Giardia: Let’s Talk SCIENCE

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?

Best,

Bill S.

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"

9
Juranek, Dennis D.: Giardiasis. Centers for Disease Control and Prevention, 1990
10
Swartz, Morton N.: Intestinal Protozoan Infections. Scientific American Medicine, 1994
11
Kerasote, Ted: Great Outdoors; Drops to Drink. Audubon, July 1986
13
Backer, Howard D.: Giardiasis: An Elusive Cause of Gastrointestinal Distress. The Physician and Sportsmedicine, Vol. 28 no. 7, July 2000
41
Ortega, Y.R. et al: Giardia: Overview and update. Clinical Infectious Diseases, Vol. 25, 1997
42
Rendtorff, R: The Experimental Transmission of Human Intestinal Protozoan Parasites. American Journal of Hygiene, Vol. 59, 1954

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."

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.

John,

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?

Best,

Bill S.

And it's wise to remember that the only two cited before/after studies I could find showed at least 5% of people were picking up giardiasis on a single camping trip.

So far "all roads lead to Rendtorff" for infectious dose experiments. As has been said, that experiment does not support Rockwell's 10 cysts minimum for an infectious dose claim which is a cornerstone of his whole paper.

> showed at least 5% of people were picking up giardiasis on a single camping trip.
I am willing to believe that giardia cysts can be found in the wild in some places – I KNOW they are there (in some places) from personal experience.

But the idea that AT LEAST FIVE PERCENT get giardia from a single camping trip??????? That I simply do NOT believe. No way at all. Our Forum threads would be drowing in reports. They aren't.

Question: did those studies actually check faecal matter for giardia cysts by microscope every time, or did they just believe the walker's claims? I have my suspicions.

Cheers

Roger,

It seems obvious you still haven’t read what I’ve written. I’ve got clickable links above at 09/27/2012 20:44:53.

From the first study: Twenty-five…campers had stools examined before and after a subsequent hiking trip in another area of Utah; none had Giardia cysts before, but 6 (24%) had them after return

A quote from my post about study #2, which also tested campers before AND after their trip:

[Welch] goes on to say: “An excellent effort at such a study, however, was recently reported by ZelI and Sorenson [17]. Although 16% of a cohort they studied developed transient gastrointestinal illness following a visit to an area of high use, none developed symptomatic giardiasis.” I agree that it is one of the very best studies of it’s kind. However, what Welch doesn’t say is that one person was empirically diagnosed with giardiasis, was treated, and quickly recovered. Giardia is often not detected with a single test. Two more people tested positive for giardia but were asymptomatic at the time of the last test. About 2/3 of Giardia carriers are asymptomatic which would match this study exactly. So it is certain at least 5.7% got Giardiasis, and it seems more than likely that 3 of 35 got giardiasis, for a total of 8 1/2%. On a SINGLE TRIP. To me, Welch went far beyond a little spin in trying to make a point.

A major goal of these studies was to see if people actually WERE getting giardiasis in the wild. Skeptics were arguing there was no proof it was happening. They argued that “it probably wasn’t giardiasis” and “almost no one gets tested.” They still do so even after these studies were done.

See this poll

In that poll almost 22% have had giardiasis. Almost half of those cases were confirmed by lab tests.

With apologies to those for whom this may be review, let's go back to the question of what the relevant scientific literature says about whether you can get giardiasis from ingesting one cyst and the six papers that were cited above.

* Juranek, Dennis D.: Giardiasis. Centers for Disease Control and Prevention, 1990


This is no longer available on the CDC website. Current references on the CDC website all go back to Rendtorff (see below).

* 
Swartz, Morton N.: Intestinal Protozoan Infections. Scientific American Medicine, 1994


I do not have access to this, but it appears to be a collection of reviews, not original research.

* Kerasote, Ted: Great Outdoors; Drops to Drink. Audubon, July 1986


Kerasote is an author. I don't have access to the article, but it seems reasonable to expect that this was not a publication describing independent experimental investigation of infectivity. If anyone really doubts this, we could ask him.

