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Giardia: Let's Talk SCIENCE
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Dave U
(FamilyGuy) - F

Locale: Rockies
Re: Re: Science and Giardia on 09/15/2012 14:46:17 MDT Print View

"It's all a crapshoot."

No pun intended....

Harald Hope
(hhope) - M

Locale: East Bay
the science on 09/15/2012 18:48:31 MDT Print View

Someone asked me a question that made me think of the science of giardia yesterday, or the day before, so I read up on all the government pdfs and pages.

I wish I'd seen this thread, so I could post all the urls for the pdf stuff I read, cdc etc, but the overall thing that all the real sources agreed on were simple:

backpacking is a key infection vector, or activity. All sources mentioned it specifically, without exception.

The reports and articles all stated that 1 cyst is all that is required. Something like 1 to 10 cysts per liter can be expected in running water (assuming infected matter upstream or in your area), that's a number all the sources seem agreed on, and would account for the dilution factor. I kept looking for that 10 cyst ingestion requirement number I've seen referenced in backpacking sources like bpl, but not one of the gov webpages or papers mentioned that higher count of 10 being required, in fact, the only place I saw it mentioned was places like bpl or other backpacking oriented information pages/sites. BPL is not a health science resource, so I don't care what people believe or don't believe, but if that's a myth, it should be terminated and stopped before people get hurt.

One page was kind of interesting, and was discussing why beavers were associated with it, and it noted that association was almost always caused by beavers being close to campgrounds, and getting infected by campers, then their fe ces contaminates the water, and so on.

The notion I come across that one can 'see' if water is good or not good struck me as particularly fantasy based, if there is some fecal matter in the water upstream from where you are, it's not good, and you certainly cannot see that fact, or smell it, maybe you can intuit it if you have a particularly excellent connection with nature and it's flows and patterns, but for average use, you certainly cannot do any such thing.

Again, I wish I'd seen this thread, I'd post every article I read, but they really seemed to agree in general. The two vectors were water and spread fe ces, which actually explains the observation that people make about handling other backpacker's food and getting giardia after that, that fits exactly, poor sanitary habits, then touching the food, then you touching it certainly could spread it, and that seems to be a very common vector.

The thing I was looking for, as scientific fact, not speculation, was specifically, how long giardia cysts can live out of the water, when it's dry. What confuses that question is a common fector, fe ces on doorknobs, trailmix etc, in trace amounts, which I guess lets giardia move to your body. I didn't find anywhere that told me that, but I did find some interesting stuff about temperature and cyst survival, in water, it's a few months when cooler, but drops steadily when warmer, if i remember right, 54C water they lived only 10 minutes, for example, boiling water I believe killed them almost immediately.

I just footnote this by observing that the cdc does not have a wonderful track record, and often lags behind the times, but they lag from what I have seen in underestimating, not overestimating, risk factors. Every single government document listed backpacking specifically as a source of infections, without exception.

This fits nicely with what Edward Abbey noted many years ago in Desert Solitaire, when he observed that prior to 1965 (or was it 1955? been a long time since I read it) or so, every free flowing water source in the USA was drinkable, talking about streams and so on, not of course rivers that flow through farming and urban areas. It took only some 10 years for that situation to change, due to live stock grazing on leased national park lands and backpackers practicing poor sanitation. Since we can read long threads covering people's experiences with that poor sanitation, ie, toilet paper on trails, non buried or inadequately buried waste, I'd certainly consider that to be a primary vector at this point, along with animals also spreading it.

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?

As correctly noted in the sources, you can't test a stream for good or bad status, it's good when there's no contaminated fe ces upstream, and bad when there is. The beaver thing above mentioned that issue directly, and it's kind of obvious anyway, in cooler water giardia cysts only lived some 2 months max. Cysts floating in the water have no smell or flavor. Heavily contaminated water of course would be suspect as dirty, but water being clean shows you almost nothing.

