In North America, many people filter water when they are in the backcountry. A risk frequently cited in outdoor and hiking guides is giardiasis. For example, Frank and Annes Canyon Country Notebook notes:

A tiny protozoan, Giardia lamblia, flourishes in streams and lakes throughout the West, making the water unsafe to drink unless boiled or treated. If you develop symptoms such as diarrhea, cramps, and bloating, consult a physician. To avoid giardiasis, carry adequate water for short hikes, and on longer trips, boil water from streams or lakes for at least three minutes before drinking or cooking with it.

However, other sources state that this risk is exaggerated, such as Cascade designs:

At MSR, we believe the likelihood of contracting Giardia from backcountry water sources in North American wilderness areas is not especially high. A number of experienced outdoor professionals drink untreated water regularly without experiencing symptoms.

A lot of discussion inputs are based on personal testimony, such as I have never been sick. The latter is my own experience as well. However, personal testimony or examples don't prove anything — some people are chain-smokers all their life and live healthily until 90.

Even sources that cite “the science” are at odds with each other, as illustrated by conflicting answers to this question at Outdoors.SE. This article, by Ben Crowell, notes it a myth. On the other hand, Buck Nelson in this blog post concludes from “the science” that the risk is significant.

Is there any proven risk (i.e. shown to be significant) of contracting giardiasis due to drinking unfiltered backcountry water in pristine areas in North America?

There may be some studies showing an increase in incidence for backcountry hikers, but one would need to show the sample sizes are sufficiently large to establish statistical significance.

If it is significant, it is still interesting to put those numbers into the context of other risks associated with backcountry hiking.

By pristine areas, I mean official wilderness areas (in the USA) or similar (in Canada) with no industry or agriculture upstream.

  • See also this chat discussion.
    – gerrit
    Commented Mar 11, 2016 at 15:03
  • Research by Welch in 2000-ncbi.nlm.nih.gov/pubmed/10737847 states that "Education efforts aimed at outdoor recreationalists should place more emphasis on handwashing than on water purification." Commented Mar 12, 2016 at 12:08
  • I think hand-washing is important, but Dr. Welch's statement is based on speculation and not good data. I think his research is deeply flawed, and the CDC appears to agree. This blog post I wrote discusses some of problems with Welch's research: bucktrack.blogspot.com/2012/09/…. pericles316, I appreciate your thoughtful feedback on this topic. Commented Mar 12, 2016 at 13:14
  • Always welcome and many thanks for the reference to your blog post which is very thought provoking when reading through the other research papers as well as the comment section agreeing about certain flaws in that meta analytic paper! Commented Mar 12, 2016 at 17:13
  • Just pointing there are two ways to define "significant risk" as it's potentially life-threatening due to the speed at which you dehydrate, so there's significant as in "likely to be contracted" and significant as in "possibly resulting in your bleached bones finally being found by someone wandering the wilderness 20 years from now" I got it from water in the back country in Yellowstone Park. I was around 9% body fat before I got it and things got ugly very quickly, so when I first read "is it a significant risk..." I was thinking "OH YEAH!"
    – Dan Haynes
    Commented Sep 28, 2017 at 22:55

2 Answers 2


Is there any proven risk (i.e. shown to be significant) of contracting giardiasis due to drinking unfiltered backcountry water in pristine areas in North America?

Giardia is considered ubiquitous. "[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.[a]

There are many, many examples of giardia being found in pristine waters, for example Ongerth said Giardia cysts appear to be continuously present, though at low concentrations, even in relatively pristine rivers. Ongerth found giardia cysts in 43% of samples from pristine watersheds.[b] Roach found giardia cysts in 32% of remote, pristine waters of the Yukon. [h]

The best science says there is ~2% risk of infection with a single cyst. [c]

I know of only one study that tested wilderness backpackers for giardiasis before and after their trip into the wilderness. 5.7% definitely picked up giardia, another hiker was treated for giardiasis before testing could be completed. The confirmed cases did not get sick. Giardia was found in the water in low concentrations. [d]

There have been confirmed OUTBREAKS of giardiasis from wilderness water [e] [f] ...these giardiasis outbreaks illustrate the risks associated with consuming untreated surface water, even water that might appear pristine. (CDC) [e]

The CDC says: Annually, an estimated 1.2 million cases occur in the United States [I] They also say: Anyone may become infected with Giardia. However, [among] those at greatest risk are... Backpackers or campers who drink untreated water from lakes or rivers [j]

Less than 1% of giardiasis cases are part of outbreaks. [g] That is likely even more true for wilderness backpackers, where a day or weeks later people would normally seek treatment one at a time, and thus be "off the radar."

a: Environmental Protection Agency “Giardia: Drinking Water Health Advisory”

b: ONGEHTH, JERRY E. "Giardia cyst concentrations in river water." (1989)

c: Rose, Joan B., Charles N. Haas, and Stig Regli. “Risk assessment and control of waterborne giardiasis.” American journal of public health 81.6 (1991): 709-713.

d: Zell, S. C., and S. K. Sorenson. “Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe.” Journal of Wilderness Medicine 4.2 (1993): 147-154.