Backer, Howard D.: Giardiasis: An Elusive Cause of Gastrointestinal Distress. The Physician and Sportsmedicine, Vol. 28 no. 7, July 2000

This is a review, to which I do not have full text access, but as a review, it would not normally be expected to contain new experimental findings, and there is nothing in the abstract to indicate new findings by the author.

FWIW, the abstract concludes by saying ”Prevention for people involved in outdoor sports or recreation and for international travelers includes treatment of all surface water and rigorous personal hygiene.”

* Ortega, Y.R. et al: Giardia: Overview and update. Clinical Infectious Diseases, Vol. 25, 1997


Ortega and Adam is a review article, which I have read. They are commendably circumspect in their interpretation of Rendtorff's findings: “Infections may result from the ingestion of 10 or fewer Giardia cysts."

FWIW, their concluding statements include “Since G. lamblia is frequently found in lakes and streams —
even in remote areas — hikers and backpackers should be warned to boil or filter water prior to ingestion.”

* Rendtorff, R: The Experimental Transmission of Human Intestinal Protozoan Parasites. American Journal of Hygiene, Vol. 59, 1954

In the absence of evidence to the contrary, it appears that this is the only paper that investigated infectious dose. 0/5 people who got one cyst were infected (subsequently showed cysts in their stools). 2/2 people who got 10 cysts were infected. 6/20 who got 25 cysts were infected.

That's it – the sum total of all the scientific evidence as to whether a low number of cysts can cause infection. Maybe it's my own biases, but when I hear things like "theoretically possible," I'm thinking maybe one in a thousand, or even one in a million. Things that people have looked very hard to find. What does five people tell us? Suppose, for a moment, that the real rate infection for a single cyst is 10% (if I had to bet, I'd bet it's lower, but let's just suppose that for a moment). Could Rendtorff have expected to see an infection among just five people? Nope, he would have had a better chance of getting a hit by spinning a roulette wheel and picking red or black.

He doesn't deal with it explicitly, but Rendtorff understood this. His conclusions had to be based only on what he *had* seen, and he wrote in his conclusions, “The fact that only a few cysts will establish the infection indicates that avenues of infection where it is probable that only a small number of cysts would be transferred may be of importance.”

I'm not looking to dissect every potential flaw in Rockwell's and similar assessments of Giardia risks, but I do think it's valuable for people to understand that there are some common misconceptions and misunderstandings that can arise from a casual reading of it *and* other writings (on both sides of arguments as to whether Giardia should be a major concern for backpackers) that deal with the subject.

If you read Rockwell, for instance, as saying that you're probably not going to get giardiasis from a given drink of reasonably well-selected Sierra water, then you need read no further.

But if you read it as saying any of the following, caveat emptor:

1) If you read it as saying that a significant body of scientific evidence shows that if you haven't ingested at least 10 cysts, your chances of getting giardiasis are effectively nil, you might want to look at the evidence. There's one paper, with five people who were given one cyst each. Five people.

2) If you read through a set of calculations quantifying your astronomically low chances of ingesting 10 cysts within given amounts of water, you might want to look at whether those are the right calculations. Is 10 cysts really the right number to be using (see above)? Even if there is a threshold, should we be making calculations as if cysts are randomly distributed? (Credit to Rockwell for pointing out this caveat himself.) Even if there is a threshold, are there any relevant data as to the time-frame within which the cysts must be consumed? Even a few tens of liters sounds like a pretty ridiculous amount to drink, unless you're talking about a week or two.

3) If you read it as saying that untreated Sierra water is safer than what millions of people in SF and LA drink, you might want to look at the evidence. That SF and LA water has been treated in ways that inactivate some proportion of the cysts (potentially a very large proportion of the cysts, depending on some details that are not readily available) so that they're not infectious.

If I've misinterpreted the state of scientific evidence wrt any of these three things, I welcome having those misinterpretations pointed out and look forward to re-examining these issues. In the course of this discussion, I've learned quite a few things I didn't know, and I am eager to learn more.

Thanks sbill9000 for all the hard work. Again, that's the kind of data-based discussion I wanted to see here.

Seemingly convincing quotes like the one below are extremely misleading because they use bogus infectious dose numbers. They are also based on dated studies done in limited areas.

