Tuesday, March 01, 2011

Waterborne Giardia for Backpackers: No Myth

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.

Here are some undisputed facts. Giardia is very common. There are about 2 ½ million cases a year in the United States. Giardia cysts may be ingested with contaminated food or water, or acquired from unwashed hands. Most sickness experienced by backpackers is undoubtedly caused by things other than Giardia. The incubation period of Giardia is usually 3 to 25 days or longer.

What are the odds of getting Giardia while out backpacking? Anecdotal evidence is extremely undependable, but here are two relevant studies:

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. This source goes on to say Questionnaires returned by 133 of the campers showed that 5% had an illness compatible with giardiasis within 2 weeks after their trip. These surveys show that campers exposed to mountain stream water are at risk of acquiring giardiasis.

In another excellent study, 2 of 35 people got Giardia, but were asymptomatic. A third had Giardia symptoms, was treated for Giardia, and quickly recovered. Giardia is often not detected with a single test. So it is certain at least 5.7% got Giardia, and it seems more than likely that 3 of 35 got Giardia, for a total of 8 1/2%.

Another study shows a strong correlation between not treating mountain water and getting Giardia:

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. 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), and had responded to a course of metronidazole or quinacrine. 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. Also identified was a correlation between the seasonal distribution of cases and degree of fecal contamination of mountain streams. These results indicated that G. lamblia is endemic in Colorado and that drinking untreated mountain water is an important cause of endemic infection.

(Note, after I wrote this post I ran a poll on the Alaska Outdoors Forum. That poll also showed that people who didn't treat water contracted Giardiasis about 3 times as often. What actually happens in the real world trumps theories of what should be happening.)

The skeptics of waterborne Giardia transmission for backpackers tend to rely on the low numbers of cysts usually shown in backcountry drinking water, and statements to the effect that 10 or even 20 cysts are required for an “infectious dose.” But those statements are all based on one very limited study, (cited in Waterborne Transmission Of Giardiasis Proceedings Of A Symposium Held At Cincinnati Ohio On Sept 18-20, 1978 starting on page 64) a key part of which is shown below:

There is no biological reason why single cysts of Giardia would not also be infectious. The FDA says: one or more cysts may cause disease. One cyst compared to the 10 cysts Rockwell used to calculate the risk for water in the Sierra. By the time Zell did his study a few years later they were adjusting  for a cyst recovery rate of only 10%. If  we similarly adjust Rockwell's cited cyst counts and change the minimum infectious dose to one, that's two orders of magnitude! Something to ponder is that An infected person might shed 1-10 billion cysts daily in their feces...

The people who claim to have drank untreated water for years without getting sick are probably right. About 2/3 of Giardia carriers are asymptomatic. Others are lucky or resistent. Nonetheless, getting Giardia in the backcountry is very common.

Anyone may become infected with Giardia. However, those at greatest risk are:
(Giardia) Cysts have been found all months of the year in surface waters from the Arctic to the tropics in even the most pristine of surface waters...In Wisconsin, Archer et al (1995) found that all 18 streams sampled were positive for Giardia; the highest level was 2,601 cysts/100 L from a stream characterized as pristine.
That's 26 cysts per liter. In other words, you'd probably get Giardia if you were drinking from that stream. The EPA goes on to say Hancock et al. (1997) collected 463 groundwater samples from 199 sites in 23 states in the United States; Giardia cysts were found in 14% of the springs. Pick your water sources and you'll be safe? Maybe not.

One of the most egregiously misleading papers I've seen on this topic is Giardiasis as a threat to backpackers in the United States : a survey of state health departments
As quoted by Robert Rockwell: “Neither health department surveillance nor the medical literature supports the widely held perception that giardiasis is a significant risk to backpackers in the United States. In some respects, this situation resembles (the threat to beachgoers of) a shark attack: an extraordinarily rare event to which the public and press have seemingly devoted inappropriate attention.”

Giardiasis is widely under-reported because backpackers and campers normally seek medical attention one at a time... "Several backpackers appear weekly at Centinela Mammoth Hospital in Mammoth Lakes sick enough with giardiasis to need urgent care, said Dr. Jack Bertman, an emergency physician, who noted, 'We publicize it a great deal more in Mammoth.' " As rare as shark attack Dr. Welch? You know how many cases of Giardiasis were reported in that county (Modoc) in 2008-2010? Zero. Including my case. In that single county backpacker giardiasis was, technically, infininitely under-reported.

