Walking Well: Giardiasis

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One of the most feared health problems on hiking trails is giardiasis, an intestinal ailment caused by an amoeba-like protozoan that has in recent years contaminated so much backcountry water that it is now recommended that hikers filter or purify all water before drinking it. The resulting gastroenteritis is one of the most frequently occurring waterborne diseases in the United States. In addition to hikers, it strikes city dwellers with faulty water filtration systems, international travelers, and the children and staffs of day-care centers. Of course gastroenteritis can be caused by other protozoa, bacteria, viruses, other microorganisms, and various chemical toxins.

Giardia lamblia occurs in two forms, trophozoites and cysts. The active trophozoite looks like a flagellated amoeba, which attaches to the lining of the small intestine, causing damage. This form is very susceptible to environmental damage. Adverse conditions convert it into an infectious capsulated cyst form. Gastric acidity converts the cyst back into the active form. Cysts are larger than bacteria and can be excluded by most filters. Extensive tests have revealed how cysts are killed by heat and iodine. Cysts can remain viable in cold water for two months. I have been asked, "how long can the cysts live outside of their water environment, on the surface of my dish, on my towel, or on my skin?" Unfortunately, unless there is very recent data, we do not know the answers, but we do know that infection only occurs if the cysts enter the mouth and then the intestine. "Will an alcohol swab on my dish kill them?" Probably yes. It takes at least 10 cysts to cause human infection.

Although beavers are often blamed for contaminating water with Giardia cysts, they probably do not spread them to new areas, but are rather victims downstream of U.S. National Forest campgrounds. Feces of human carriers are washed by rain into streams. Beavers are then infected and then transmit cysts by defecating in or near the water. Beavers in remote areas have a zero infection rate; beavers and muskrats living near campsites and campgrounds have high infection rates, as do humans themselves. Some Giardia infections have been reported in coyotes, deer, elk, cattle, dogs, and cats, but not in horses and sheep.

My first acquaintance with Giardia came when after running the Moscow marathon in 1989, we visited St. Petersburg (at the time it was called Leningrad). CDC advised us to only drink tea and bottled water and brush our teeth with Pepsi because more than half of American visitors who did not take these precautions became positive for Giardia because the water supply was insufficiently treated. We avoided the disease. However, I drank suspicious water on my first thru-hike and had severe diarrhea in Damascus. Though the lab test was negative, I received quinacrine as a precaution. That experience convinced me to only drink safe water.