…of those sources that had cysts present, the amount was ridiculously low – nowhere near enough to make you sick, considering you must ingest approximately twenty viable cysts to develop giardiasis…Therefore, you would have had to
drink 132 gallons of untreated Sierra Nevada water in 24 hours to get giardiasis… (Giardia Myth-Buster: How Rumor and Paranoia Have Created a False Industry Standard) [my bold]

Another thing is that the whole focus here seems to be on cysts. There are trophozoans as well. Of course they are much easier to filter out, but if they get in you, you can get sick.

I can see this coming. In a few years we will be trying to do DNA testing on giardia cysts.

–B.G.–

Hi Buck

First of all, please remember that a few anecdotes do not make for credible science, even if they are true. It is far too easy to cherry-pick such data, and that happens all the time. Very often it is not even intentional.

> From the first study: Twenty-five…campers had stools examined before and after a
> subsequent hiking trip in another area of Utah; none had Giardia cysts before, but
> 6 (24%) had them after return
I took this to mean they were all on one trip. OK, so that's one case where giardia was spread. However, I wonder where it came from? Perhaps there was an asymptomatic carrier in the party, and a general lack of hygiene on everyone's part led to others getting Giardia from him? An entirely believeable scenario, but valueless for scientific purposes.

> Although 16% of a cohort they studied developed transient gastrointestinal illness
> following a visit to an area of high use, none developed symptomatic giardiasis."
I would interpret that in the same way: E coli infections spread from one person by poor hygiene. Happens all the time.

> what Welch doesn’t say is that one person was empirically diagnosed with
> giardiasis,was treated, and quickly recovered.
Empirically? They guessed? And the gastro infection went away in the few days while he was being treated for Giardia? Entirely believable, common for gastro, but certainly NOT a proof of Giardia.

> So it is certain at least 5.7% got Giardiasis, and it seems more than likely that 3 of 35 got
When I see a figure like 5.7% quoted for a study involving just 35 people, I get very cynical. With 35 people you can't have an accuracy any better than ~3%, so quoting to one decimal place is futile.

> In that poll almost 22% have had giardiasis.
A meaningless result. What happens in such polls is that the only respondants are those who thought they did get Giardiasis. The vast majority who did not get sick don't bother to reply. It might make good headlines, but it is not science.

Sorry, but …
Cheers

Good stuff as usualsbill9000. You said:

3) If you read it as saying that untreated Sierra water is safer than what millions of people in SF and LA drink, you might want to look at the evidence. That SF and LA water has been treated in ways that inactivate some proportion of the cysts (potentially a very large proportion of the cysts, depending on some details that are not readily available) so that they're not infectious.

The reality is the SF/LA city water is going to be far, far safer than random untreated Sierra water.
Beginning in 2011, UV light will be implemented for primary disinfection of
Hetch Hetchy water source in addition to chlorine. Second, chloramine is formed to prevent microorganisms from growing in the pipes, which distribute water to the customers. http://www.sfwater.org/modules/showdocument.aspx?documentid=960

Here's how Rockwell put it: the US Environmental Protection Agency and the California Department of Health Services have granted Hetch Hetchy water a filtration exemption, meaning that filtration treatment to ensure its safety from Giardia and other organisms is not required

Makes it sound like the raw water is safe as is so Sierra Mountain water must be safer yet. That's clearly not the case.

Here's how the Metropolitan Water District of Southern California treats it's water:

1.Screening with relativelly coarse sieves removes larger debris such as sticks and leaves.
2.Primary disinfection with chlorine. Several of Metropolitan's treatment plants are now using ozone instead of chlorine as a primary disinfectant.
3.Addition of flocculant chemicals such as ferric chloride and organic polymers to encourage smaller particles to clump together.
4.Sedimentation to allow the clumps to settle out of the water.
5.Filtration to remove remaining particles.
6.Next, more chlorine is added to the water to disinfect and prevent any illness due to water-borne pathogens.
7.Lastly, ammonia is added to the water to react with the chlorine to form chloramines. Chloramines are used to maintain a residual disinfectant throughout the water distribution system.
8.After purification, sodium hydroxide is added to adjust the pH level to protect the pipes and plumbing fixtures, and fluoride is added to the water to help prevent dental caries in children.

http://en.wikipedia.org/wiki/Metropolitan_Water_District_of_Southern_California#Purification_and_treatment

Roger,

My two cited scientific studies are not cherry-picked anecdotes so please don't refer to them as such.

