On April 8, 2022, NPS-OPH and Grand Canyon staff were notified of seven cases of AGE in a commercial rafting trip. At that time NPS-OPH and CCHHS shared norovirus infection control recommendations with outfitters, and CCHHS collected pooled groover samples for testing. CCHHS and NPS-OPH distributed a supplementary questionnaire for trip-level characteristics (trip report form [TRF]) to all affected commercial rafting trips. Outfitters and trip guides reported contact information, dates, means of transport (row or motor), food and beverage suppliers, menus, trip plans including stops and overnight stay locations, exchanges of passengers at Phantom Ranch, other group contact, and illnesses. By May 10^th, five additional commercial rafting trips reported cases, leading NPS-OPH, CCHHS, and Arizona Department of Health Services (ADHS) to contact Centers for Disease Control and Prevention (CDC). By May 21, 13 additional trips with 102 cases were documented, and 27 backpackers also reported AGE. A specific source for the case increases had not been identified; therefore, NPS-OPH and CCHHS requested CDC aid in conducting an outbreak investigation.

During April 1–June 30, 2022, the Colorado River NPS Backcountry Permit Office (NPS-BPO) permitted 142 private trips (1,824 persons) and 317 commercial trips (7,182 persons) for a total of 9,006 persons across 459 trips. The NPS-BPO reported 1,206 hike-in permits (3,976 persons) were issued for backcountry campers with at least one night outside of the main canyon corridor (developed campgrounds along popular trails linking North and South Rims).

We sought to determine outbreak transmission contributors and individual AGE risk factors while rafting or backpacking in Grand Canyon by comparing behaviors (e.g., hand hygiene practices) and environmental exposures (e.g., drinking water source and treatment) among trips with ≥1 AGE cases during trip or ≤7 days of trip end with trips that reported no cases.

Since norovirus was suspected at investigation onset, we defined a probable case as a person who participated in Grand Canyon rafting or backcountry backpacking during April 1- June 16 and experienced any vomiting or diarrhea (≥3 loose stools in 24 hours) within the 24 hours before trip start through three days after their trip end. Unaffected trips were identified using the Grand Canyon online river trip launch calendars (11). Well persons were defined as persons who did not report AGE, regardless of whether they were on an affected or unaffected trip.

We developed an online survey (Supplemental Materials) available through Qualtrics during June 13–July 8 and provided an email template with a survey link to river outfitters for distribution to passengers on affected and selected unaffected trips. As we were unable to obtain contact information for commercial passengers, rafting companies were asked to distribute the survey to participants of selected trips. All 14 commercial trips launched during the time of interest with ≥1 AGE case were selected, and 14 unaffected trips with a similar distribution of launch dates, group size, and trip duration were selected using information from NPS trip launch calendars (11) and rafting permits. We partnered with NPS-BPO to distribute the survey on June 16, to all private rafting trip participants age ≥16 years and backpacking trip leaders with routes reported outside the main canyon corridor during April 1-June 16. Survey distribution was limited to backpacking trip leaders due to the permit issuance process; leaders were asked to share the survey with their party members. The survey asked about symptoms, meals consumed, drinking water sources and treatment methods, and environmental exposures. We do not know the number of people who received the survey; however, completed surveys represented an estimated 14% of rafters and 12% of backcountry backpackers who were permitted for their respective activities during April–June 2022.

IRF, TRF, and survey data were merged using name, age, sex, home state or country, and trip dates and analyzed using SAS v.9.4 (Cary, NC). Epidemic curves and frequencies of participant characteristics and exposures were stratified by activity type (commercial or private rafting or backpacking). Individuals with AGE onset in the 24 hours before trip start were excluded (n=5) for risk factor analysis, as their exposure preceded their backcountry trip. Cohort analysis was limited to persons who completed a survey and reported a trip start date during April and May. We generated Kaplan-Meier curves describing differences among rafters between cases and well persons by known risk factors including age, encounters with ill persons, and hand hygiene. Univariable and multivariable Cox proportional hazard models (12) were used to assess the statistical significance of risk factors among rafters only. To improve sample size, commercial and private rafters were collapsed into a single multivariable model; commercial versus private trip type was included as a predictor in the model to account for the difference in participation of healthy participants due to survey distribution methods. Survival time was defined as days from trip launch to symptom onset; those with no symptoms were censored at three days after trip end. Variables were selected using backward elimination. Month of trip launch was forced into the model to adjust for differences in risk over time.

In total, 484 backpackers completed the survey and met either the case (n=31) or well person (n=453) definitions (Supplemental Table 1). Backpacker age ranged from 16 to 82 years (median 40 years) and 329 (69%) were male. Three reported AGE symptoms in the 24 hours before the start of their hike. Among cases, hiking routes included most designated trails into the canyon, including North and South Rims, and hiking areas both east and west of the main park corridor. Most cases used the same toilet facilities or camping locations as other ill backpackers or visited river areas shared by rafters; however potential exposures were spread out in time, and no single location was associated with all cases. Due to low case numbers among backpacking survey respondents, no additional analyses were conducted.

Our analysis included 286 commercial and 470 private rafters (Table 1). Participants’ ages ranged from 18– 82 years (median 52 years), and 440 (58%) were male. Most commercial trips lasted 1–2 weeks (median [interquartile range (IQR)] = 7 [6–13] days), while private trips lasted 2–3 weeks (16 [15–20] days). Among survey respondents, commercial trip participants had a higher proportion of AGE cases (n=136, 47%) compared to private rafters (n=55, 12%); however, in context of the number of the 7,182 commercial rafters and 1,824 private rafters permitted to raft during April–June, the overall attack rate detected in the investigation was somewhat lower on commercial trips (2%) than private trips (3%).

