Giardia: Let’s Talk SCIENCE

Zell, Steven C. Epidemiology of wilderness-acquired diarrhea: implications for prevention and treatment. J Wild Med 1992;3:241-249.

"In the 1950s, Rendtdorff [18] studied the innoculum of Giardia lamblia cysts necessary to establish infection in federal prison volunteers. Known numbers of cysts were placed along with a small amount of saline into gelatin capsules ingested by subjects. Stool specimens from prisoners were examined for up to 165 days after exposure and infection defined as ability to identify Giardia lamblia cysts in the subject's stool, regardless of symptomatology. No person exposed to one cyst became infected, whereas 36.4% of 22 men receiving from 10-25 cysts established infection. All subjects receiving 100 cysts or more developed infection. Of note, no individual developed symptomatic giardiasis and there was a tendency for infections to disappear spontaneously over time [18]."

Rendtorff, R The experimental transmission of human intestinal protozoan parasites. Am J Hyg 1954; 59, 209-20.

http://www.cdc.gov/parasites/giardia/epi.html

"However, swallowing as few as 10 cysts might cause someone to become ill[2,6]."

2. Yoder JS, Harral C, Beach MJ. Giardiasis surveillance – United States, 2006-2008. MMWR Surveill Summ. Jun 11 2010;59(6):15-25.
6. Rendtorff RC. The experimental transmission of human intestinal protozoan parasites. II. Giardia lamblia cysts given in capsules. Am J Hyg. Mar 1954;59(2):209-220.

http://www.cdc.gov/mmwr/preview/mmwrhtml/ss6105a2.htm?s_cid=ss6105a2_w

"The infectious dose is low; ingestion of 10 cysts has been reported to cause infection (12)."

12. Rendtorff RC. The experimental transmission of human intestinal protozoan parasites. II. Giardia lamblia cysts given in capsules. Am J Hygiene 1954;59:209–20.

My eyes aren't what they used to be, age catches us all, so I can't count how many cysts are in each liter I drink. Nor do I know what is upstream from me. Possibly others out there can, more power to them. So there's the 10 count, good, from a very old study. So that's one single fact now that's somewhat established. Unless it's been disproved in a more recent study, something that happens constantly, with newer methods and newer testing tools etc. So really what we are looking for is up to date data, but an old test is better than none. So did that liter I just downed have 0, 1 or 10 in it? See the absurdity? Hopefully yes. this is why I use a filter, and these discussions are what totally convinced me to always do that from now on, especially when I see how thin the actual research is.

Odd to find the CDC still keep noting 1 is all that is required on most of its documents, it's not like them to err on the side of care and caution, but figuring out what makes them decide x or y is a very challenging thing, I will happily admit, if they trust one doctor over others, and that doctor is corrupt, or lazy, or stuck in old habits, it leads to problems. Lots of ego, careerism, profiteering, corruption, etc, involved with these areas too, things change, and people who made their careers being experts in the old way, keep resisting change because the new way makes their studies less valuable.

However, I am left with the problem that I have no way of counting the cyst count per liter I drink today, here, from this water source I am at now, assuming I'm backpacking somewhere or other. And neither does anyone else. I'd be a fool to follow higher risk actions re pathogens when I can easily and with no effort follow lower risk actions, with exactly zero negative repercussions for me. Basically what we have is one cranky old physics professor, who has his pet peeve, and writes on it. That's not a new or rare thing in science, by the way, lots of professors do that, with the same lack of discipline.

But again, I truly do not care what other people do as long as they don't expect me to help them if they run into troubles based on their decisions. No common sense I can locate suggests that not filtering is a better idea than filtering, that simply makes no sense, so I will happily filter. But the added information is good, I wasn't sure about the cyst sizes, so that's clearly much less grounds for concern, and being aware of other backpackers as possible vectors is a good thing to know too, so that's a plus.