[e] Yoder, Jonathan, et al. "Surveillance for waterborne disease and outbreaks associated with drinking water and water not intended for drinking--United States, 2005-2006." Morbidity and mortality weekly report. Surveillance summaries (Washington, DC: 2002) 57.9 (2008): 39-62.

[f] Gretsch, Stephanie. “Outbreak of Giardiasis Associated with Drinking Surface Water Along a Hiking Trail.” 2015 CSTE Annual Conference. Cste, 2015.

[g] Yoder, Jonathan S., et al. "Giardiasis surveillance–United States, 2009-2010." MMWR Surveill Summ 61.5 (2012): 13-23.

[h] Roach, P. D., et al. "Waterborne Giardia cysts and Cryptosporidium oocysts in the Yukon, Canada." Applied and Environmental Microbiology 59.1 (1993): 67-73.

[i] Painter, Julia E., et al. "Giardiasis surveillance–United States, 2011-2012." MMWR Surveill Summ 64.Suppl 3 (2015): 15-25.

[J] CDC, Parasites - Giardia: Sources of Infection & Risk Factors

  • 1
    Welcome to Skeptics.SE! Your view that "Since I have personally contracted giardiasis three times after drinking water from three different wilderness areas in three different states, (I believe two were official wilderness areas, one area was in Alaska far more remote than any wilderness of the ("Lower 48") I'm convinced there is significant risk. :)" is not encouraged out here due to the community rules for avoiding personal anecdotes. Have a look at the help section out here-skeptics.stackexchange.com/help/how-to-answer. However, I feel your other research references are great! Commented Mar 12, 2016 at 11:12
  • You can also add the CDC's (cdc.gov/mmwr/pdf/ss/ss5709.pdf) view that 'Both of these giardiasis outbreaks illustrate the risks associated with consuming untreated surface water, even water that might appear pristine." as proof of your summary for risks associated with drinking untreated pristine water. You have already excellently summarized the research material for your answer here-outdoors.stackexchange.com/questions/11100/…! Commented Mar 12, 2016 at 11:57
  • I like the "no anecdotes" rule. I removed edited my post. I also added your suggested CDC citation. Thanks! Commented Mar 12, 2016 at 13:09
  • 1
    Yet another problem with this answer is that you use the Zell paper as evidence that people got (asymptomatic) Giardia infections from water, but I don't see any evidence of that. It seems much more likely that the people in the Zell study got their infections by hand-to-mouth contamination. From their methodology, I can't imagine how they could have ruled that out.
    – user4216
    Commented Mar 13, 2016 at 21:45
  • 1
    @BenCrowell The Rose paper discusses outbreaks from water containing as little as 0.6 cysts per 100 liters. How could this be possible unless people are getting infected from single cysts?
    – DavePhD
    Commented Mar 14, 2016 at 19:25

In order to interpret the scientific evidence properly, it's necessary to understand some scientific background about Giardia and giardiasis.

The human gut is naturally teeming with microorganisms. These are known as your gut flora or gut microbiome. Most of these are bacteria, but quite a few are other organisms, including protozoans such as Giardia. Some of these critters in your intestines are beneficial or even necessary for your metabolism, while others may be neutral or harmful. People tend to develop tolerance for their own gut flora, but can get sick from other people's. The gut microbiome tends to be more diverse among people in the developing world, less so in the developed world, and this decreased diversity may actually be a bad thing.

Giardia is present in about 3-7% of adults in the US, about 30% in the developing world.[Auerbach 2012] Among toddlers in the US, roughly a third have it.[Ish-Horowicz 1989] Most people who have Giardia as part of their gut flora have no symptoms at all. Among people who do have symptoms, the condition is normally mild and self-limiting. For unknown reasons, there is a small portion of the population that tolerates Giardia badly if newly introduced to it, and these people have unpleasant diarrhea for some period of time.

Giardia is present in surface water in the form of dormant cysts. These cysts tend to resist being killed by chlorine. You can pick up Giardia by drinking water that contains cysts, but what was not realized in the 1970s, during the initial Giardia public-health panic, was that Giardia is also transmissible through hand-to-mouth contamination. In the context of backpacking, this would something like the following. A and B go backpacking together. A has Giardia in her gut microbiome, and she tolerates it and has never had any symptoms. B doesn't have it. A poops and doesn't wash her hands, and then A and B have dinner together and share pots and pans. B eats food that is contaminated with A's Giardia.

