I'd read that chart to suggest that giardia detection is not perfect. Also that fewer than 10 could be infectious for many people. Depending on how reliable detection is, it could be a lot fewer than 10.

"I'd read that chart to suggest that giardia detection is not perfect."

Exactly.

The infection rate from 25 cysts is lower than the rate from 10 cysts. It seems illogical.

In another reference, it was stated that 10-25 cysts are capable of causing clinical disease in 8 of 25 subjects. What was not stated is that [maybe] 1-9 cysts are capable of causing clinical disease in [maybe] 2 of 25 subjects. I just don't like those odds.

The whole subject is difficult to analyze with any precision.

Maybe it is best to just treat your own water and hike your own hike.

--B.G.--

"The infection rate from 25 cysts is lower than the rate from 10 cysts. It seems illogical.

In another reference, it was stated that 10-25 cysts are capable of causing clinical disease in 8 of 25 subjects. What was not stated is that [maybe] 1-9 cysts are capable of causing clinical disease in [maybe] 2 of 25 subjects. I just don't like those odds."


As a non-epidemiological research scientist I would have to say that these are all very small sample sizes (N). The lower rate for the 25 cysts is probably a fluke or maybe it's from a different study? The chart has a majority of tests where N = 2, so if one person didn't get it, then it's 50%. But that's a very poor statistic because it relies on a single count. Roughly, the error on 2 is about 1 1/2 so it would be statistically the same to have 1 infections, 2 infections or even none. It's like the old joke: "One third of the lab rats improved, one third didn't improve and the other one got away." You would have to test with about 100 people to get the infection rate accurate to 10%.

From your reference, 10-25 cysts caused disease in 8 of 25, so the uncertainty on 8 is about 3 (it goes by the root of N) From this, the odds are about 50-50 with 10-25 cysts. Again, I wouldn't read any more into these numbers, the point is that very small numbers of cysts (on the order of 10) can cause illness.

I largely agree with what you fellows are seeing. There are sweeping conclusions based on a very tiny database form a single study. What you absolutely cannot conclude based on that study is you can't get Giardia with less than 10 cysts. With a little critical thinking one can conclude it certainly does happen. How often, who knows.

But the "debunkers" say things like this: "you must ingest approximately twenty viable cysts to develop giardiasis." (Schlimmer) Rockwell's odds are based on 10 cysts minimum, as are those of Welch, Derlet, and apparently all the major "skeptics." (Usually they are based on old, inefficient water testing methods, also.) However, even Rockwell says How many cysts does it take to get the disease? Theoretically, only one.

Giardia is also highly infectious—one cyst has a 2 percent probability of causing
giardiasis. http://www.wcponline.com/pdf/0902warwick.pdf

Anyone want to bet they won't drink one cyst when about 1/3 of the water sources in the Sierra, for example, were found to have Giardia, and an infected person might shed 1-10 billion cysts daily? http://www.cdc.gov/parasites/giardia/epi.html

http://www.modbee.com/2010/05/08/1158938/fouled-waters-sierra-lakes-streams.html#

Here is an article by the Professor mentioned in the article

http://www.sierranaturenotes.com/naturenotes/DerletWater.htm

Edited by gg-man on 03/01/2011 13:01:27 MST.

Comments welcome

A few items are missing but I don't remember where I read them, sorry.
1) More beaver get the disease from humans than humans get it from beaver.
2) Nothing was mentioned of aquired imunity. I know of several families in the ADK's that regularly drink water from spring boxes and shallow wells. Yet they don't seem to have trouble from the water. The water sources are highly suspect.
3) You mention treatments, but do not mention specific types. Some is more effective than others.
4) You do not mention if there were any specific types of minerals/salts in water that may contribute to the eventual infectous ability of the critters.

Just a few thoughts, nothing wrong with it as a light intro though.

"4) You do not mention if there were any specific types of minerals/salts in water that may contribute to the eventual infectous ability of the critters."

I've never seen any mention of giardia with dissolved minerals or salts in the water.

Salts such as sodium chloride are extremely soluble. When they leave the rock deposits (that maybe were ocean bottoms 200 million years ago), they tend to wash down into low elevation basins where we find them today. However, I tend to be found backpacking at high elevations where the water is cold and hard and relatively pure. But I treat it anyway.

--B.G.--

"Perhaps they picked it up in the very same pool two weeks before."

That still begs the question of how the person(s) who originally infected the pool acquired giardiasis. Or how senior citizens, not exactly a backpacking crowd, acquire it. As far as agricultural workers are concerned, I was wondering if they might vector it in from Mexico, where the water is not exactly pristine in many areas, not whether they acquire it in the fields after they arrive. In other words, how do those with giardiasis other than backpackers, who account for a relatively small number of the total cases, acquire it? Anybody know?

"Anybody know?"

Tom, you seem to have a problem with this.

They get it by oral ingestion. Every time.

Anybody can get it if they drink contaminated water, or if they drink or eat out of a contaminated vessel. If they get food from somebody else whose hands are unclean and possibly contaminated, that is just one of many ways.

The first time that I was around a person contracting giardiasis, it was on a trek in Nepal. I can safely say that the person did not drink any open water. The Sherpa cook and cook boys were washing cook gear, plates, and all that. Maybe somebody rinsed something with impure water. Then one week later, that person thought that death was near.

--B.G.--

"Tom, you seem to have a problem with this.

They get it by oral ingestion. Every time."

