Robert L. Rockwell’s paper Giardia lamblia and Giardiasis With Particular Attention to the Sierra Nevada is widely quoted on internet forums and, on the surface, makes a very convincing case. I believed it myself, and was quickly rewarded with a case of giardiasis, in the Sierra.
I will present his most convincing arguments, and explain why each argument is untrue. I encourage questions and comments and invite you to share this information with other outdoors enthusiasts. Quotes from Rockwell’s paper appear in italics. By the way, Rockwell’s article is NOT a peer-reviewed scientific paper.
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.
FALSE. That quote comes from an extremely poorly designed and misleading study. It is debunked here. There are numerous peer-reviewed scientific papers* that have concluded that backcountry water often causes giardiasis. (See below) The CDC (Centers for Disease Control and Prevention) has looked at the health department surveillance and the medical literature and concluded: Although the advice to universally filter and disinfect backcountry drinking water to prevent disease has been debated, the health consequences of ignoring that standard water treatment advice have been documented.
The FDA, EPA, and the CDC, tell us that the best data shows that those drinking untreated backcountry water are in a high risk group. The CDC says [among] those at greatest risk are:… Backpackers or campers who drink untreated water from lakes or rivers
Rockwell: In some respects, this [giardia] situation resembles (the threat to beachgoers of a) shark attack: an extraordinarily rare event
Absolutely false. I am aware of only two scientific studies which tested campers/backpackers before AND after a trip. One study found at least 5.7% got giardiasis, the second study found 24% got giardiasis! These groups must have been unlucky, (and less than half became symptomatic) but it’s very common for backpackers to be unlucky with giardia. (I’ve had it three times.) In this large poll about 22% of the outdoors people responding have had giardia, and the infection rate was about TRIPLE for those who don’t treat water.
Rockwell: Sierra Nevada water has fewer Giardia cysts than, for example, the municipal water supply of the city of San Francisco… Water collected in Hetch Hetchy already meets governmental standards for drinking water and is not required to be filtered before distribution
Absolutely false. Here is what the city of San Francisco says: Water from the Hetch Hetchy Reservoir receives the following treatments to meet appropriate drinking water standards: disinfection by ultraviolet light and chlorine, corrosion control by adjustment of the water pH value, fluoridation for dental health protection, and chloramination SF water is THOUSANDS of time safer from pathogens than backcountry water.
Rockwell: Sierra Nevada water has far too few Giardia cysts for you to contract an infestation from it.
Ridiculous. Giardia is commonly found in Sierra water and one giardia cyst sometimes makes people sick. (A 10 cyst minimum infectious dose is commonly misquoted, even by sources that should know better.) It is absolutely certain there are countless billions of giardia cysts in backcountry water, including the Sierra.
* Here are just a few of the peer-reviewed scientific papers showing a link between untreated backcountry water and giardiasis.
Giardiasis in Colorado: an epidemiologic study
“drinking untreated mountain water is an important cause of endemic infection”
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.”
“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.”
Ok, I’ll take the bait on refutation.
By my rough count, you’ve got about nine key assertions, with seven (?) sources, at least four of which you misuse.
You start off with a bang by interpreting the very plain conclusions of TP Welch’s meta-analysis completely BACKWARDS as if it supported your point of view (which it contradicts).
You go on to cite a study of non-backcountry water which concerned eight outbreaks of disease, nearly all of which involved contaminated wells. The study isn’t relevant to backcountry water at all.
You effectively ignore San Francisco water utility’s own measurements as well as those of the USGS on backcountry water, and then declare Rockwell’s comparison of the two data sets to be “simply untrue.” Why you would assert this isn’t supported beyond mere bluster (no source).
You cite a page from the CDC, which itself cites the TP Welch report above. Welch finds “minimal evidence” to associate giardiasis with backcountry water.
You cite a 40-year-old report from Utah, about which the Journal of Wilderness & Envtl Medicine has raised substantial doubt (December 2004).
You assert ,without sources, that “one cyst can make people sick” which contradicts ALL published information on the topic (ten is the accepted number).