This is why I carry a real filter, mechanical. What one paper explicitly stated was that bleach, chlorine, does NOT kill giardia, that's why people use aquamira, it's a tweaked bleach, so it would be good for that myth to be terminated once and for all, bleach does not kill giardia, unless the science is wrong, their outsides are too tough. Bleach could be a good option post filtering however, to kill smaller things, that's probably a good idea in very bad water.

Speaking for myself, I don't care what people do, but since I do somewhat try to follow the science of such matters, I'm not into pointless risks (for example, if I am carrying a filter already, why would I not use it? Sawyer squeeze is light and leaves no taste behind) when it comes to contracting long term weird parasite stuff, it's really not fun, trust me.

You can find all the cdc and other gov stuff easily, just google giardia and pick the authority sites, read the pdfs, etc. Maybe there's some other science out there, could well be, but that would be in scientific journals and medical journals, which usually are not free to read.

By the way, the bpl word filter really needs to be updated, it's kind of silly, this is an adult oriented site, grownups can handle the word fe ces.

Edited by hhope on 09/15/2012 18:59:10 MDT.

John S.
(jshann) - F
Re: the science on 09/15/2012 19:37:33 MDT Print View

"The thing I was looking for, as scientific fact, not speculation, was specifically, how long giardia cysts can live out of the water, when it's dry."

From Rockwell's paper
"Cysts can survive for as long as 2 to 3 months in cold water, but they cannot tolerate drying or freezing.12, 13, 15, 16, 17, 18 They are also destroyed by UV radiation, heat and biocides such as bleach.15"

"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.8 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"

Edited by jshann on 09/15/2012 19:39:40 MDT.

Harald Hope
(hhope) - M

Locale: East Bay
not science, not peer reviewed on 09/15/2012 20:08:58 MDT Print View

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.

I can point you to countless scientists writing outside their fields of expertise who say the darnedest stuff, doesn't mean it's wrong or right, but it's not technical science, it's not peer reviewed, science is sort of like a medieval guild system, has its rules and so on, and each discipline has its own methods and preferences.

The area he is right in, and in full agreement with the science, is that spreading via touch of contaminated materials is very likely.

I'm happy for people who do this practice and who can never get sick from water, I really am, but that's not science, it's just sort of anecdotal.

The CDC does not, as he suggests, take a 'very conservative stance', just follow their horrible record re Lyme disease, where they consistently have failed to take such a stance, nor does the general US system, which likes to err on the side of error, not caution.

I really wish this guy's paper would stop getting cited as some type of valid science, it's just an article he wrote. Can't we do a bit better? Plus, so he hikes in a few areas of the Sierras, and doesn't get sick, that proves exactly what, regarding other areas? Nothing. Maybe he's fond of high country, where most water is almost direct snow runoff, I don't know.

The thing of value though is to note that people are getting contaminated, and that they are spreading that contamination by touch.

I've read a lot of stuff on different topics over the past year, so the way data spreads is interesting to watch, but there's some common sense logic stuff that's not really happening with this guy, start here: is high country grazing leasing increasing, decreasing, or is it about the same? How about backpacker numbers? Higher, lower, or about the same? Science is VERY expensive to do, testing, etc, and it's very tedious, so it often takes a real kick in the pants to get it done, this guy Rockwell isn't doing it though.

to me the entire discussion is moot, it's a risk benefit thing, it costs me n othing to filter since I always will carry the filter, and if I carry it, I will use it.

The thing that does interest me however is another question, about using poison on living water, that to me is another matter altogether, there's hordes of good organisms in things we eat and drink, and poisoning them seems to be a very bad idea in terms of actually benefiting from being out there in the wilds, so I think in that sense I somewhat agree with Rockwell, drink the water as near fresh as you can manage, don't kill it. Giardia cysts are big and easy to filter out, so who cares if there are x or y infection rates, the Sierras are just one place, and the high sierras part of that place.

