Norovirus Outbreaks Among Colorado River Rafters in the Grand Canyon, Summer 2012

Objective To investigate the incidence and causative agent of the recurrent outbreaks of acute gastrointestinal illness (AGI) among different rafting groups on the Colorado River in the Grand Canyon National Park during the 2012 summer season. Methods Confidential illness reports were completed by all individuals with symptoms of AGI, and samples of fecal matter and vomitus, surface swabs of rafting equipment, and environmental swabs at stops along the hiking corridor were collected and tested for the presence of norovirus using reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Results During the active outbreak period between May 9 and July 9, 2012, 97 rafters (1.4%) from 10 trips (2.9% of all trips) declared AGI symptoms. AGI incidence within the 10 infected trips varied from 6% to 88%. Outbreaks occurred in 3 distinct temporal clusters that involved 2 different genogroups of norovirus. All available toilet fecal samples (5 samples) were positive for norovirus RNA: 1 with genogroup I (GI) and 4 with GII. The vomitus sample tested positive for GI. None of the fomite samples from rafting equipment or from the hiking corridors were confirmed for norovirus. Conclusions The results suggest that norovirus may have been introduced by ill or asymptomatic individuals actively shedding the virus in their vomitus or feces, and spread within, or between, river trips by different modes of transmission. This study reinforces the importance of appropriate guidance and practice regarding norovirus prevention and the necessity of postoutbreak containment in relatively isolated groups of individuals.