Over the past two months, roughly two dozen people have fallen ill with flu-like symptoms following river rafting trips through Grand Canyon National Park. Symptoms have been severe enough in some instances to require hospitalization, and public health officials have so far been unable to identify a cause.
Outbreaks of illness on the Colorado River are not unheard of, although they remain rare. Most recently, in 2022, more than 200 rafters and backpackers contracted norovirus, a highly contagious virus that causes acute gastroenteritis. Even less common, though, is for the cause of an outbreak to baffle medical experts.
The first cases in the recent outbreak appear to be linked to private rafting trips that traveled the Lee’s Ferry to Diamond Creek corridor of the Colorado River between May 18 and June 22. Four of the 16 members of one expedition reported symptoms within a week or two of returning home, including fever, chills, fatigue, weakness, and joint and muscle soreness progressing to severe pain. Several people later sought treatment for a buildup of fluid in their lungs, and at last one developed a raised rash and bruising on their shin that felt similar to a broken bone.
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At least two people experienced symptoms severe enough to require hospitalization, including one who lost consciousness while in the doctor’s office, according to one member of the group, who posted about his experience on Facebook.
Matthew Wappett, the executive director of the Institute for Disability Research, Policy & Practice at Utah State University, was one of those people. An experienced rafter who was rafting the Grand Canyon for the first time, Wappett became ill shortly after returning home from a 16-day trip that ran May 18 to June 2. It began, he told SFGATE, with a painful swelling in his knee that quickly spread into a full-body ache and fever. By June 7, he was in the hospital.
Matthew Wappett, an experienced rafter who was rafting the Grand Canyon for the first time, became ill shortly after returning home from a 16-day trip, which ended up with him in the hospital.
Matthew Wappett, an experienced rafter who was rafting the Grand Canyon for the first time, became ill shortly after returning home from a 16-day trip, which ended up with him in the hospital.Courtesty Matthew Wappett
“It’s been a surreal experience to have to go through something like this, where you don’t even know what’s wrong with you,” Wappett said, adding that now, weeks later, he continues to struggle with pain, weakness and general malaise. His family recently left for another rafting trip on the Salmon River, but Wappett chose to stay home. “We live in a day and age where we think we’ve got everything figured out and we can diagnose anything, and I think this is proving that we really don’t,” he said.
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As these early cases have been shared on social media, other people are now speaking out, bringing the full scale of the outbreak into focus. Shae Blackwell, for example, spent 16 days on the river in late June. She told SFGATE that she shared one of the original Facebook posts about the illness with her group as a matter of curiosity and soon learned that one member had become ill after their trip. He has yet to receive a diagnosis, she said, but has since recovered.
Public health officials have not provided a total number of cases, but the current count hovers somewhere around a dozen. Wappett told SFGATE that when he spoke to National Park Service epidemiologists earlier this week, they shared an unofficial estimate of between 18 and 24 people. Requests for comment to two Park Service epidemiologists assigned to the outbreak went unanswered prior to this article’s publication.
In a statement shared with SFGATE, Grand Canyon spokesperson Joelle Baird confirmed that the park is aware of the outbreak and that the National Park Service Office of Public Health is leading an investigation in coordination with public health officials. “At this time, the investigation is ongoing, and we are not able to comment on the extent of the illnesses, potential diagnoses, or other details while the investigation is underway,” Baird said. “We will share additional information with the public as it becomes available.”
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FILE: Rafters in Marble Canyon at Arizona’s Grand Canyon National Park.
FILE: Rafters in Marble Canyon at Arizona’s Grand Canyon National Park.Terray Sylvester/VW Pics/Universal Images Group via Getty
The race is now on to identify the cause. In the weeks since cases first appeared, people have been tested for a variety of diseases, including COVID-19, rabies, dengue fever, Rocky Mountain spotted fever, West Nile virus, chikungunya, hantavirus, and a rare fungal disease called valley fever. The variable nature of these conditions — which include bacterial, viral and fungal pathogens, and which are spread through contaminated water, mosquitos, ticks and dust — hints at just how flummoxed health officials are.
So far, all tests have returned negative or inconclusive, such that Wappett says private genetics testing companies have reached out to him offering to run free tests to solve the riddle.
