Saturday, September 08, 2012

Backpacker Giardia: Debunking a Skeptical Paper

Water on the Desert Trail: Mexico to Canada

The paper Giardiasis as a threat to backpackers in the United States: a survey of state health departments (Welch TR, Welch TP) contains this statement: Thus, neither health department surveillance nor the medical literature support the widely held perception that giardiasis is a significant risk to backpackers in the United States. In some respects, this situation resembles that recently described by Campbell and Smith in reference to shark attacks [18]: an extraordinarily rare event to which the public and the press have seemingly devoted inappropriate attention.

That conclusion is convincing, misleading, and untrue. In Risk of Giardiasis from Consumption of Wilderness Water in North America: A Systematic Review of Epidemiologic Data, Welch, TP himself says: Published reports of confirmed giardiasis among outdoor recreationists clearly demonstrate a high incidence among this population.

A good example of the laughable research in this paper is this sentence: Giardiasis and similar enteric illnesses in developed nations are overwhelmingly spread by direct fecal-oral or food-borne transmission, not by contaminated drinking water. Verifiable facts from unbiased sources say just the opposite. Giardiasis outbreaks in the United States, 1971–2011 found 98% of individual cases were waterborne, and 2.1% were foodborne or person-to-person.

Another example of the easily disprovable "research" in this paper is this statement: Only two departments considered water-associated giardiasis to be a problem for backpackers in their jurisdiction, and neither had any data to support this concern. Baloney. Only two departments? No data? Nearly all the states I just looked up clearly contradict this "research". Here is just the tip of the iceberg:

"In Wyoming, [Department of Health] officials see cases of giardiasis in people who drink untreated water from streams, lakes or reservoirs

Minnesota Department of Health: This was a waterborne outbreak of giardiasis associated with drinking surface water at multiple points along a hiking trail.

Montana Department of Public Health: On average, there are close to 200 cases of cryptosporidiosis and giardiasis reported in Montana every year. Of those who reported risk, 51% of cases had recreational water exposure or drank untreated water in the days before they became ill.

 Washington Department of Social and Health Services: "Clusters of [Washington giardia] cases were linked to... backpacker groups... Factors which appeared to put a person at increased risk included consumption of untreated water.

Oregon Public Health Division: During 2016, 99% of the [Oregon] cases were reported as “sporadic” Rates of [giardia] infection tend to be higher in the summer months with transmission related to outdoor activities in or near untreated water... Do not drink untreated surface water.

Below are just some of the studies refute the authors' claim that medical literature does not support the perception (waterborne) transmission of giardia for backcountry travelers is a significant risk:

Giardiasis in Colorado: an epidemiologic study
"drinking untreated mountain water is an important cause of endemic infection"

An outbreak of giardiasis in a group of campers
"These surveys show that campers exposed to mountain stream water are at risk of acquiring giardiasis."

Factors associated with acquiring giardiasis in British Columbia residents "the authors concluded that consumption of local water while participating in outdoor activities, such as camping, was associated with a higher risk of giardiasis than in controls who participated in such activities but did not ingest local waters."

Acute Giardiasis: An Improved Clinical Case Definition for Epidemiologic Studies
"an outbreak of waterborne giardiasis occurred in a group of 93 university students and faculty participating in a geology field course in Colorado. All cases occurred in one subgroup of persons who were heavily exposed to untreated stream water on a field trip, and the risk of illness was strongly related to the amount of untreated stream water consumed."

Giardia Prevalence among 1-to-3-Year-Old Children in Two Washington State Counties
 "Significant increases in [giardiasis] prevalence were observed, however, if the child had a history of drinking untreated surface water (from streams or lakes during recreational activities)"
While the EPA, FDA, CDC and the Mayo Clinic say that waterborne giardiasis for backpackers is a concern, this paper claims hygiene is the primary culprit for giardiasis in backpackers while implying there is little if any risk from waterborne transmission. If Drs. Welch or anyone else knows of studies that support that claim, similar to the above, I hope someone will point me to them and I will include a link. I have already debunked the well known Rockwell paper, and part of the paper being discussed now, here.

This paper 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 From that it would be easy to conclude that giardia must not be very dangerous to backpackers. However, an outbreak is "the occurrence of cases of disease in excess of what would normally be expected in a defined community, geographical area or season." Backpackers are commonly treated for giardiasis, so it rarely gets reported as an outbreak. Why ask a question in a survey which will guarantee misleading results?