You cite an Internet poll in which respondents self-select and self-report, and which is obviously not scientifically useful.
You cite two very small, apparently scientific telephone polls in BC and Colorado, which do appear to support your viewpoint..
Perhaps I’ve missed or mistaken some of your points here?
All in all, the intelligent reader would be much more impressed by Rockwell’s essay and his 60+ citations. Perhaps that’s why National Geographic Magazine, et alia, have quoted him…
“You start off with a bang by interpreting the very plain conclusions of TP Welch’s meta-analysis completely BACKWARDS as if it supported your point of view (which it contradicts).”
Obviously Drs. Welch and I take opposite views on the issue, and our views contradict each other. Why else would I write several posts refuting their conclusions?
You go on to cite a study of non-backcountry water which concerned eight outbreaks of disease, nearly all of which involved contaminated wells. The study isn’t relevant to backcountry water at all.
Are you referring to my citation of Surveillance for Waterborne Disease and Outbreaks Associated with Drinking Water and Water not Intended for Drinking — United States, 2005–2006 It covers all waterborne outbreaks and says “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.” Note: backpacker giardiasis will very rarely meet the definition of “outbreak.”
You effectively ignore San Francisco water utility’s own measurements as well as those of the USGS on backcountry water, and then declare Rockwell’s comparison of the two data sets to be “simply untrue.” Why you would assert this isn’t supported beyond mere bluster (no source).
I do nothing of the sort. Taking both data sets at face value, I’m pointing out the obvious. One measurement is for raw water where a backpacker would be drinking viable (living and hazardous) cysts. For San Fransisco water the measurement is in the water supply. But here’s what Rockwell completely misrepresents. THE CYSTS ARE DEAD. They’ve been through a modern water treatment plant. Something like 99.999% would be killed. Water treatments including disinfections by ultraviolet light and chlorine, pH adjustment for corrosion control, fluoridation for dental health protection, and chloramination
You cite a page from the CDC, which itself cites the TP Welch report above. Welch finds “minimal evidence” to associate giardiasis with backcountry water.
What some people fail to understand is that citing a paper doesn’t mean that it supports your view. The CDC has specifically refuted the conclusions of Welch, and in their conclusions and recommendations contradict both Welch and Rockwell. I once showed someone that virtually none of Rockwell’s citations, other than those of Drs. Welch, support his conclusions.
You cite a 40-year-old report from Utah, about which the Journal of Wilderness & Envtl Medicine has raised substantial doubt (December 2004).
You mean Welch’s opinion piece supporting his own conclusions? It’s called a “false consensus.”
You assert ,without sources, that “one cyst can make people sick” which contradicts ALL published information on the topic (ten is the accepted number).
I linked to a source, which links to other sources. The 10 cyst claim is one of those errors commonly repeated, even by experts.
I have studied this issue in depth. In the only study of it’s kind, there was a 100% infection rate among humans at the 10 cyst level. No one has ever been tested at the 2-9 cyst level. The definitive study found that there is risk all the way down to the single cyst level. Risk assessment and control of waterborne giardiasis [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.] I am making progress in getting various authorities to correct this common “10 cyst” error.
You cite an Internet poll in which respondents self-select and self-report, and which is obviously not scientifically useful.
You are right that it is not a scientific poll. However, usually people argue against me with cherry-picked anecdotal evidence. Rockwell does this extensively. To put things in a real world perspective, I included this large poll. It does clearly refute, for example, the common claim that people are rarely lab-tested for giardiasis, and that giardiasis is “as rare as shark attack.”
All in all, the intelligent reader would be much more impressed by Rockwell’s essay and his 60+ citations. Perhaps that’s why National Geographic Magazine, et alia, have quoted him…
People seem impressed with Rockwell’s blizzard of citations. Again, when those papers directly address Rockwell’s claims in their conclusions, they overwhelmingly refute him. And the very fact that you have to cite a layman’s magazine shows how little respect his writing has gotten from the scientific community. Yes, a magazine respects his opinion. Epidemiologists don’t.
Gunks, you might ask yourself if you’re looking for an argument, or the truth.