assessment be made.

I think the Rockwell paper has been solidly debunked. Not nitpicking at little points, but the very heart of the report:

Rockwell says"
*Giardiasis is "an extraordinarily rare event" in backpackers. False. It's common.
*"The minimum infectious dose is ten cysts." False.
*(to paraphrase) "It would be nearly impossible to drink enough water in the Sierras to contract giardiasis." False. You could get it with one drink. (Plus the study is decades old.)
*Rockwell says “Almost always, Giardiasis goes away without treatment”
and “If you get a Giardia infection, you are unlikely to have
symptoms.” The Colorado survey of 256 infected people showed they were sick an average of 3.8 weeks and lost an average of about 12 pounds. AVERAGE.

Here's a quote from a newspaper article from a recent Derlet Sierra water survey:
Nowhere is the water dirtier, he discovered, than on U.S. Forest Service land, including wilderness areas, where beef cattle and commercial pack stock — horses and mules — graze during the summer. There, bacterial contamination was easily high enough to sicken hikers with Giardia, E. coli and other diseases. Rockwell tells us to "drink freely" in the Sierras.

I have never seen where someone has tried to debunk the Colorado study we've discussed.

I'm not sure who is saying "giardia is everywhere." The EPA has said Cysts have been found all months of the year in surface waters from the Arctic to the tropics in even the most pristine of surface waters I don't think they literally mean ALL waters, just in a surprising variety of surface waters.

Even the Rockwell surveys show that there was giardia in 1/3 of the sources tested in the Sierra. If the true minimum infectious does is 1-10 cysts that's enough to make a whole lot of people sick. And those tests are OLD.

You aren't likely to get giardia from drinking the water. Has that been debunked? Yes. It was a speculative claim to begin with. The Colorado survey alone shows drinking untreated water increases the giardiasis risk by 5 times according to a post above.

Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe does not prove what some people claim it does. It showed from 5.7 to 8.5% of backpackers got giardiasis on one trip and that there was giardia in the local water. It did not prove where they got their giardiasis but inferred it couldn't have been from the water due to the "10 cysts minimum infectious dose" myth. That has been debunked. Also unless the water they actually drank was tested, and it wasn't, there is no way of knowing how much giardia was in the water they actually ingested.

If you get giardia, you likely won't even know it. I've cited one study already that supports that claim. I think that's true. But if you get sick with it, you'll likely get very sick indeed. See the 3.8 weeks/12 pound weight loss quote above. Those people definitely noticed it.

"even an argument with a couple of flaws can still lead to a valid conclusion." The Rockwell article and the Welch papers don't have "a couple of flaws." The central points of their conclusions don't stand up to scrutiny. See the below if you'd like to try to refute specific points I've made.

I think it's interesting that Rockwell says "Cysts can survive for as long as 2 to 3 months in cold water, but they cannot tolerate drying" yet argues people aren't getting giardiasis from drinking water, but from dirty hands.

Debunking the "Giardia Myth"
Debunking a Welch Giardia Aricle

Edited by Colter on 09/28/2012 12:54:00 MDT.

Bob Shaver
(rshaver) - F

Locale: West
My experience on 09/27/2012 21:36:05 MDT Print View

I've hiked a lot, or at least for a long time: 40 years. I've never had giardia, but my partner got it on a trip to the North Cascades, drinking out of the same water sources as me. He definitely did not get an e coli infection from hygeine issues. He was sick for weeks, and weak for months, and I don't think he ever regained the muscle mass he lost.

After that I almost always filter water in the backcountry.

That is my only statistic, I have not done a survey or anything.

Paul Johnson
(johncooper) - F

Locale: SoCal
Rockwell report and Giardia:Let's Talk Science on 09/28/2012 15:09:58 MDT Print View


Has someone debunked Rockwell or is your statement "I think the Rockwell paper has been solidly debunked." simply based on the comments you are making?

I find the Rockwell study well written, data driven and well supported by references.
For reader convenience

If you want to debunk Rockwell, and do it under your thread of "Giardia: Let's Talk Science", then can you provide data or science to refute Rockwell's data?

I find his data regarding Giardia concentration in the Sierra's and his references to studies showing that 10-25 cysts per liter are needed to have a reasonable (1/3) likelihood of contracting Giardia to be compelling.