I was just checking to see if there was any actual new science on this question, there isn't apparently, so that's fine, the question is still relatively the same, except for worsening ecosystem degradation, which stresses things in all types of ways, most far too rapidly evolving to actually keep track of unless you specialize in the area. But one thing can be stated with absolute certainty: things are getting worse, and they are getting worse quickly. Which parts of our ecosystem react in which way, that is something being studied with more energy now than in the past, for obvious reasons. So anyway, there's no debate I can find here, just some slightly improved understanding of the question. As I thought.

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, ie, a drop of water dries? However, if the 10 count is accurate, I'd say that question is also somewhat mooted, would be more relevant if 1 is sufficient. It's odd how hard it is to find answers to even simple questions like that, I think that reflects the state of the research fairly well is my guess.

And, other questions, is the count of cysts cumulative, ie, over 2 days and 4 liters, do 10 total count? Or does it have to be 10 per ingestion? Has anyone ever tested this? I seriously doubt it. There's a fair degree of wishful thinking going on here about what is actually being said I suspect. So much not known here, zero grounds to make any statements about safety in my opinion, and I suspect that is what makes the CDC err, as they should, on the side of caution. I'm leaving this one alone, categorizing it as wishful thinking, best argument for filtering I've seen yet, thanks.

"the 10 count"

I think that is because different humans have immune systems of different powers. One person might be fragile, and one cyst might be able to set up housekeeping in the gut and make a living at it. One person might be tough and have an excellent set of biochemical acids, so ten cysts can be wiped out before they make a go of it. Maybe one person had a strong dose of coffee and that helped to devitalize the critters. We will likely never know.

Where I was this week is where a close friend contracted Giardia 25 years ago (swimming in the lake). So, I filter every drop for consumption.

–B.G.–

This has been an interesting thread, but as others have observed, there are some real gaps in the science.

Harald asks a key question, perhaps *the* key question: "is the count of cysts cumulative, ie, over 2 days and 4 liters, do 10 total count? Or does it have to be 10 per ingestion?"

I might place the emphasis slightly differently – does the probability of infection from a given exposure increasingly linearly with the number of cysts ingested? (The alternatives being that there is a key threshold or simply a greater than linear increase in probability due to "cooperativity." And if the answer's that it's non-linear, then we have to move on to Harald's question about the timing.)

Everything I've seen (big caveat – I have not looked exhaustively) is consistent with a linear response and a human infection rate of ~ 1-2%/viable cyst (n.b., viable is very important here, if/when we start parsing data for consistency with this). In the most commonly cited human study, one cyst didn't infect anyone because they didn't test enough people to overcome the odds. Unfortunately, everything I've seen is also consistent with the other two possibilities I've posited above!

Given that concentrations of cysts in most contaminated wilderness water sources is low, this is what I would most want to know. It seems pretty clear that my chances of getting infected from a single half liter drink of reasonably well-chosen wilderness water are low. Hypothetically, let's say that I'm backpacking in an area where I can consistently limit my exposure to the order of 1 cyst/20 liters by choosing my water well. If there's a threshold or a significant non-linearity to infection rate/cyst (and cumulative effects are relevant only on the order of a day), I can expect to drink happily for a long time without becoming infected. But if it's linear or approximately linear, my luck will run out, and on a timescale that's relevant.

If there's a good study that addresses this question, please, someone point it out! And if not, for now, we sift through the clues that may exist within other available information.

Best,

Bill S.

The cited Rendtorff study seems to be the basis for most or all of the infectious dose numbers. Most people got giardiasis at the 10+ cyst level. Five men got one cyst one time and didn't get giardiasis. No one was tested for 2,3,4,5,6,7,8, or 9 cysts. Obviously the 10+ infectious dose number isn't valid statistically yet it is a vital part of the argument against waterborne giardiasis for backpackers.

Someone said I disagree with your conclusion that the cited references establish "…there is more scientific evidence to support the spread of Giardiasis through water than there is through hygiene. I didn't mean which cause is the more common. I'm not sure what the ratio is and I don't think anyone does. What I mean is there are several studies saying something along this line:
"drinking untreated backcountry water is an important cause of endemic giardiasis infection" with no studies that I'm aware of making this sort of claim:
"poor hygiene in the backcountry is an important cause of endemic giardiasis infection." Welch and Rockwell have made statements approaching the latter, but only after basing those statements on lack of evidence for waterborne giardiasis.

