Background.—Every year over 22 000 people raft the Colorado River through the Grand Canyon in Arizona. Since 1994, over 400 rafters in 6 separate outbreaks have become ill with norovirus while rafting this stretch of the river.
Objectives.—To assess potential causes of these outbreaks, Colorado River water, water from nearby wastewater treatment plants, and a drinking water source were sampled and tested for norovirus and other water quality indicators.
Methods.—Colorado River water was collected and sampled during the 2004 rafting season. Water from wastewater treatment plants near the Lee's Ferry launch site and drinking water from the Lee's Ferry launch site were also examined during the 2003 and 2004 rafting seasons. Stool samples from ill rafters and composite stool samples from onboard toilet-cans were tested for the presence of norovirus during the 2003 and 2004 outbreaks. Parameters examined included the following: norovirus by reverse transcriptase– polymerase chain reaction, coliforms, Escherichia coli, temperature, turbidity, and pH.
Results.—No norovirus was detected in the Colorado River during the 2004 field sampling. Norovirus was detected in the Glen Canyon Dam Wastewater Treatment Plant on one occasion in 2004. Drinking water from the Lee's Ferry launch site was negative for norovirus in 2003, and Colorado River water from the Lee's Ferry launch site was negative for norovirus in 2004. In 2003, 3 of 10 stool samples from ill rafters or onboard toilet-cans were positive for norovirus. Neither of 2 stool samples collected in 2004 was positive for norovirus.
Conclusions.—Colorado River water tested during nonoutbreak periods was negative for norovirus, indicating that there is not an ongoing high level of norovirus contamination in the river. No source or sources of contamination could be identified from the testing. Potential sources of norovirus outbreaks among rafters include drinking contaminated river water, consuming contaminated foodstuff, rafter importation of the virus and subsequent person-to-person spread, and contaminated fomites, campsites, or equipment. It is likely outbreaks are the result of more than one source of norovirus, and the exact source remains unknown for several outbreaks.