Consensus is narrowing, however, around leptospirosis as a likely candidate. The disease is caused by the Leptospira bacteria, which infects humans and other animals through contact with soil or water contaminated by infected urine. The bacteria enters the body through cuts, scrapes or mucous membranes such as in the eyes, nose and mouth.
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Harmony Tyner, an infectious disease specialist at Dartmouth Health in New Hampshire, stresses that she can only speculate pending an official diagnosis but told SFGATE that leptospirosis lends itself to certain details of the case.
“Leptospirosis is an interesting disease in that it’s very common, but because it’s hard to grow the bacteria in a lab, it doesn’t make it onto our diagnosis list a lot of the time,” she said. “But anytime I hear fresh water, my list automatically includes leptospirosis.”
A dryer-than-usual winter across the West means that bacteria could have built up in stagnant pools or other freshwater sources that rafters swim in or drink. And while many of the reported symptoms are general, joint pain and pain that feels like a broken bone can be indicative of leptospirosis. Symptoms typically appear up to two weeks after exposure — matching the existing case timelines — and sometimes come in waves, with an acute, flu-like phase followed by an inflammatory phase after starting antibiotics. This later phase is caused by toxins released by the rapidly dying bacteria that cause the body’s immune system to go haywire.
Tyner’s deductions also line up with details shared by other affected rafters. James Schnepel was part of an eight-person private trip on the Grand from June 3 to June 18 and was the only member of his group to fall ill. Shortly after arriving back home, Schnepel, like others, began to experience muscle soreness, achy joints, a runny nose and general malaise. His symptoms, which have since cleared, lasted for about a week, he told SFGATE.
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FILE: A group of rafters, from Alaska, push off from the banks at Lees Ferry for a 25-day rafting trip down the Grand Canyon on Jan. 1, 2023, in Marble Canyon, Ariz.
FILE: A group of rafters, from Alaska, push off from the banks at Lees Ferry for a 25-day rafting trip down the Grand Canyon on Jan. 1, 2023, in Marble Canyon, Ariz.RJ Sangosti/Denver Post via Getty Images
Looking back, Schnepel says he can think of one instance that could be linked to his illness. Early in the trip, the group planned a hike through a slot canyon called the Silver Grotto. Accessing the canyon requires scaling a slick rock face, and when the group arrived, there was a stagnant pool of water at the base. Schnepel was the only one to wade through and attempt the climb.
“I made it partway up before the group decided no one else was interested, and in trying to down climb, I slid down into the pool and totally submerged myself,” he said. “I have no idea if that stagnant pond had anything to do with my later conditions, but it comes to mind as I read about others.”
For her part, Tyner said such an incident is “a classic infection scenario,” adding that “if it was my patient, until we have an alternative, I’d be treating everybody with doxycycline,” a drug often used on bacterial infections including leptospirosis.
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Several people have since started taking the drug, and some, including Wappett, have responded well to the treatment. One physician told SFGATE over Facebook that she plans to bring doxycycline with her on an upcoming Grand Canyon trip, but Tyner said that’s not something everyone needs to do.
Doxycycline is a great antibiotic, she said, but it causes sensitivity to sun exposure and can cause nausea when taken on an empty stomach. Because of this, it isn’t something that should be taken as a preventive measure.
“Instead, I’d just encourage people to be aware of the risks. Wear bug spray and sunscreen and keep good hygiene. Maybe reconsider swimming in any standing water,” she said. “I’m a big fan of experiencing life with open eyes, and if it were me considering a trip, I wouldn’t let this stop me from going.”
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July 17, 2026
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Southwest Contributing Parks Editor
Amanda Heidt (she/her) is the Southwest contributing parks editor at SFGATE, covering parks and public lands across Utah, Colorado, New Mexico, Arizona and Nevada. She is also an award-winning freelance science journalist, with bylines in The New York Times, National Geographic, Nature, Science and NPR, among other outlets. When away from her desk, you can find Amanda kicking around in the desert near Moab or otherwise chasing beautiful scenery. Send story tips to amanda.heidt@sfgate.com, or message her securely on Signal at aheidt.16.