You said: I took this to mean they were all on one trip. OK, so that's one case where giardia was spread. However, I wonder where it came from? Perhaps there was an asymptomatic carrier in the party, and a general lack of hygiene on everyone's part led to others getting Giardia from him? An entirely believeable scenario, but valueless for scientific purposes. Pure speculation on your part about hygiene. The scenario is perfectly valuable scientifically because it directly refutes your opinion But the idea that AT LEAST FIVE PERCENT get giardia from a single camping trip??????? That I simply do NOT believe. No way at all

You said Empirically? They guessed? And the gastro infection went away in the few days while he was being treated for Giardia? Entirely believable, common for gastro, but certainly NOT a proof of Giardia. Nobody said it was proof for that single case. And yes, the physician "just guessed." Based on years of medical school and subsequent experience. That's why I called it empirically treated. Of course, the other two people had lab confirmed giardiasis, (5.7% of the group) which also refutes your opinion But the idea that AT LEAST FIVE PERCENT get giardia from a single camping trip??????? That I simply do NOT believe. No way at all

You said > [In that poll almost 22% have had giardiasis.]
A meaningless result. What happens in such polls is that the only respondants are those who thought they did get Giardiasis. The vast majority who did not get sick don't bother to reply. It might make good headlines, but it is not science

78% percent of responsdents said they have never had giardia, thus refuting your statement. The poll also refutes your earlier statement "very rarely is the patient actually tested for the presence of giardia cysts." If it's not science, it certainly is of more value than any single person's usubstantiated opinion, including mine.

"My two cited scientific studies are not cherry-picked anecdotes so please don't refer to them as such."

Buck, if you read Roger's words more carefully, he did not refer to your anecdotes as cherry-picked.

–B.G.–

I can add a little something regarding clinical diagnosis of Giardiasis.
I warn you though.. IF YOU ARE EATING OR ABOUT TO EAT DON"T READ THIS NOW!

In 2009 while on a long hike i contracted an intestinal illness.

The first symptoms were loss of appetite(despite hiking 25 miles per day), lethargy, and uncontroled diarhea with greasy black stools.
The first symptoms lasted a few days and lessened but returned again even stronger with new symptoms.
After ten days there was sulphurous gas, bloating, intense stomach cramps, clear mucousal diarhea and severe weightloss and dehydration.

After 20 days I had lost 30 lbs and the symptoms became so severe i sought medical help.

When i described the symptoms to the doctor one symtom caused him to clinch his diagnosis: Acute Giardiasis and perscribe metronidazole(Flagyl) right there on the spot without a fecal test.

That symptom was the clear, thick, mucousal discharge of diarhea.
The night before i went to the clinic i was hunkered down in my tent on the ski slope above hwy 50 in Tahoe.
I got up to relieve myself for the 10th time that hour and when i squatted down I filled the hole I had dug but could not stop the flow in time to dig another hole.
I estimate nearly a gallon of clear mucous came out of my body.
It really scared me.

The doctor said that in the acute phase the Giardia parasites can colonize nearly the entire small intestine.
The mucous they secreted combined with anything i ate or drank literally went right through me undigested.

okay.. here is the REALLY gross part and also the final factor in his clinical (best guess) diagnosis:
I had lost so much weight i forced myself to eat macaroni and cheese that final night.
The following morning I passed that macaroni and cheese and it was the same color and texture as if i had simply poured it out of my cup even after passing all the way through my gi tract.
Giardia had completely coated my intestine in mucous and nothing could get through.. i was starving.

My case was extreme. Most cases are not that cut and dried he said.

After 4 days on the Flagyl I was eating normally and back on trail.
By day 5 i got to the Soda Springs store near Donner pass and ate over 15,000 calories of food at one sitting.
By the end of that long hike I had regained all 30 lbs.

