The explosive outbreak of gastroenteritis that broke out in the Grand Canyon earlier this year was likely caused by multiple people bringing in norovirus infections, according to a recent study published by the Centers for Disease Control and Prevention in the Morbidity and Mortality Weekly Report. The contagious explosion ended with violent hollowing out of at least 222 visitors to the geological wonder over a brisk, but brutal, three-month period.
As Ars readers may recall, the National Park Service warned early in the summer that an outbreak had wiped out river rafters and hikers. But the new study, led by local and CDC officials, offers a more detailed look at the outbreak that caused outdoor adventurers to grip the edges of basins much smaller than those of the Colorado River.
The problem appears to have started in early April, with the first identified case hitting a backpacker on April 4. On April 8, a commercial rafting company reported to the National Park’s Office of Public Health (OPH) that seven people fell ill on a rafting trip. with vomiting and/or diarrhoea. The cases continued to pour in until April and broke out in early May. The OPH contacted the CDC on May 11, after collecting dozens of case reports. On May 21, the OPH received reports of another 102 cases from 13 rafting groups and several backpackers.
Local officials tested samples of portable toilets used by nine affected rafting groups. They found the norovirus in all of them. Officials also tested toilet samples from two unaffected rafting groups, which were negative for norovirus. Of the nine positive samples, two came from rafting groups that visited the park in April, while the other seven were from May.
Genetic research showed that the strain of norovirus that caused illness in April was different from the strain found in May. In addition, at least five people reported that their symptoms started more than 24 hours before their trip to the canyon. All of this suggests a “potential for multisource introduction of norovirus into the river corridor,” the authors wrote.
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To date, it is still unclear how the virus spread among the various groups and park visitors. But the authors note that “[b]Because many trips use the same campsites and place portable toilets in the same locations, particles may have been transferred to surfaces, beach sand or river water where new groups may encounter them, then passed the virus from person to person and journey to journey.”
However the virus moved, it did it efficiently. The attack rate among rafting trip groups ranged from 10 percent (ie, three rafters in a group of 31 got sick) to a horrific 83 percent (29 out of 35).
In response to the cases, the OPH sought to improve sanitation around drinking water taps at campgrounds, including daily chlorine disinfection and mechanical devices that prevent backflow. They also announced the outbreak and advised visitors to wash their hands with soap and water, not share food, stay home if they feel sick and immediately isolate anyone who gets sick. The authors emphasized that many people “didn’t know that alcohol-based hand sanitizers are not effective at reducing norovirus transmission,” and that washing with soap and water was the most effective prevention.
The efforts seemed to pay off. After warnings were posted in mid-May, cases slowed to a trickle by the end of the month. Only three cases were reported in June, the last of which occurred on June 17. In total, the authors counted 222 cases between April 1 and June 17, 2022, but they say cases are “probably underreported.”
Looking more broadly, the authors note that the outbreak in the Grand Canyon reflects a nationwide surge in norovirus outbreaks in the spring of 2022. Like many infectious diseases, the highly contagious gut-wrenching virus died during the pandemic, but came back roaring. as pandemic health restrictions eased. A study published last week by the CDC found that norovirus outbreaks nearly tripled in the 2021-2022 surveillance year compared to the 2020-2021 surveillance year.
Park visitors should therefore remain vigilant, the authors warn. “With norovirus rising nationwide and visitor rates returning to near prepandemic levels, the potential exists for a resurgence of norovirus outbreaks among visitors to the Grand Canyon hinterland,” they conclude.