I believe that most of these studies are going about it the wrong way anyway. Retrospective surveys seem wisest to me, like the Colorado study where they surveyed 256 giardiasis cases and determined that the infection rate was triple for those not treating. That is significant. It didn't matter how many cysts there were or how viable the cysts were or when and where the tests were done or even what the infectious dose was. At the time we are drinking from backcountry sources it is essentially untested water so viable cyst counts aren't available and the big picture stats are more valuable for risk analysis.

Harald said So my quest for the science of it showed almost no variation in the data or information, and all of it said the same thing. Where exactly is the alleged 'science' going on that is disproving categorically the cdc and other government findings, making for an actual scientific disagreement? It's just one guy, right, who publishes some stuff online, correct? Typical internet stuff, right? Or is there real science going on somewhere?

Much of the "science" comes from the paper I am currently trying to debunk. It is the cornerstone of the infamous Giardia Myth-Buster paper which I find appalling, especially in his heavy use of cherry-picked anecdotal evidence. He posted that article all over the internet to the delight of people who had just learned that science had proven that untreated drinking water is safe to drink.

In a forum post about what he considers excessive paranoia about giardia he actually said nearly all warnings state that all water is polluted with Giardia. He also said Someone wrote about other threats in the water: "…Giardia is not the only thing to worry about in the back country. with the increased human waste problem in the high peaks, bugs like e-coli, VRE, and Hepatitis are real possibilities too."

These are not real possibilities — no studies have shown these threats to be of concern to backpackers who do not treat their water. That is some highly irresponsible "science."

Of course Rockwell did a lot of research and cited many legitimate scientific sources but his conclusion to "drink freely" was based largely on the Welch paper currently being discussed.

That's just a guy, talking about one area, that he likes to hike in. It's not science, even though he's a scientist.

Can be good to start a conversation though, I'd certainly not follow anything he says beyond that.

Good point. I stopped reading your (lengthy!) post at this point :-).

That paper maybe hasn't been "peer reviewed" in the sense you imply, but in terms of being read, discussed, and contemplated on by fellow travelers in the Sierra, it most certainly has. And in some ways, that's worth a lot more than some academic critique.

"Put your money where your mouth is," if you will.

Go read the "peer reviewed" studies he cites if you don't believe him.

The OP (so far as I know) hasn't had his blog post peer reviewed. Shoot; he's asking us here to review it for cripes sake.

That paper maybe hasn’t been “peer reviewed” in the sense you imply, but in terms of being read, discussed, and contemplated on by fellow travelers in the Sierra, it most certainly has. And in some ways, that’s worth a lot more than some academic critique.

Go read the “peer reviewed” studies he cites if you don’t believe him.

The conclusions of the Rockwell paper seem so plausible largely because of all the peer reviewed papers he cites. It’s easy to conclude that with all those citations that they must agree with his “drink freely” philosophy. But if you read them, almost none of them do, except for those written by Welch or other paper’s Rockwell has written. If cited sources take a stand on the issue, they almost always say waterborne giardiasis in the backcountry is a concern. Anyone can read through piles of peer reviewed papers and pick and choose certain data to support what might be invalid conclusions.

You are right that the Rockwell paper is likely the most read and discussed paper on the topic. That doesn’t make it accurate of course. Apparently many readers don’t buy his conclusions because most people in the Sierras still treat their water.

I devote most of this post to debunking the Rockwell paper. The orignal post here was directed at a Welch paper which covers less ground, making it simpler to refute and discuss.

Not surprisingly my blog posts have not been peer reviewed in the conventional sense. That’s why I came here to be reviewed by my peers! I want to identify the flaws. There are educated people here with a critical eye who will not hesitate to point out logical fallacies, I’m sure.

This continues to be one of the best, most rational discussions I’ve ever seen on the topic. Keep it coming!