Humanity and its livestock are spreading, not shrinking, the ranges we occupy are increasing, not shrinking, the biosphere damage is getting worse, not better, to believe otherwise is to be somewhat oblivious to modern reality, so I bring a filter, it's easy and simple, I truly do not care what others do, nor do I care if people invent their own stories to make themselves happy. The sad thing is it would be so hard to actually do real science on this question, it would take years and even then it wouldn't be conclusive.

Some things in modern backpacking chit chat are fun to argue about, some to me there's no argument needed, this is definitely for me the latter category. But it's getting less and less likely real science will get done on this, we are running out of money and time, things are getting cut left and right, particularly in California, that will include various testing and regulatory bodies.

I also believe, by the way, that certain people if they do what they have always done tend to avoid problems, and are somewhat justified to note that it works for them. The idea is simple, ride a bike your whole life, no accident, get in a car, bang, you're not in the same groove, not following the same way.

Anyway, I just wanted to note the fact that there really is no science beyond this one guy's non peer reviewed article, and I'm glad to see that's the case, there isn't. Not that I worship science as some kind of deity that cannot be wrong, by the way, that's certainly not the case, but if we are in the world science defines, the rules there sort of have to be followed at some point. And science can be so hard and tedious, easier to just chat on forum threads.

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

Locale: Silicon Valley
Re: not science, not peer reviewed on 09/15/2012 20:12:04 MDT Print View

"science is sort of like a medieval guild system"

Egad. Don't let Roger Caffin read that.

Maybe he is out looking for a job as an alchemist.

--B.G.--

John S.
(jshann) - F
Re: not science, not peer reviewed on 09/15/2012 20:39:01 MDT Print View

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.

John S.
(jshann) - F
Re: the science on 09/15/2012 20:47:48 MDT Print View

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.

John S.
(jshann) - F
Re: Re: the science on 09/15/2012 20:50:42 MDT Print View

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.

Harald Hope
(hhope) - M

Locale: East Bay
old stuff on 09/15/2012 21:17:43 MDT Print View

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.

Edited by hhope on 09/15/2012 21:36:43 MDT.

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

Locale: Silicon Valley
Re: old stuff on 09/15/2012 21:41:18 MDT Print View

"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.--

Bill Segraves
(sbill9000) - F - M
infection rate per cyst on 09/16/2012 05:27:26 MDT Print View

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.

Buck Nelson
(Colter) - MLife

Locale: Alaska
Giardia Cyst Counts, etc on 09/16/2012 14:24:21 MDT Print View

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.

Edited by Colter on 09/16/2012 14:32:14 MDT.

Bill Law
(williamlaw) - M

Locale: SF Bay Area
Re: not science, not peer reviewed on 09/16/2012 16:24:38 MDT Print View

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.

Buck Nelson
(Colter) - MLife

Locale: Alaska
Peer review on 09/16/2012 17:38:15 MDT Print View

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 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!

Edited by Colter on 09/16/2012 17:43:40 MDT.

Bill Law
(williamlaw) - M

Locale: SF Bay Area
Re: old stuff on 09/16/2012 17:42:18 MDT Print View

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."

Katy Anderson
(KatyAnderson) - F
math on 09/16/2012 17:49:47 MDT Print View

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.

Bill Law
(williamlaw) - M

Locale: SF Bay Area
Re: Peer review on 09/16/2012 18:06:43 MDT Print View

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.

Buck Nelson
(Colter) - MLife

Locale: Alaska
Control Groups on 09/16/2012 18:23:22 MDT Print View

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?

Katy Anderson
(KatyAnderson) - F
Re: Control groups on 09/16/2012 18:42:02 MDT Print View

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

Buck Nelson
(Colter) - MLife

Locale: Alaska
re: re: Peer review on 09/16/2012 19:58:21 MDT Print View

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]."

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?


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!