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…

Simply untrue. San Francisco is required by law to treat their Sierra water to make it safe. They use UV light, and chloramine. Each of those treatments, properly employed, will kill at least 99.9% of giardia cysts. 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.”