I'd be a fool to follow higher risk actions re pathogens when I can easily and with no effort follow lower risk actions, with exactly zero negative repercussions for me.

A negative repercussion for me is to be unable to walk along a trail in the wilderness and simply dip my cup into a free-flowing stream and enjoy a refreshing, cold, drink. I'd be a fool to give that up to protect against a risk that does not appear to exist.

Basically what we have is one cranky old physics professor, who has his pet peeve, and writes on it.

Your arguments might carry more weight if they omitted such ad hominem attacks. Attacks, BTW, which blatantly ignore what the man has contributed by sharing his experience and expertise, not to mention his life-long public service to the community of like-minded Sierra travellers.

It makes you come across as a bit, well, "cranky."

I have a different reading of the math.

Bruce said:
"Retrospective surveys seem wisest to me, like the Colorado study where they surveyed 256 giardiasis cases and determined that the infection rate was triple for those not treating."

This is how they word it in the summary for the study:
"A statewide telephone survey of 256 cases and matched controls identified: 1) and increased incidence of giardiasis in persons between the ages of 16 and 45, p less than .001, with males and females equally affected; and 2) a higher proportion of cases than controls who visited Colorado mountains (69% vs. 47%), camped out overnight (38% vs. 18%), and drank untreated mountain water (50% vs. 17%), p less than .001."

I read that as: the giardia infected group was three times more likely than the control group to drink untreated mountain water, twice as likely to camp out overnight and somewhat more likely to visit the mountains.

What you did with these numbers was to say: if three times as many people in the infected group drank untreated water then it follows that untreated water is three times as infectious as treated water. I think that is an incorrect conclusion.

What, precisely, might you mean by the phrase "is a concern?"

One can find studies that show incredibly low numbers of giardia cysts being found in Sierra water sources. Way lower than municipal water supplies, apparently (I don't know if the cited levels are at the source, or coming out of the tap).

One offhand statement in a study cited above is this: "Of note, no individual developed symptomatic giardiasis and there was a tendency for infections to disappear spontaneously over time [18]."

Headline: Studies show that 100% of giardia infectees exhibit no symptoms, and it cures itself!

So what's the risk we're talking about? Getting a disease that nobody will notice, and which goes away without doing anything?

I need *more* diseases like that one, not less.

Katy said:

What you did with these numbers was to say: if three times as many people in the infected group drank untreated water then it follows that untreated water is three times as infectious as treated water. I think that is an incorrect conclusion.

Hmmm. That's something to ponder. "(Matched control) In statistics, matching is comparing groups in which each subject is matched by a comparable subject (called matched or paired control) in terms of age and all other measurable parameters, optimally being equal in every aspect except the exposure of interest."

My reading, accurate or not, is that the matched control would be as equal as possible in every way except for the treatment of water.

If I'm wrong I wonder how the control group was chosen? Would they possibly have people in the control group that had been drinking untreated water?

The way I read it is that the control group are uninfected by giardia.

Bill, by "waterborne giardiasis in the backcountry is a concern" I mean that giardia is often present and drinking the water might result in an infection.

No offense intended, but you really need to read through the thread so we don't have to keep rehashing the same stuff. The "city water is dirtier than Sierra water" has been debunked.

One offhand statement in a study cited above is this: "Of note, no individual developed symptomatic giardiasis and there was a tendency for infections to disappear spontaneously over time [18]."

So what's the risk we're talking about? Getting a disease that nobody will notice, and which goes away without doing anything?

This is a classic example of the type of disinformation that's out there.

In the large Colorado study they found this: The majority of infected residents who were surveyed had experienced an episode of chronic watery diarrhea (median duration 3.8 weeks) with bloating, flatulence, and weight loss (averaging 5.1 kg)

So sick for an average of 3.8 weeks and losing over 11 pounds. Not exactly 100% of giardia infectees exhibit no symptoms, and it cures itself!