AGE onset was reported from April 4–June 8, with five (4%) of the 191 total cases reporting onset after May 22 (Supplemental Figure 2). Most cases occurred among park visitors (161; 84%) and the remaining (30; 16%) among professional guides. AGE onset was geographically widespread (122 [IQR: 82 – 183] miles from Lees Ferry) and occurred a median of seven days (IQR: 4–10) into the rafting trip. Potential exposure locations occurred throughout the river corridor including multiple popular attraction sites (Supplemental Figure 3). Symptoms lasted a median of 24 hours (IQR: 18–42 hours) and were characterized by vomiting (n=166, 87%), non-bloody diarrhea (n=146, 76%), and fatigue (n=145, 76%). Other commonly reported symptoms were headache (n=72, 38%), chills (n=69, 36%), and muscle aches (n=63, 33%) (Table 1).

Univariable Cox regression was used to examine the influence of single characteristics on time to AGE. All characteristics shown in Table 1 were assessed for statistical significance or excluded due to small strata size (n<5); significant predictors and those of particular interest to the investigation are shown in Table 2. Among commercial participants, younger age groups had higher rates of AGE compared to older groups (<40 years: HR [95%CI] = 1.5 [1.2–1.9] and 1.6 [1.1–2.4] for those 40–59 years, compared to persons aged ≥60 years). Guides (who are concession employees) had higher rates of AGE than visitors (2.2 [1.5–3.3]). Sharing dishes or utensils (1.9 [1.2–3.1]); sub-optimal hand-hygiene (i.e., use of hand sanitizer, wipes, or only water as compared to washing with soap and water) before snacking (1.9 [1.1–3.3]); and known contact with ill persons, such as being on a trip with an ill person or reporting contact with ill individuals from other affected trips (3.0 [1.8–4.8]) were all associated with AGE. Of note, 99% of respondents reported washing hands with soap and before all meals, while only 65% reported doing so before snacks. Among private trips, only contact with ill persons (4.1 [2.2–7.3]) was associated with AGE. No association was identified for specific food or water exposures, or for exchanges (e.g., being on a trip in which new individuals joined the trip halfway through). There was no difference in AGE occurrence by sex or race/ethnicity. The survival curves in Figure 1 show the timing of illness onset by age and known contact with ill persons. All age groups became ill at around the same rate early in the trip, but younger individuals continued to fall ill later in the trip after cases plateaued in the older age groups. Respondents who reported known interactions with ill persons became ill at a faster rate throughout their trips than those who did not.

Our multivariable Cox regression analysis included 119 cases and 505 well persons with complete survey data. The final model included age, contact with known ill persons, hand hygiene before snacks, the trip launch month, and whether the trip was commercial or private. Results were like those of the univariate analysis, highlighting contact with ill persons, inadequate hand hygiene, and younger (≤60 years) age were associated with AGE (Table 3). Of note, significant differences in reported behaviors by age group were observed (Supplemental Table 2). When excluding age from the model, sharing dishes remained significant (Supplemental Table 3).

This study has several limitations. First, we were unable to assess statistical significance for several factors due to small strata sizes, particularly among private rafters. The small number of ill backpackers who responded to the survey precluded statistical modeling of hiker data. We were also unable to apply trip-level matching in our analysis of commercial rafters, as we were only able to reach a few unaffected trips. Second, the total number of cases associated with this outbreak is likely underreported among more casual recreators in the park as an increase in norovirus activity was observed at a national level in Spring 2022 (4). There is also the potential for recall bias or misclassification, since the survey asked about experiences that took place up to two months before the survey was available, and case classification was symptom-based. Third, we were unable to collect clinical specimens from ill individuals for lab confirmation, so we cannot rule out additional pathogens as reasons for symptoms. A 2013 AGE outbreak among rafters in Idaho was attributed to both Giardia and norovirus (16). Fourth, the detection of two norovirus genogroups suggests there were likely multiple introductions of norovirus into Grand Canyon during Spring 2022. Finally, norovirus can spread multiple ways, and it is impossible to assess all potential sources of contamination in Grand Canyon.

Norovirus cases are returning to near pre-pandemic levels (4) as are Grand Canyon visitation numbers (6). Therefore, it is likely that there may be future outbreaks among rafters and backpackers in the park. This investigation’s recommendations for control and prevention measures remain broad (7, 8, 10, 27) with a focus on public messaging, screening for AGE, surface decontamination, hand hygiene, and limiting contact with ill persons unless as a necessary caregiver. Since AGE prevention and control guidelines are already tailored to this unique environment, interventions for mitigation could focus on modifiable behaviors in patrons like enhancing norovirus education to all backcountry permit holders. Persons could consider taking steps, such as carrying biodegradable soap for handwashing when possible. Outfitters could consider implementing a pre-arrival symptom screening process and encouraging rebooking for people with AGE onset within 72 hours of trip start. Moreover, information including plain language recommendations of isolation and hand hygiene (emphasizing soap and water when available) could be distributed following any illness onset. At the time of this publication, NPS-OPH developed a “Norovirus on the River” handout and web page for outfitters, which includes a description of mitigation measures recommended during an outbreak (27). Additional signage with information regarding AGE outbreaks could be posted on bulletin boards throughout the park and restroom signs about the importance of proper hand hygiene and reporting illness. During periods of increased AGE, if staffing allows, parks could increase the regular cleaning and disinfection of high-touch points like restrooms, water fountains, and spigots at the launch sites and trail heads to reduce spread. Beyond launch sites, the Colorado River is accessible only by rafting, hiking, pack animal, and helicopter, emphasizing the importance of public health messaging before trip start in addition to existing measures. Despite norovirus’s environmental stability and remarkable transmissibility, infection control mitigation measures are effective in other settings and can be readily applied to the wilderness environment to reduce spread in future outbreaks.