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.”
Your source does indeed say the “theoretical infective dose” is one cyst.
But interestingly, it also cites estimates that “an exposure to an annual geometric mean of 0.0007 cysts per 100 liters would result in a 1/10,000 annual risk of infection, assuming that 2 liters of water are consumed daily.”
Probably, if you look into your sources more carefully, and cleaned up your post on Rockwell, you’d seem less foolish.
Yes, 1/10,000 chances you’d get giardiasis if you stipulate relatively clean water. Your odds aren’t so good drinking backcountry water in the real world:
The geometric mean was 6.5 cysts/100 L (maximum = 156/100 L) for polluted lakes and 0.5 cysts/100 L (maximum = 7/100 L) for pristine lakes
In Wisconsin, Archer et al. -5- (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
In three pristine watersheds in Washington, Ongerth (1989) found assessed Giardia cysts in 43% of 222 samples during a nine-month period. Cyst levels ranged from 10 to 520 cysts/100 liters
If you take the MEAN of the first example your odds of getting giardia, drinking untreated water for a year, would be over 9,000 times higher than your cited example. In other words, you probably would get giardia. If you were just drinking from the pristine lakes you’d have about a 1 in 14 chance of getting giardiasis.
In the second example you’d theoretically get giardiasis drinking a single liter of water from that “pristine” stream.
In the third example, if you drank the wild waters from the pristine watershed all year long, untreated, you’d probably get giardiasis from the cleanest source.
Source of the above, the EPA.