Here's a good one I found by a guy who was stupid enough to fall for that Rockwell paper when hiking the PCT:
http://bucktrack.blogspot.com/2011/03/waterborne-giardia-for-backpackers-no.html

Years ago I twice contracted Giardiasis, in consecutive years. Since the second time I have been very careful about treating water, and had no trouble for many years. However, before I left on my five month thru-hike of the Pacific Crest Trail last year, I read this article. Giardia lamblia and Giardiasis With Particular Attention to the Sierra Nevada. It made a very convincing case that the odds of getting Giardia from backcountry drinking water was so low it didn't pay to worry about, at least in the High Sierras, as long as some attempt was made to select water sources. Instead, a focus on hygiene was stressed. So on the PCT I usually didn't treat my water. But I was the most careful I've ever been with hygiene in the outdoors. Unfortunately I got so sick in the Sierras that I was curled up on a mountainside, and it took me most of a day to make it the four miles (only four, luckily) to the nearest road. I partially recovered, after a full day of rest, then a week later became very sick again. Luckily I was at Mammoth Lakes. My physician diagnosed Giardia. He told me he treats many backpacker Giardia cases and doesn't report it. He laughed when I told him about the article I read. Other hikers have told me their doctors told them the same. A cursory scanning of Pacific Crest Trail sources shows about 12 thru-hikers a year (out of around 300) report getting giardiasis. And despite statements to the contrary, most are diagnosed by lab tests and/or physicians. With 2/3 of people being asymptomatic, the true number is likely much higher.

This guy lists some recent studies as well that show the Rockwell thing to be nothing more than wishful thinking and pseudoscience.

I read some other more scientific stuff, the only real takeaway there is things like it takes from 1 day to 10 days to come down with it. the amount of totally wrong information in this community on this topic is somewhat staggering, maybe think about not citing articles that cherry pick their data to confirm their premise?

Anyway, as I noted, I'm now totally sold.

I like this guy's blog posting because he's no longer engaging in wishful thinking, and probably no longer thinks that saving 3oz of pack weight is a very good idea, I could have told him that if he'd asked, of course.

So people reading this thread, be warned, you're looking at a myth that people are trying to spread, not science, keep filtering, boiling, treating whatever, and ignore people who make claims about it not being necessary.

http://download.journals.elsevierhealth.com/pdfs/journals/0953-9859/PIIS0953985993711729.pdf
http://www.ncbi.nlm.nih.gov/pubmed/937629

I mean, really, let's move on to debating silnylon vs cuben, bpl is good at that, clearly when wishful thinking gets involved, something goes wrong. I had my suspicions when I first came across people in the backpacking scene trying to spread this myth, it had that smell of pseudoscience about it, ie, primary source not science, relies on anecdotal evidence and cherry picking of data, all the telltale warning signs.

It is good however to now and then re-examine such things, but the notion that backcountry is getting healthier is almost too far fetched to take seriously now, best to adjust to reality and move on to trying to drop pack weight without skipping water treatment.

Another telltale is just how easy it is to find stuff that shows the myth as myth, and how hard it is to find real science that supports it.

But bpl is a good place to hash over such things, but when it comes to medical stuff, I'm looking elsewhere for good advice, just like that guy I quote above wishes he did.

His article is good, he goes through almost everything people in this thread are citing, including that single study on number of cysts required. I'd stop spreading this garbage if I were you guys who keep promoting that junk science of Rockwell's, it's bad for people's health and it really isn't cool. I like the part about his doctors laughing when they saw Rockwell's paper, that kind of says it all, don't you think? I'd be embarrassed personally to spread this nonsense, and will make a point of noting the fact it's nonsense in future threads on this topic to help save some uncritical souls who might believe the fake non research so they can break that SUL barrier or whatever other reason people come up with for having to cut those 3 ounces.

But we do learn, and one thing I learned is that in fact, some people don't get this, only about 30% or so do. That means, for those counting, that a bit over 10% of PCT thruhikers get it every year, or maybe even more, though only 1/3 of them get sick from the stuff. That also explains the people who note they've never gotten sick, seems like there's variations in how sensitive one's body is to it, or something, those accounts can well be true, but have no meaning for your body.