Welch looked at outbreaks. He says Nineteen of these outbreaks were attributed to consumption of contaminated drinking water; only two outbreaks were reported among individuals identified as campers or backpackers Giardiasis, although common among backpackers, rarely meets the definition of an outbreak. The Welch paper is very, very misleading. In Rockwell's paper he says this about water in the Sierra: One conclusion of this paper is that you can indeed contract giardiasis on visits to the high mountains of the Sierra Nevada, but it almost certainly won’t be from the water. So drink freely and confidently. From a more recent survey in the high Sierra which included seemingly pristine water: Nowhere is the water dirtier, he discovered, than on U.S. Forest Service land, including wilderness areas, where beef cattle and commercial pack stock — horses and mules — graze during the summer. There, bacterial contamination was easily high enough to sicken hikers with Giardia, E. coli and other diseases." "Drink freely" is bad advice.

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. Been there, done that, more than once. Won't do it again.

It's also important to recognize there are other protozoa, viruses and bacteria that can and have sickened backcountry travelers drinking untreated water. Some people have gotten by for long periods of time without treating, but the odds have a way with catching up with people. Here's a quote from an article about Scott Williamson, who has hiked the Pacific Crest Trail 13 times or more (!!!) Williamson does not filter or treat his water. 'I’ve been sick multiple times, I have had giardia…' (and then he ticks off a list of other parasites, but your trusty reporter was too dumbfounded to write them all down).'I am very selective about my water. If it looks like a heavily used area I will try to find cleaner water, but I have had to drink some nasty water. It saves time, I just dip and drink.' He does add, 'If you don’t want to be sick at some point, you have to always treat your water.'

Although giardiasis makes some people very sick indeed, it is treatable. If I somehow got in a position where I had a choice between drinking suspect water or getting dangerously dehydrated, I'd choose the former. Most stomach upsets resulting from backpacking are not Giardia, and hand washing before eating is important.

You should make your own choices about water treatment, but it's important to realize that the risks of getting Giardia from backcountry water are no myth.

Please comment with your corrections and feedback. I will edit this article as needed. If you like this article, please click the “Like” button!


  1. Anonymous6:54 PM

    Good job Buck. -John Shannon

  2. Thanks Doc! (I'm assuming you're a physician based on your comments at Backpackinglight.com.)

  3. It's interesting that the doctors don't report all cases. Good post! It's nice to see a logical approach to the myth and supporting data.

  4. Thanks BJ. Giardia is one of those topics that people often debate with an almost religious fervor.

    For many doctors Giardia is so commonplace they probably feel no need to report cases. I suppose it's not like the plague or something in consequence, and like most of us they like to limit paperwork as much as possible.

    No surprise that health departments have so little stats on Giardia amongst backpackers.

  5. Good article.
    Sounds like backpackers and other outdoorspeople should be more aware of the importance of clean/safe drinking water... and HOW to get it, before leaving home.

    Maby focus is needed on this topic, in backpackermedia aso...

    Will keep it in mind myself :o)

  6. Hi Brian,

    Thanks. I think it's important that people get the facts and decide for themselves what precautions to take.

  7. Hi, Buck,

    Your article prominently features a 1976 case report from Utah and a 1977 case-control survey by Wright. You might want to look at some more recent data, e.g., this recent meta-analysis: Welch, T.P. "Risk of giardiasis from consumption of wilderness water in North America: a systematic review of epidemiologic data," Int J Infect Dis. 2000;4:103100, http://download.journals.elsevierhealth.com/pdfs/journals/1201-9712/PIIS1201971200901024.pdf?refuid=S1080-6032(04)70498-6&refissn=1080-6032&mis=.pdf Welch thinks the Utah case report is weak evidence for contaminated drinking water as opposed to hand-to-mouth contamination. He surveys four different case-control studies, of which Wright is one, and concludes that the sum of the available evidence indicates that the risk from hiking is "trivial or even insignificant."

    I'm sorry to hear about your own experience, but it's anecdotal evidence.


    Ben Crowell

  8. Hi Ben,

    I appreciate your response. Ben is the author of one of the "myth" papers.

    Yes, my experiences with Giardia are anecdotal, but they didn't seem anecdotal at the time.

    Dr. Welch's paper discusses Giardia outbreaks. To quote myself A disease outbreak is defined as "the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area or season. Thus, in 1991 there were only 80 [giardiasis] outbreaks reported, or at least studied, out of 34,348 reported cases.

    Backpacker Giardia is very, very rarely an "outbreak." No surprise it's not getting reported as such. Thus it would seem to me it's a classic case of "garbage in, garbage out." Welch is relying on the absence of data to make his point. Welch is, I think you'd agree, a recognized "giardia skeptic."

