Volume , Issue

A typical river beach kitchen with dishwash setup. Photo credit: NPS file.

Grand Canyon (GC) National Park Service (NPS) officials reported more than 150 cases of acute gastrointestinal illness (AGI) among river runners and hikers between April and May, 2022. Local NPS and Coconino County Health Department officials are investigating the source(s) of the largest outbreak in the GC.  So far, fecal samples have confirmed Norovirus (NoV) during at least 8 raft trips.

In 1998, as part of a four-year study of the Colorado River and its tributaries in GC between Lees Ferry and Diamond Creek (River Mile 226), University of Arizona researchers and County Public Health officials concluded:  Low occurrence and concentrations of Cryptosporidium and Giardia suggests that the waters in the GC are relatively pristine.  Furthermore, human enteroviruses were not detected.  This finding was later confirmed in a 2004 study.  The current NoV source(s) is almost assuredly from human-to-human transmission, and not waterborne from the Colorado river or side streams in GC.

Currently, the NPS allots for more than 28,000 river runners annually. Rowing, paddling, or motorized boating trips in GC average between 7-14 days in length.  Most meals are served cafeteria style, with shared serving utensils.  NPS requires hand washing stations with soap and water for use before eating and after toilet use.  In addition, portable toilets are required, and all fecal (and vomit) waste must be transported out.  River runners are also required to urinate directly into the river or into “pee buckets,” which are subsequently emptied into the river (without feces or other solid waste).  This is to prevent beach contamination by urine and a harmful change in soil pH.  The volume of the river immediately dilutes the urine.  River water is used as the main source of drinking and cooking water after filtration and disinfection.

Common beach handwashing station on the Colorado River. Photo credit: Down River Equipment.

Clinical Epidemiology

Norovirus is the leading cause of vomiting and diarrhea in the United States. Humans can acquire AGI from microorganisms in water from protozoan parasites, enteric bacteria, and viruses (human or animal) during outdoor recreational activity, and through consuming water that is potentially polluted by fecal waste. Each year, the CDC reports that the number of NoV cases in the US are approximately 20 million vomiting and diarrhea illnesses, 900 deaths (mostly in adults 65 years and older), 109,000 hospitalizations, 465,000 emergency department visits, and 2.3 million outpatient visits (mostly children).


Norovirus infection can be acquired at any time of year. Transmission is typically human-to-human, requires only a small inoculum (<100 viral particles), and is spread via several mechanisms. Most common is the fecal-oral route. Spread may also occur through aerosolized droplets containing viral particles (usual during vomiting), direct contact with an infected person or consumption of contaminated food and water.

Signs and Symptoms

The symptoms of NoV infection are nonbloody diarrhea, vomiting (nonbloody, nonbilious), and stomach pain. Severe dehydration, resulting from diarrhea and vomiting, is the most serious complication of NoV illness. Low-grade fever, nausea, and stomach cramping may also be present. Average incubation is 24 to 48 hours and symptoms usually last 12 to 72 hours. Viral shedding in stool is maximal over the first 24 to 48 hours after onset of illness; with a mean duration is four weeks. Immunocompromised hosts may have persistent viral shedding in feces for several months.

Norovirus Diagnosis and Treatment in the Backcountry

A possible NoV infection should be suspected in all patients with AGI symptoms. Confirming the diagnosis with stool testing is impractical (especially in a wilderness setting) and generally not necessary. Fortunately, viral AGI is self-limited and is generally treated with simple supportive measures, most important of which are fluid repletion and nutrition. No specific antiviral or vaccines (in clinical trials) are currently available and antibiotics are not indicated. Patients without active vomiting or signs of significant volume depletion should be rehydrated with fluids, e.g., water, sport drinks, diluted fruit juices, etc. Fluid repletion can be augmented with saltine crackers and broths or soups. Once vomiting has stopped and appetite returns, patients can be encouraged to eat small meals as tolerated. Restricted diets are generally not necessary, but bland, low-residue foods may be more tolerable. Evidence to support the BRAT diet (Bananas, Rice, Applesauce, and Toast) is lacking, as is proof to support exclusion of milk and dairy products. The value in use of probiotics or zinc in AGI is not well established. Antiemetics (ondansetron) and antimotility medications (loperamide) may be useful for excessive vomiting or excessive fluid loss from diarrhea, respectively.

When to Evacuate Patients

Most individuals with AGI can be managed with supportive care in the outpatient setting, including wilderness. Indications for hospitalization include any individual with signs of severe dehydration; ongoing, intractable vomiting; excessive bloody stool or rectal bleeding; severe abdominal pain; prolonged symptoms (more than one week); AGI in patients 65 years or older with comorbidities (e.g., diabetes mellitus, immunocompromised), and pregnancy. GC backcountry air evacuations may be initiated by communications to GC EMS agencies via satellite phone or digital texting devices such as InReach, cell phones (functional in very limited locations), or in person to the NPS Ranger Station at Phantom Ranch (River Mile 88). The GC Regional Communication Center emergency line is (928) 638-7805.

Grand Canyon National Park Service medical evacuation team. Photo credit: NPS file.


Unfortunately, and again, NoV is very contagious, able to be shed in feces for weeks by those previously infected. The virus has the ability to survive in the environment, resisting freezing temperatures, and heating to 60°C (140°F). Therefore, hand hygiene is extremely important to minimize spread. It should consist of a thorough washing for at least 20 seconds with soap and water, rather than use of alcohol-based hand sanitizer, especially to prevent NoV infection.

Common portable toilet (known as the Groover) on the Colorado River. Photo credit: frankg, Sand Dollar Adventures.

Contaminated surfaces should be disinfected with chlorine bleach solution, then followed by soap and hot water. In addition, individuals with AGI infection should not prepare food for others until at least two days after resolution of symptoms. River rafting vendors provide many practical recommendations to prevent AGI such as tips for going to the bathroom on the river. Also, the GC NPS has posted recent NoV prevention guidelines.

Water Disinfection Procedures

Based on the recent NoV outbreak, the NPS recommends that all collected water in the GC should be disinfected. Since silt particles inhibit disinfection, any turbid water should be allowed to settle undisturbed for several hours. Adding a small amount of a clearing agent, such as aluminum sulfate, may help facilitate this process. Very cold water (e.g., Colorado River) should rest even longer. Once the silt has settled, filter the clear water through a minimum of an absolute 0.3-micron filter. Water filters are appealing for backcountry ease of use, but by itself will not prevention potential acquisition of NoV and other viruses that are smaller than 0.3-micron filters. As technology rapidly changes, seek out ongoing reviews for the best commercial water filters.

Safest disinfection methods use one of these options: 1) Heat – use boiling water to purify water in a roiling boil for at least 60 seconds. At elevations above 6500 feet (2000 meters), increase the boiling time to three minutes; 2) Ultraviolet (UV) light - treat non-turbid water for 60 seconds using a commercial UV light; 3) Chemical disinfection with halogens (iodine and chlorine). We recommend to review an evidence-based discussion on water disinfection techniques published by the Wilderness Medical Society.

Summary of water disinfection techniques. Photo credit: CDC


Norovirus, as the cause for AGI, should be suspected in patients with acute onset of vomiting and/or watery diarrhea. Most commonly, norovirus transmission is usually human-to-human. NoV is self-limited and is generally treated with simple supportive measures. Prevention is key and best done with vigilance to hygiene, especially hand washing with soap and water, and alcohol-based hand sanitizers are suboptimal to control the spread of NoV.

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