Looking further at the abstract for the 1977 study in Colorado, I see some interesting numbers.

"A one-year retrospective laboratory survey in Colorado revealed that 691 (3%) of 22,743 stool examinations for ova and parasites were positive for Giardia lamblia, a higher percentage than that reported from surveys outside of Colorado."

That means that out of 100 people who had persistent stomach upset, went to the doctor, and convinced the doctor to order an ova and parasite test, only 3 of them were found to have giardia.

"a higher proportion of cases than controls who visited Colorado mountains (69% vs. 47%), camped out overnight (38% vs. 18%), and drank untreated mountain water (50% vs. 17%)"
So if 17% of the Colorado population of about 4 million in 1977 drank untreated water that would be 680,000 people. Out of these 691 tested positive for giardia, that is a 0.1% lab identified infection rate. This is assuming that all giardia cases in Colorado were caused by drinking untreated water, which of course is an overestimate.

So if you drank untreated water in Colorado in 1977 you had less than 1 in a 1000 chance of catching giardia so bad that you went to the doctor and got it lab tested and confirmed.

I'm a nerd but not a statistician, so if I am reading this wrong let me know.

Katy said: A one-year retrospective laboratory survey in Colorado revealed that 691 (3%) of 22,743 stool examinations for ova and parasites were positive for Giardia lamblia, a higher percentage than that reported from surveys outside of Colorado."

That means that out of 100 people who had persistent stomach upset, went to the doctor, and convinced the doctor to order an ova and parasite test, only 3 of them were found to have giardia.

I don't think that can be true. 3% is, as they noted, "a higher percentage than that reported from surveys outside of Colorado" [who have giardia] so it must be cross section of the public in general. I believe something over 2% of people in the U.S. have giardiasis. Most backpackers I know who are tested for giardia come up positive.

Oh, coincidentally, I was the guy Harald quoted above who was stupid enough to believe the "giardia myth" papers. That is what triggered my research.

> The "city water is dirtier than Sierra water" has been debunked.

Can you direct me to where this has been "debunked"? It's one of the three key pieces of evidence, and perhaps the most germane.

Thanks.

Bill S.

To quote what I've written earlier: A major theme in the Rockwell paper is that Sierra water on average is purer than San Francisco and LA water. Except for two big factors: these cities filter and/or treat their water supplies, so they are undoubtedly much safer than untreated Sierra water. And hikers don't drink an "average" canteen of water. Their water bottle might contain no Giardia cysts, or they might get enough cysts to send them to the hospital.

http://bucktrack.blogspot.com/2011/03/waterborne-giardia-for-backpackers-no.html

I would also add that the giardia numbers used for the Sierras are decades old and undependable.

Bruce,

In my opinion, if you want to refute the validity of the comparisons among Sierra and California city water supplies, you need to dig deeper into the data and show exactly where and how they're flawed. I have only looked at a few of the reports in detail, but if I have understood and recall them correctly, the Hetch Hetchy water feeding the Bay area was considered clean enough not to require filtration. If that's the case, then the count comparison per se may well be valid.

What you may want to look into is whether there's evidence that would bear upon the viability of those cysts. If I've understood correctly, the water does get chlorinated, and while the relevant chlorine concentrations would be inadequate for purifying a more highly contaminated source in the field, they may be enough – over an extended period of exposure – to knock the viable cyst count down significantly (i.e., to a level well below that of contaminated Sierra sources). Just to be clear, I'm speculating as to what you might find, but I think that's the place to be looking for clues.

The other place, as I've intimated before, is the original data on the number of cysts required for infection. Some oft-cited sources have taken the data to say that infection cannot occur below a 10 cyst exposure (or that the probably is dramatically lower at lower exposures), a clear mis-interpretation of the findings. The implications for the potential infectivity of water with low level contamination are huge. Finding ten in a given liter of water at typical measured Sierra levels is enormously improbable. Finding one is going to happen a lot more often.