"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? 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, infinitely under-reported.

Actually, approximately 99% of giardiasis cases go unreported (based on official reported figures vs estimated infection rates) By extrapolating reported rates of cases vs unreported, and camper/backpacker outbreaks vs other outbreaks, I come up with a number of 62,000 giardiasis cases a year for waterborne transmission in the backcountry. Fuzzy numbers? Absolutely, but much more reflective of reality that the “2 outbreaks a year” reported in this paper.

This paper says It appears to be common wisdom among outdoor recreationists in the United States that there is widespread fecal contamination of wilderness waters. That common wisdom is proven: (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.

Attempting to refute An outbreak of giardiasis in a group of campers the paper says No cysts were identified in suspect water, Well, the water testing filter didn’t work.  Doesn't that seem relevant to mention in a scientific paper?

The paper goes on to say. there was no association between water consumption rates and the likelihood of disease. The authors discounted food or fecal-oral spread, commenting that the former had never been reported. To quote from that paper's abstract: The temporal distribution of cases and the absence of clustering among food preparation subgroups suggested a common source exposure. They discounted food preparation for good reason, because it didn’t correlate to food sharing groups! It DID correlate to one common source, the drinking water. And water consumption rates?? Was there a correlation between food consumption rates and infection? Apparently not. The conclusion of water-borne giardiasis made sense then, and it makes sense now.

There is an even more egregious "mistake" in Giardiasis from Wilderness Water. Citing Giardiasis in Colorado: an epidemiologic study it says Responses indicated that 38% of cases vs 18% of controls had camped overnight in backcountry areas. He ignores the very next line in the abstract to that paper which says and drank untreated mountain water (50% vs. 17%.) There it is, in black and white. The infection rate was TRIPLE for drinking untreated mountain water in this large group. I think it's a great example of confirmation bias and is bad science, at best.

The paper 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 the authors don't say is that one person was empirically diagnosed with giardiasis, was treated, and quickly recovered. Historically, giardiasis was often not detected with a single test. Two more people tested positive for giardia but were asymptomatic at the time of the last test. Many giardiasis cases are asymptomatic. 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%. On a SINGLE TRIP. To me, the authors went far beyond a little spin in trying to make a point.

The CDC SPECIFICALLY cited a TR Welch paper and said Although the advice to universally filter and disinfect backcountry drinking water to prevent disease has been debated (62), the health consequences of ignoring that standard water treatment advice have been documented in WBDOSS [Waterborne Disease and Outbreaks Associated with Drinking Water and Water]  In other words, there is documentation, with data, by the Centers for Disease Control and Prevention, that Dr. Welch is wrong.

My conclusion: Giardiasis as a threat to backpackers in the United States: a survey of state health departments is deeply flawed and misleading for the reasons outlined. For backpackers, there is more scientific evidence to support the spread of Giardiasis through water than there is through hygiene. That said, there is good scientific evidence that hygiene plays a significant role in gastrointestinal health for outdoors people. Logic tells me there are some cases of giardiasis in the backpacker community as the result of poor hygiene despite lack of studies to properly support that claim. Having been diagnosed with giardiasis three times, I am treating my water and washing my hands. Others are welcome to make their own risk assessment.
[Note, there are two authors named Dr. Welch who have written on giardiasis. Welch, TR, and Welch, TP. I've attempted to keep them straight in this blog post.]

Please click the like button if you like this article. I invite your comments and corrections. Have a great hike!


  1. What caught my attention immediately in Welch's abstract was the depth, or lack thereof, in his results: "Of 104 articles identified in the initial screening, nine Met the inclusion criteria." So, after locating 104 reports/studies in his literature search, Welch eliminated 95 of them; so his meta-analysis was based upon data deferred from 9 reports/studies (that's about 8.6% of the sources). And from that, he still seems able to draw a statistically significant conclusion? Not so, IMO.

    What the most reasonable conclusion would have been is that there just wasn't enough data upon which to draw-- and thus there would be a very compelling call made for additional reporting/research.

    Since Welch's paper was published in 2000, I would be interested in seeing if there have been follow-ups performed in the recent past.

  2. Thanks for the comment, Dr. C.

    I want to trigger a scientific debate beyond the usual "I've never had it so I don't think it's a real problem" vs "I have had it and I can assure you it is."

    It would be great if someone would write a new, balanced paper on the topic.