    Welch says ...Further studies should attempt to separate the specific risk factor of drinking water from backcountry.

    I cited two studies that show a link between drinking backcountry water and Giardiasis. Welch is admitting he doesn't know where all the backpackers are getting Giardia, otherwise he could provide data proving it. The scientific papers that show the link between water and Giardia, he dismisses. I would be curious how he explains the "50% vs. 17%" for Giardia for untreated vs treated water.

    Do you have one paper showing the breakdown of how backpackers ARE getting Giardia? Do you have one paper showing backpackers CAN'T get Giardia from drinking water? Can you find one expert in Giardiasis that will go on record saying it's a myth? I've linked to evidence showing Giardia levels have been found to be DOUBLE the infectious dose even in "pristine" waters.

  9. Great article. I'm going to show this to my friend who worked for a whitewater rafting company. He believed the whitewater had a purifying effect so he simply drank the water straight from the river bed. He became incredibly sick. We will never let him live it down.

  10. Thanks Matt. I suppose it's all about risk assessment, in this case weighing the effort of treating water with the odds of getting sick and the consequences if you do.

  11. Hi. Wow - is there anything you HAVEN'T done?? The AT, PCT, CDT and more. Geez! You're an animal!

    Anyway, I'm not sure if I should be commenting somewhere else, as I don't have thoughts on giardia. :)

    My husband and I thru-hiked the AT in 2008 (walkingtomaine.blogspot.com) and we've been dreaming about paddling the Mississippi. When your blog came up in my Google search I just had to reach out and say hello, and thanks for such a great resource of information!

    I haven't read all of it, but I wanted to ask if you had any problem stealth camping, or if you have to actually pay to camp in a lot of places along the river. In our minds, it seems incredibly populated and not like an escape to nature.

  12. Hi Betsy,

    Thanks! I think I could count the places I had to pay to camp along the Mississippi on one hand. Stealth camping was easy. One big surprise to me is that the river seems much wilder than you would think along the lower river. That's because of the "no man's land" inside the levee where the river runs, often largly shielded from the outside world by trees and the levee. Many times I'd beach the canoe, crawl up on a levee, and see another world. Of course there are many places where the cities and towns are "right there."

    No, there's not much wilderness like Maine on the AT. But it's definitely a true adventure, and one of my favorites. Check out my Mississippi blog page if you haven't already.

    I checked out your blog about the AT. Nice. Good photos, too. It's a difficult experience to define, isn't it?

  13. Yeah, debriefing the AT can last for years! We are ready for another adventure, but this one we think will come at a time in our life when we have kids, and when we talk it through there are so many unknowns. Your blog will be a huge help!

    Thanks again.

  14. Anonymous8:45 AM

    My son is currently in Colorado with the Rocky Mountain Youth Corps. His crew was given Seventh Generation Bleach (which doesn't have chlorine) to purify their water. They drank this water for several days before they realized the problem. It has now been over a week and none of the crew has come down yet with symptoms. I am wondering if you have any suggestions on what to do pre-symptoms... Fasting? Herbs? Take a stool test? Wait for symptoms to see if you're one of the lucky ones who are resistant? Also, was it flagyl that you took to clear the giardia? Any help is appreciated. I have been online for two days now researching many different viewpoints.

    1. Hi Sunny, I was treated with Tinidazole.

      If I were in your sons position, I'd just wait to see if I got symptoms. Odds are he won't from this one trip.

  15. Anonymous8:54 PM

    Well I read the Welch papers and then stumbled on this while I was trying look for any rebuttals.

    I'm also an academic, a biologist no less. From a biological perspective, Welch's papers make intuitive sense and they are well-researched. He's very careful not to overstate himself either.

    While your anecdotal evidence is unfortunate and also somewhat interesting, I'd just note that you don't mention ever being tested for Giardia, therefore there's really no way that your doctor could prove that this is actually what you had. Furthermore, unless you'd been tested previous to your hike, it's impossible to know whether it came from your hike.

    My other quibble is that while your rebuttal is interesting, it's terribly devoid of concrete sources. Citing a few weak studies and not providing any non-anecdotal data isn't encouraging. Nor is it very convincing. I'm fairly unbiased, and you haven't convinced me. The burden of proof isn't on showing what IS making hikers sick (a nearly impossible task from a scientific perspective), but on showing that it's giardia and that secondarily that it's coming from the water.

    I leave you with another source for this saga, and this one with some actual data. You can argue the merits of Welch all day, but it's hard to discount this (see bottom):

    77 participants would make this the largest (albeit unofficial) study to date.