Bob,

For a guy who seems to know everything about darn near everything, you seem to have unexpected difficulty understanding relatively simple questions in plain English. More likely, you're twisting my questions for reasons I don't understand, nor do I care. Now here's one you should be able to understand: If you don't have the answer, why not just shut up and stop wasting my time? Nobody's going to think the less of you for not knowing the answer to every question.

Tom, you are trying to confuse us here. I did have the answer and I described the answer. I didn't want to get too graphic about it. You seem to reject that answer and the similar answers in other threads here. Now you can seek another answer that you prefer.

--B.G.--

Grandma gets it fed to her by the college volunteer who lived abroad for a year and caught it from sharing a bicycle with hairdresser from Italy that had a boyfriend who climbed everest with a Contaminated Sherpa who caught it from a cook who diluted the soup with unclean water.

So obvious.

"Tom, you are trying to confuse us here. I did have the answer and I described the answer."

You do sound confused. I apologize, Bob. I'll withdraw my question, because I just don't know how to make it any simpler. Guess I'll have to go out and do the digging myself. Night, night.

"Grandma gets it fed to her by the college volunteer who lived abroad for a year and caught it from sharing a bicycle with hairdresser from Italy that had a boyfriend who climbed everest with a Contaminated Sherpa who caught it from a cook who diluted the soup with unclean water."

Elementary, my Dear Cameron. ;-)

Something along those lines would be my hypothesis as well but, if I want to know for sure, I'll probably have to get out on the web and do some research. I was hoping someone here would have beat me to the punch. I'm getting a bit lazy, I guess. I was hoping our resident oracle could help me out, but he seems to be hung up on proximate causes.

What about pets as a vector for giardia transmission ?
A dog swims in water, then interacts with owner while they share a sandwich, for example.

Any mammal can harbor the organism.

I have no proof of this, but a park ranger told me that was the reason why dogs are not allowed in the backcountry of Yosemite and most other national parks. Of course, also they threaten wildlife. Dogs would drink raw water where they find it and defecate where they choose. The other national park safeguards about domestic animals were previously mentioned.

I like dogs, but I don't touch them when I have no convenient place for hand washing.

--B.G.--

Buck,
You didn't mention where in the Sierra you picked up the giardia. I would guess that is was south of Kennedy or north of Sonora. Much of your manifesto talks about your experience which frankly is a single data point and can be refuted by others who have their own "single data points." The researcher below suggests that there is little danger of giardia in the National Parks due to the restrictions on cattle. Unlike all of our antedotal evidence he has done the research and has the best data on water quality in the Sierra that I have seen.

http://www.sierranaturenotes.com/naturenotes/DerletWater.htm

Would love to hear your perspective on his conclusions.

"The researcher below suggests that there is little danger of giardia in the National Parks due to the restrictions on cattle. Unlike all of our antedotal evidence he has done the research and has the best data on water quality in the Sierra that I have seen."

Greg,

An excellent article, by far the best I have seen on the subject. It cleared up my last major concern, which concerns the presence, or not, of giardia in horse manure. Thanks for posting it.

North of Sonora. Does hiker hygiene go down on certain parts of the trail, or would it be because there's more Giardia in the water on that stretch of trail? That is the stretch where PCT hikers tend to get Giardia, isn't it?

I absolutely agree about anecdotal evidence and my story proves nothing standing on it's own. But most articles like what I wrote start with a personal lead-in. My story about believing the "debunkers," then not treating, and subsequently getting Giardia is about as relevant as it gets. I think the rest of my manifesto stands on it's own, with two scientific studies concluding hikers are getting giardia from drinking water.

I read the Derlet article. If you read a skeptical article about the dangers of giardiasis, they will likely be written by, or sourced to, Delret, Welch, Zell, or Rockwell, who in turn usually source each other. I think Zell is the most objective and has come closest to doing what needs to be done. Actually, it might not even be fair to call him a skeptic.

No one is ever going to prove whether hikers are getting Giardia from testing a few dozen sites and trying to extrapolate the results to all other untested water sources. For one thing, they never seem to take into account that any time you have one Giardia cyst that there were likely MILLIONS in a concentrated area when they first entered the water. Derlet himself said on REI.com Here's a fact I recently uncovered: Giardia-infected cattle excrete nearly 100 million Giardia cysts per day, and pack animals excrete about 12 million per day. Twelve million! So be careful if you drink in those watersheds. Well, jeez, thanks! I read that after I was convinced that it was paranoia to drink the water untreated in the Sierras. As I've related, my best guess is I got Giardia from a place where cattle had been grazing. Hadn't run across cattle grazing in probably a month. Didn't know they were there until after I drank. Of course, infected people excrete up to 10 BILLION a day.

What needs to be done is more extensive studies like Zell's to determine who is getting Giardia, and HOW they got it. Zell tested the hikers before and after in the Desolation Wilderness study. Between 5 and 8% or so got Giardia on their Sierra trip, except it didn't determine how they got it.

Seems to me some much larger, more carefully controlled studies like this informal poll could really help to paint a picture of what's going on: http://forums.outdoorsdirectory.com/showthread.php/94691-Giardia-Poll-Please-vote!/page2 The Colorado study I talked about on my blog where 3 times as many untreated-water drinkers got Giardia was good, because they studied 691 people who actually HAD laboratory confirmed Giardiasis. Less theories and more new research. Find out what actually DID happen and WHY.