"The highest concentration of Giardia cysts (in 69 Sierra locations) was 0.108 per liter of water in Susie Lake, south of Lake Tahoe. The next highest was 0.037 per liter near Long Lake, southwest of Bishop. Samples taken in the Mt. Whitney area varied from 0 (most sites) to 0.013 (Lone Pine Creek)"

"Summary figures
Units are cysts per liter.
Concentration Comment
~1000 Typical swimming pool contamination
~100 Giardiasis is plausible
~10 Minimum needed to contract giardiasis
~1 Some wilderness water outside California
0.12 Some San Francisco water
0.108 Worst Sierra Nevada water
0.030 Some Los Angeles Aqueduct water
0.013 Mt. Whitney at Trail Camp
0.003 Mt. Whitney at Whitney Portal"

Per the earlier posts, most of us are trying to make a risk assessment. These low concentrations lead to an assessment to drink untreated Sierra water. Especially, if you practice BPL self styled "good" sources criteria.

Bill Segraves
(sbill9000) - F - M
Re: Rockwell report and Giardia:Let's Talk Science on 09/28/2012 16:11:57 MDT Print View

For any who are still game, let's go back to some of the key science:

1) What does the relevant scientific study say about whether you can get giardiasis from ingesting one cyst?

2) What does Rockwell say about whether you can get giardiasis from ingesting one cyst?

3) Upon what key assumption does Rockwell's analysis of the probability of contracting giardiasis from drinking lightly contaminated water depend?


Bill S.

Paul Johnson
(johncooper) - F

Locale: SoCal
Re: Re: Rockwell report and Giardia:Let's Talk Science on 09/28/2012 19:02:15 MDT Print View

From the Rockwell report.

"How many cysts does it take to get the disease? Theoretically, only one. But there are no documented cases of giardiasis being contracted from such low levels. Volunteer studies have shown that 10 or more are required to have a reasonable probability of it, with about one-third of persons ingesting 10 – 25 cysts getting detectable cysts in their stools. 9, 10, 11, 13, 41, 42"

Juranek, Dennis D.: Giardiasis. Centers for Disease Control and Prevention, 1990
Swartz, Morton N.: Intestinal Protozoan Infections. Scientific American Medicine, 1994
Kerasote, Ted: Great Outdoors; Drops to Drink. Audubon, July 1986
Backer, Howard D.: Giardiasis: An Elusive Cause of Gastrointestinal Distress. The Physician and Sportsmedicine, Vol. 28 no. 7, July 2000
Ortega, Y.R. et al: Giardia: Overview and update. Clinical Infectious Diseases, Vol. 25, 1997
Rendtorff, R: The Experimental Transmission of Human Intestinal Protozoan Parasites. American Journal of Hygiene, Vol. 59, 1954

Buck Nelson
(Colter) - MLife

Locale: Alaska
My opinion doesn't debunk Rockwell, the science does on 09/28/2012 19:26:04 MDT Print View

One thing that makes the Rockwell paper dangerous (and it was dangerous enough for me personally that I ended up at a hospital) is that it cites so many scientific papers that it's easy to conclude that the conclusions are scientifically supported. But I believe my blog posts, which reference many of those same scientific papers, show that most of Rockwell's main conclusions simply aren't supported by the underlying science.

John Cooper, please read through my two blog posts cited above because otherwise I'm presenting my case in bits and pieces. If you had read through them you'd see why the seemingly impressive Rockwell cyst count data is extremely misleading.

Bill S., I like the way you think. The answers to #2 and #3 are clear. At least one statistician has told me that the answer to #1 would be "not enough data to draw a reliable conclusion."

Bill Segraves
(sbill9000) - F - M
Re: Re: Re: Rockwell report and Giardia:Let's Talk Science on 09/28/2012 19:35:10 MDT Print View

I have read Rendtorff in full, but not all of the others. Are any of the others independent experiments, or all just references back to Rendtorff?

Bill S.

Bill Segraves
(sbill9000) - F - M
Re: Rockwell report and Giardia:Let's Talk Science on 09/29/2012 05:18:17 MDT Print View


While I and others work on whether the other references provide any independent info, may I ask two more questions?

You wrote "I find his data regarding Giardia concentration in the Sierra's and his references to studies showing that 10-25 cysts per liter are needed to have a reasonable (1/3) likelihood of contracting Giardia to be compelling. "

Did you mean to say "10-25 cysts per liter"?

Is a 1-3% likelihood during any given trip a "reasonable" likelihood, or something that you would say no one needs to be worried about?


Bill S.