  16. Anyone who takes Welch's giardia papers seriously should read my other post Backpacker Giardia: Debunking a Skeptical Paper. His papers don't make intuitive sense, nor are they well designed, and he over-states himself greatly, repeatedly. He has said, for example that there "is no evidence that wilderness waters in the United States are unsafe for consumption." That is absolutely false and a deeply irresponsible and unscientific thing to say.

    My rebuttal is "terribly devoid of concrete sources" and I "didn't provide any non-anecdotal sources"? Are you serious? Read through my post again, and my other post I linked above. There are numerous "concrete sources" and I've cited many peer-reviewed scientific papers, so your statement is simply untrue.

    It's interesting you cite the Schlimmer paper. He's a personal friend of Welch. The Myth Buster "paper" is maddeningly unscientific. For example he says By my 200th quart I was wholly convinced of Giardia’s absence in backcountry water So despite scientific studies with lab tests that show Giardia is common in backcountry water, he is convinced by his own anecdotal experience (without lab tests) that it isn't true.

    As for the anecdotal experience of other people, he selects those that don't get sick, and ignores those that do. The ones that don't get sick haven't been lab tested so there's no way of knowing whether they acquired Giardia or not. His method is not scientific and as far as I can tell it isn't taken seriously by a single expert in the field.

    Compare the Schlimmer paper to this study that LAB TESTED 22,743 people and concluded drinking untreated mountain water is an important cause of endemic infection.

    That peer-reviewed study is almost 300 times bigger than the non peer-reviewed Schlimmer "paper."

  17. Hi Anonymous,

    Two more things:

    Welch claims that poor hygiene is the primary culprit for Giardiasis for backpackers and then designed a study to prove it. Except his study found backpackers hands were actually cleaner in the field than they were heading into the field. Oops.

    And, contrary to what you insinuated, I have had LAB-CONFIRMED Giardiasis.

  18. Buck, good information, thanks for sharing. What would we do without studies? I've been hiking in the Sierras for more than a decade, have always treated the water, and have never had a problem. Results speak, don't they. Happy trails!

  19. I agree, Bill. Solid data interpreted with good, critical thinking is essential for seeing things as they are.

  20. Gregory9:46 AM

    Bottom line: Don't treat your water and you WILL get sick. Treat your water and practice good hygiene and it is pretty safe to say you WON'T get sick. The choice is yours. Thanks for all you do Buck. Big fan.

    1. Thanks for the comment Gregory! It sure seems like the evidence is abundantly clear that treating water will increase the odds of staying healthy. :)

  21. The human and animal "asymptomatic carriers" are polluting the waters of remote alpine wilderness areas around the world. Cattle and sheep run into the high country each summer chasing green pastures also contribute their pathogen loads acquired at lower elevations under crowded winter conditions. The end result is that over time, many remote and relatively inaccessible bodies of water have become contaminated with Giardia and Crypto. Even so, the odds of an individual being able to state with confidence that drinking untreated water from so and so lake or stream made them sick will remain an unlikely conclusion. Treating all drinking water is a significant barrier to infection and helps stop the spread of these pathogens into our wilderness areas.

  22. Joshua Moody5:13 PM

    Hello Buck and thank you for your efforts here. One question that has been plaguing me is how much contaminated water intake would be needed to affect me. Obviously, then answer will be, "It depends on many factors." The list of people at greatest risk to getting Giardia includes, "People who swallow water while swimming and playing in recreational water where Giardia may live, especially in lakes, rivers, springs, ponds, and streams," When I backpack, I love to swim in the water. I love lakes, streams, and rivers. I don't "swallow" water. But I have to imagine that every time I enter the water some small amount of droplets end up in my mouth or on my lips and face and hands and could end up in my mouth. If I am swimming in contaminated water (alpine lake) and that little bit is enough to come down with Giardia, then I wonder if filtering water is pointless. I would hate to spend all that time, money, and effort filtering water just to get infected from swimming. Do you have any indication or do you know of any research that would suggest just how much one needs to swallow to be infected?

    1. Hi Joshua, you are of course right that it depends on many factors. All things being equal drinking unfiltered surface water is dramatically more risky than accidentally swallowing a few drops of water while swimming. For example there are about 20,000 drops of water per liter, so it would be 1,000 times more risky to drink a quart of untreated water than to accidentally ingest 20 drops. And there is a huge difference between swallowing mouthfuls and a few accidental drops.

      I think it’s definitely worth the small risk of swimming in an alpine lake AND definitely worth filtering/treating backcountry water.