You may hear backpackers claim that they "got Giardia," when all they really know is that they got sick. They could have had some other condition, they could have had giardiasis but contracted it somewhere else, or they could have gotten a bug through hand-to-mouth contamination from their hiking partners.

Often people will make this claim when they got sick during the hike. This is unlikely. When an animal is infected by a parasite, there's a prepatent period, which is the time from infection with a parasite to when the bugs reach a life stage where they can be detected by a lab test. There's also an incubation period, which is the time from infection to symptoms. For most parasites, the prepatent period is shorter than the incubation period, but for Giardia it's often longer. A 1954 study on prison volunteers showed an average prepatent period of 9 days, but there's a wide range of variation, and the incubation period can be as long as months. In a study of travelers to the Soviet Union, the typical time until acute symptoms occurred was found to be a couple of weeks. In about two thirds of patients, the prepatency period was longer than the incubation period by a week or more. In summary, if someone gets backpacker's diarrhea while on a weekend backpacking trip, it's very unlikely that it was caused by giardiasis that they acquired during the trip.

The question asks specifically about pristine backcountry areas in North America. Some studies have surveyed water in these areas for Giardia.[Suk 1986],[Jaret 2003] The water was extremely clean, and huge volumes of it had to be filtered in order to pick up any detectable number of Giardia cysts. For example, there were sites in the Sierra where they filtered 100 gallons of water and didn't detect a single Giardia cyst left over in the filter. In most of the locations where cysts were detected, the concentrations were so low that they have to be expressed in scientific notation. In low-use areas, they ranged from zero to about 5x10^-3 per liter, while one high-use area had about 0.1 per liter. Elsewhere in the U.S., similar testing also found extremely low concentrations all of the backcountry locations tested: West Beaver Creek, AZ; Merced River, CA; Chattooga River, NC; Neversink River, NY; White Pine Lake, UT; Greenwater River, WA; and Renard Lake, WI.

From these studies, it appears that if you spend a weekend drinking untreated water in pristine backcountry areas in the US, you will typically not ingest a single Giardia cyst. We then need to do a risk-benefit analysis. This raises the question of how many cysts you need to swallow in order to have a certain chance of getting infected, as well as the chance that this will cause symptoms. The best single source of information on the first question is a 1954 study by Rendtorff that used prison volunteers; the data are summarized and analyzed further by [Cox 2002] and [Rose 1991]. Roughly speaking, you have to ingest about 20-30 cysts to be likely to get an infection.

It would be interesting to know whether there is a threshold effect, i.e., whether or not there is some chance, however small, of getting sick by swallowing a single cyst. Since Rendtorff never tested such small doses, the data are not sufficient to determine this directly.[Rose 1991] Rose introduces a mathematical model in which it is assumed that each cyst has some probability p of setting up shop in your gut, but this is an assumption of the model, and is not testable based on the data, which used higher doses. Rose's model is also not consistent with data showing that even when people drink very highly contaminated water, there is still only about a 50% chance of contracting giardiasis.[Wilkerson 1992] In this type of study, one of the confounding factors is that the minimum infectious dose can vary depending on the strain of the microorganism.

If you do pick up a giardia infection, and if you weren't already an asymptomatic carrier, then it appears that your chance of developing symptoms is about one in 10.[Wilkerson 1992]

Based on these numbers, we can make at least a rough order-of-magnitude estimate of the risk associated with drinking backcountry water. Suppose you go on a weekend hiking trip in the Sierra and drink 6 liters of untreated water. In low-use areas, the concentration of Giardia cysts in your water appears to average about 3x10^-3 per liter. Based on Rose's model, take the probability of infection to be about p=.02 per cyst. If infection occurs, the chance of getting symptoms (which are in most cases mild) is on the order of 0.1. Multiplying these factors, we arrive at a probability of about 4x10^-5 that you will get Giardia symptoms. That is, under these assumptions, out of a million people who do this, about 40 are expected to get diarrhea.

This estimate appears to conflict with a study by Zell,[Zell 1993] which states:

The incidence of Giardia cyst acquisition in backcountry travelers was only 5.7% (95% CI 0.17–20.2%). Mild, self-limiting gastrointestinal illness occurred in 16.7% of subjects (95% CI 4.9%–34.50%), none of whom demonstrated G. lamblia infection.

Although none of the people in the study who had Giardia got symptoms, this rate of infection is many orders of magnitude higher than would have been expected from water contamination based on the Rendroff-Rose data and modeling. Unfortunately the Zell article is paywalled, so I can only see the abstract, but it appears that he would have had no way to tell whether the people who acquired infections got them from contaminated water or from hand-to-mouth contamination. In any case, Zell's conclusion is that water treatment is of marginal cost-effectiveness, given the low risk and the fact that the infection is self-limiting and usually asymptomatic.