And you are right, of course, that it's inappropriate to draw conclusions that assume that cysts are randomly distributed in Sierra water. This profoundly alters the odds that have been calculated based on measured concentrations (interestingly, unless one cyst is a tenth as likely to infect you as ten cysts).

Bill S.

At some point enough is enough on digging too deep in a blog post. The typical person is unlikely to read all the way through a long post with citations as it is. If it were a paper for peer review, you'd probably be right.

I know Hetch Hetchy water is run through water treatment plants and is treated with Chloramine. I found this quote: "Giardia cysts can be inactivated by free chlorine at 0 to 25 degrees Celsius, or with carbon dioxide, ozone, or chloramine." (What's In the Water? Climate Change, Waterborne Pathogens, and the Safety of the Rural Alaskan Water Supply) The Rockwell paper failed to mention treatment plants at all. One skeptic, a friend of Rockwell, pointed out it was because at the time they were using chlorine and chlorine is ineffective. If that's the truth, Rockwell was wrong on that as well:Due to a misstatement in the early literature, Giardia cysts have been considered for many years to be extremely resistant to disinfection…Several studies have shown that as little as 2 mg of chlorine per liter can kill greater than 99.8% of the Giardia cysts

http://www.tandfonline.com/doi/abs/10.1080/10643388809388341

I think we can agree that the whole city/Sierra concentrion argument is invalid from the get-go anyway because high concentrations of viable giardia cysts are dramatically more likely in the Sierras compared to a city hundreds of miles away, especially after it's been run through a water treatment plant.

Sorry to drill down on this, but if we are to discuss science we have to look carefully at the numbers in these studies and try to understand what they mean.

Katy said: (the quote is from the 1977 study abstract)
"A one-year retrospective laboratory survey in Colorado revealed that 691 (3%) of 22,743 stool examinations for ova and parasites were positive for Giardia lamblia, a higher percentage than that reported from surveys outside of Colorado."

That means that out of 100 people who had persistent stomach upset, went to the doctor, and convinced the doctor to order an ova and parasite test, only 3 of them were found to have giardia.

Bruce said:
I don't think that can be true.

Katy says:
True or not, what this study found was that 3% of the stool exams were positive.

Bruce said:
3% is, as they noted, "a higher percentage than that reported from surveys outside of Colorado" [who have giardia] so it must be cross section of the public in general.

Katy says:
If it was a cross section of the public that would mean that 22,743 healthy people in Colorado had a ova and parasite stool test done for research purposes in 1977. Seems unlikely.
Instead I believe "a retrospective laboratory survey" is a study where they do a statistical analysis of existing lab results.
So these ova and parasite stool exams were done on patients rather than healthy research subjects. Yet only 3% showed up positive for giardia.

Hi Katy,

It's good that you don't just accept things as they appear to be on the surface.

One thing I've learned early on in this research is to step back and ask "here's what the numbers seem to show. Does this make sense?"

I don't doubt the 3% number. I doubt highly that it was 3% of people that had symptoms that physicians believed warranted a giardiasis test, if that is what you are saying. The overall incidence of infection in the United States is estimated at 2% of the population (FDA.) Colorado has a reputation for a higher than average infection rate. I think the ability of a physician to diagnose giardiasis is much better than random chance.

Perhaps the tests were from people with unidentifiable stomach ailments. I don't know. Too bad there isn't free public access to the full report.

Thanks for the skepticism!

Does it matter if people are infected with no symptoms? They might pass it on to other people or animals, but we should already treat fec es as though it's infected with whatever. One theory says we're too hygenic – if we are exposed to some bugs we're less likely to have auto-immune diseases.

Another question is, does treating water prevent you from getting infected? maybe with or without symptoms?

Like, people say that they got Giardiosis from swimming – but that's irrelevant to the question of treating water. Or people have said they got Giardiosis even though they treated their water – maybe that would indicate that it's hopeless so don't worry about it or you have to do it more carefully.

A good study would be to select 100 people at a trailhead, randomly either select no water treatment or either of several treatments, maybe give them good instruction how to properly treat, test them before and after a backpack trip. You'de still have a bias because only some people would agree to it.