But Roger is quite correct. Neither that doctor or I can confirm scientifically that Giardiasis was what i had.
Thankfully for me though i did not have to wait 5 days or more for a test result before starting the antibiotic that brought about much needed relief.

I mention all this to il;lustrate that there are some pretty horrific symptoms related to Giardiasis that are far beyond your average case of E Coli poisoning.

Just to leave you with one final lovely detail… Immodium controlled the diarhea alright.. during the day that is.
Imagine a gallon of mucous spilling out from "down there" into your sleeping back in the middle of the night.

Any takers on a "slightly" stained and mysterious smelling WM ultralight?
To quote the Monkeys.. "I'm a believer"..

Perhaps there was an asymptomatic carrier in the party, and a general lack of hygiene on everyone's part led to others getting Giardia from him?
That is why I don't share food on the trail, however I see that most do and that is of the "dunk your hand in the same bag and pick up some nuts" type sharing.
BTW, having drunk water from mountain creeks all of my youth (cows,sheep,goats,horses drunk and pooped in there too…) it is possible that I am a carrier so better not take food from me either.

Matthew, you are correct with your warning to us.

"Thankfully for me though i did not have to wait 5 days or more for a test result before starting the antibiotic that brought about much needed relief."

Maybe it takes five days or more for a microscopic test result from some state-certified lab. However, I watched while one physician peered through a microscope at a slide for only a few minutes before concluding a diagnosis of giardia. Sometimes the symptoms are slight, and they have to look for a while to find cysts. But, as the symptoms become more serious (like yours) the population of cysts becomes more overwhelming in the microscope slide, so they become quicker and easier to estimate and identify.

–B.G.–

Same here – seldomly share food or water – I have my allocated food so if I ate someone else's or they ate mine I wouldn't have the right amount of food.

"it is possible that I am a carrier so better not take food from me either"

I may be an asymptomatic carrier also

I think contamination is from your other end, and there are other bad organisms there so you want to use good hygene regardless : )

Actually a fellow thru hiker in 2009 actually had a confimed test. I wont reveal his name here but he was not the only one.

I drank unfiltered water all my life too up until the PCT.
I decided to use chlorine bleach drops for the SoCal section but i quit using them around Kennedy Meadows mile 704.

Personally I never shared food but that does not rule out the possibility of someone transitting it to me on trail.
A common practice among most thru hikers i saw share food was to only dump it into a persons hand.

I also began using a spoon to eat things out of my Fritos bag or ziplock.

Just got back from a 7 day sierra trip where i never filtered or treated so having had Giardiasis has not changed my behavior.
If i get sick to my stomach it's one thing, Giardiasis is totally on another level of "stomache ache".
I would say don't be afraid but if you are sick for more than a week, have stomach cramps, greasy or mucousal stool it becomes more than just sick from bad hygene.

Edit:
Ya know.. I just realized i probably scared away every single chick on the internet.

Last year I got Giardiasis – tested for and confirmed by my doctor. As the symptoms started a week after my return from a High Sierra trip during which I drank exclusively untreated water, my doctor suspects I caught it during my trip. That is of course not "scientific" proof that I contracted it in the High Sierra by drinking untreated water. But for me the experience has been unpleasant enough to now treat every drop of water. Most likely it is not necessary 99.x % of the times, but after my experience I won't second-guess any longer.

Manfred

Ditto to what Manfred said. Happened to me this year in very similar circumstances. I too will be taking precautions in future.No repeat please!

Bob said "Buck, if you read Roger's words more carefully, he did not refer to your anecdotes as cherry-picked."

Roger's post following his "cherry-picked anecdotes" comment refers exclusively to my cited scientific studies, So I think that's exactly what he was insinuating.

I have cited numerous scientific studies and haven't been relying on anecdotal evidence, cherry-picked or otherwise, so I think it was an unfair comment regardless.

Those two "before and after" studies weren't cherry-picked among studies, either, they are the only two of the type that I could find.

Rodger can weigh in, but I read it as the studies cherry picked their data points, not that you cherry picked the studies.