Another reality check we can do is to compare the concentrations of Giardia in backcountry water with the concentrations in city tap water. This is complicated by the fact that Giardia cysts, whether found in backcountry water or tap water, may be nonviable, and surveys cannot usually determine their viability. Historically, city tap water has been getting cleaner and cleaner, especially in the developed world. However, it appears to have been normal in 20th-century America for city tap water to contain concentrations of Giardia cysts that are similar to or greater than the concentrations found in backcountry areas of the US.[Rockwell 2002] It would be interesting to get more detailed information on this from someone who has professional-level knowledge of US water supplies and their history.

A meta-analysis of the literature in 2000 concluded that "the evidence for an association between drinking backcountry water and acquiring giardiasis is minimal."[Welch 2000] When people do actually contract backpacker's diarrhea from exposure during a hiking trip, by far the most common reason is hand-to-mouth contamination.[Welch 1995]

Auerbach 2012 - Paul S. Auerbach, Wilderness Medicine (6th ed., 2012), ch. 68

Cox 2002 - Cox, F.E.G. (2002). History of Human Parasitology. Clin. Microbiol. Rev. 15(4): 595

Erlandsen 1984 - Erlandsen, Giardia and giardiasis: biology, pathogenesis, and epidemiology, 1984.

Ish-Horowicz 1989 - Ish-Horowicz et al., "Asymptomatic giardiasis in children," Pediatr Infect Dis J. 1989 Nov;8(11):773-9.

Jaret 2003 - Peter Jaret, "What's In the Water?," Backpacker, Dec. 2003, p. 45.

Jokipii, The Lancet, Volume 309:1095.

Rockwell 2002 - Robert L. Rockwell, Sierra Nature Notes, Volume 2, January 2002, http://web.archive.org/web/20051026030831/www.yosemite.org/naturenotes/Giardia.htm

Rose 1991 - Rose, Haas, and Regli, "Risk assessment and control of waterborne Giardiasis," Am J Public Health 81 (1991) 709, http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1405147/pdf/amjph00206-0039.pdf

Suk 1986 - Map cited as reference 17 in S.C. Zell, "Epidemiology of wilderness-acquired diarrhea: implications for prevention and treatment," Wilderness and Environmental Medicine 3 (1992) 241, http://www.wemjournal.org/article/S0953-9859(92)71235-2/abstract

Welch 1995 - Thomas R. Welch and Timothy P. Welch, "Giardiasis as a threat to backpackers in the United States: a survey of state health departments," Wilderness and Environmental Medicine, 6 (1995) 162, http://www.wemjournal.org/article/S1080-6032%2895%2971046-8/abstract

Welch 2000 - 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

Wilkerson 1992 - Wilkerson, James A., MD: Medicine for Mountaineering and Other Wilderness Activities. The Mountaineers, 4th edition, 1992 (referenced in Rockwell, http://web.archive.org/web/20051026030831/www.yosemite.org/naturenotes/Giardia.htm )

Zell 1993 - Zell and Sorenson, "Cyst acquisition rate for Giardia lamblia in backcountry travelers to Desolation Wilderness, Lake Tahoe," Journal of Wilderness Medicine 4 (1993) 147.

  • 1
    @Dawn: Thanks for your comments. I've deleted the first two statements that you found objectionable, and added a reference for the third.
    – user4216
    Commented Mar 13, 2016 at 23:40
  • Ben says "Roughly speaking, you have to ingest about 20-30 cysts to be likely to get an infection." Citation? Here's the Rendtorff study. tinyurl.com/hrhnurx 2 of 2 people were infected with 10 cysts. No tests at the 2-9 cyst level. TP Welch said "Published reports of confirmed giardiasis among outdoor recreationists clearly demonstrate a high incidence among this population." There are confirmed OUTBREAKS of backcountry waterborne giardiasis. Can you cite backcountry hand-to-mouth giardiasis outbreaks, or even single CDC confirmed cases? Commented Mar 14, 2016 at 1:13
  • Ben I've explained why the city/backcountry cyst level comparison is bogus. The 1989: Surface Water Treatment Rule, in effect when Rockwell wrote his paper, required AT LEAST 99.9% of the giardia cysts in SF the water to be killed. At equal cyst counts backcountry water would be a thousand times more dangerous, more if you consider cyst age, even more when you consider a 99.9% reduction is the bare minimum. Rockwell's LA water cyst counts were in the aquaduct, NOT after the water had been run through a modern treatment plant. Commented Mar 14, 2016 at 1:43
  • 2
    Rendtorff wrote about his prisoner study again in 1979 and said "It seems very likely that in a susceptible host a single cyst would cause infection" at page 70 here books.google.com/…
    – DavePhD
    Commented Mar 14, 2016 at 19:53

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .