BACKGROUND: Routine surveillance interviews of two giardiasis cases conducted by the Minnesota Department of Health (MDH) in August 2014 revealed that both cases had participated in the same week-long camping trip along the Superior Hiking Trail. Availability of potable water is limited along the trail, and the individuals reported drinking water from various lakes and streams throughout their trip. Swimming in various water bodies was also reported. An outbreak investigation was initiated to determine if there was an association between a water source and illness.
METHODS: Epidemiologists from MDH interviewed campers to obtain information on drinking water sources and treatment, swimming exposures, and illness history. A case was defined as a camper who hiked along the trail and subsequently developed a laboratory-confirmed Giardia infection or diarrhea (>3 loose stools in a 24-hour period) lasting 3 or more days. Water treatment times between cases and controls were compared using two-sample t-tests in SAS 9.4.
RESULTS: Eleven campers were interviewed. Six cases were identified, including two laboratory-confirmed with Giardia. The group primarily used chlorine dioxide water treatment drops to treat their water; none reported drinking untreated surface water. The median amount of time campers reported usually waiting between treatment with the drops and consumption of water was 30 minutes (range, 20 to 45 minutes). The length of time waited did not differ significantly between cases and controls. The package instructions specify that water should stand 15 or 30 minutes before use depending on water temperature or turbidity; however, the label indicates the drops kill odor causing bacteria but does not make a claim that the product kills Giardia or other pathogens. The manufacturer of the drops was contacted and reported that the product requires 4 hours of contact time to inactivate Giardia cysts. Swimming at any water body was not significantly associated with illness.
CONCLUSIONS: This was a waterborne outbreak of giardiasis associated with drinking surface water at multiple points along a hiking trail. Chlorine dioxide can be an effective pathogen reduction method; however, the product used by the campers is not certified for pathogen reduction, although it is frequently marketed as such. Consequently, while the campers followed the package instructions, the treatment appears to have been ineffective at inactivating Giardia cysts in the water. Future efforts to prevent illness from the consumption of backcountry water need to include creation of clear guidelines recreationalists can use to choose effective water treatment methods.