    Anyone else have an opinion or insight they'd like to share?

  3. I'm not qualified to provide opinion, but based on what I have read and heard from others, I'm not taking any chances now.

    In June 1974, I spent 24 days backpacking the San Juan's of So. Colorado and drinking untreated from the small streams. Never got sick, but the sheep herders hadn't moved up to those altitudes yet. Do you think the Giardia problem has gotten worse over the years or was I just lucky?

    Also, what do you recommend for purification? I'm considering going to the Steripen.

    Buck, Thanks for keeping us informed.

  4. Thanks Carl! I think many areas have gotten much worse, and many areas have gotten better. Grazing comes and goes, human visitation spikes and fades, animal populations rise and fall. One of the points I've tried to make is that it's hard to be sure if the water is safe or not based on appearence. Personally, I use Aqua Mira drops and like them a lot. I've heard very good things about Steripen, too.

    This is a good article on water treatment:

  5. I think this review is better than Welch's.

    As far as "scholarly" research goes, Welch's paper is pretty thin, even for a review paper. Guy works at a pediatric hospital. What's he doing writing a review about parasite prevalence in surface water? You'd expect a parasitologist or a surface water scientist (or both) to write that sort of paper. Neither author of the Welch review has authored any of the cited research, so they don't have first-hand knowledge of the methods you'd use in such a project.

    Here's TR's previous research (there are other TR's in there, of course):

    A Google Scholar search for TP Welch doesn't turn up much from THIS TP Welch. What I did find was all related to the liver.

    This paper didn't show up until a few pages down Google's results.

    After digging up work from both of these authors, I'm not surprised by the low quality of the review article. TR Welch is at least well published and should know how to write a good paper, even if this one appears outside his area of expertise.

  6. Thanks Nate. I appreciate your comment and your insight.

    It is a bit surprising Dr. Welch has written these papers given his area of expertise. But my criticism has nothing to do with that, but all the other issues I brought up.

    Google Scholar is great, isn't it? Thanks for the link. He's definitely published a lot!

  7. In your article you only refer to the US. Being European I would like to know whether Giardia exists only in the US or worldwide. In your article you quote cysts from the Arctic to the tropics but the link does not work. FYI: Giardia does not get discussed in European backpacker circles and does not seem to be a big problem here in Europe. If cysts exist worldwide it would be interesting to know why they are a problem in the US but not in European backpacker circles.

  8. Hi German Tourist

    I fixed that link. I didn't research European waters and most of the studies I cite concern giardia in the US and Canada.

    In short I don't know much about the giardia situation in Europe and you are undoubtedly better informed on the situation than I am. Sorry I couldn't answer your questions.

  9. Bill Thompson8:32 PM

    In over 40 years of backpacking in the Sierra Nevada of California I have never treated water in backcountry locations and never had a case of giardia occur personally or to anyone of the many folks I camped with. I'm not a doctor or a scientist, but have read studies that report very low numbers of giardia cysts in Sierra water sources. My choice is to not carry, pump, add chemicals, or rely on electronic devices. I simply drink the delicious water from the springs, streams, and lakes that I encounter. YMMV.

  10. Hi Bill,
    Fair enough. My mileage did vary. I got giardia on my first long hike in the Sierra. Many other people get it in the Sierra as well. Unless you, and your other friends, have been lab tested there's no way of knowing if you are, in fact, carriers. I have found many accounts of people going years and even decades drinking untreated water, until their luck ran out.

    Finally, a reminder of this quote: "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...


  11. Anonymous3:15 PM

    Your note on "An outbreak of giardiasis in a group of campers " in Utah appears to link to an unrelated item concerning a ski area's in-house water system in Colorado.

    Basically you're ignoring bottom-line research that pretty clearly shows non-infective levels of Giardia in wilderness water sources, and over-reacting to an organism that is ubiquitous and not ordinarily of much concern.

    1. Anonymous, read the page 267 citation. It says that, with one exception, the "membrane filter method" had not been successful in finding giardia cysts. So if the tested source contained giardia cysts, they likely wouldn't be found.

      The infective dose of giardia can be as few as one cyst. Top experts have estimated an approximate 2% chance of infection with a single cyst. And, as you say, giardia is ubiquitous. Some people will be lucky, some won't.

      Do you think CDC epidemiologists are ignoring bottom-line research? If you want to refute my statements, you'll need to produce some data.