Background
Norovirus is a highly contagious virus and the leading cause of acute gastrointestinal illness (AGI) in the United States. From 1994 to 2011, there were 8 norovirus outbreaks on Colorado River trips through Grand Canyon National Park. Only 3 of these outbreaks had identified sources (river water or food contamination).
Objective
To determine the infection incidence, infectious agent, and possible contamination sources of norovirus on river trips during the 2012 summer season.
Methods
Confidential illness reports were completed by AGI symptomatic persons and included date and location of illness onset and recovery. Toilet samples of collective fecal matter, a sample of vomitus, surface swabs of rafting equipment, and samples of toilet and water stops along the hiking corridor were collected and analyzed using RT-qPCR to detect the presence of norovirus.
Results
From May 9 to September 28, 2012, 98 rafters from 10 different trips became ill with symptoms of AGI from norovirus. Attack rates on individual trips varied from 6% to 88%. Outbreaks occurred in 3 distinct clusters that involved 2 different strains of norovirus. All available toilet samples were positive for norovirus strains: 1 with the GI strain and 4 with the more common GII strain. The vomitus sample tested positive for GI. One of 12 fomite samples from rafting equipment was positive for GII norovirus. Swab samples collected along the hiking trail to the half-way point were negative for norovirus.
Conclusions
Norovirus strains from the summer of 2012 outbreaks in Grand Canyon were not from a shared source such as contaminated food or river water. The results suggest that the virus was introduced by ill or asymptomatic persons actively shedding norovirus in their feces, and spread within, or between, river trips by different modes of transmission. This study reinforces the importance of education regarding prevention of norovirus and containment after outbreak.