In the last month, about 50 people have reported getting ill while rafting or working on the famed River of No Return.
Epidemiologists are trying to determine what is causing the gastrointestinal illness that has affected commercial and private rafters on the Middle Fork of the Salmon as well as U.S. Forest Service and fire personnel, said Mike Taylor, an epidemiologist with the Eastern Idaho Public Health District.
"Even river guides have reported getting ill. A Forest Service weed control crew had to be flown out" after becoming ill, Taylor said.
Based on the symptoms nausea, vomiting and diarrhea and the rapid onset with recovery in 24 to 48 hours, Taylor suspects it may be norovirus, the notorious cruise-ship virus.
The highly contagious intestinal bug can be spread by contact with an infected person, food or surfaces and is especially virulent among people in close quarters.
Despite the Middle Forks isolation and ruggedness, the circumstances in which people traverse the 104-mile wilderness river are similar to those in a cruise ship a lot of people close together using shared facilities. Last summer, about 9,500 people floated the stretch.
That's about 800 people a week sharing the same campsites, water stations and toilets. The average commercial raft group has about 23 people; the average private raft group has about 11 people, according to the U.S. Forest Service.
So far, Taylor said, the only thing the victims have in common is being on the Middle Fork; they came from different rafting parties, and floated at different times. Taylor is not aware of anyone requiring hospitalization
To learn more about the outbreak, the health district is surveying people who floated the Middle Fork of the Salmon River since July 1.
Of the 238 survey responses officials have received since Tuesday morning, 51 people reported getting ill on the trip. The district has already tested three people one came back with norovirus, one with E. coli and one with giardia. The district is working to collect samples from other people to confirm if the broader outbreak is of norovirus or some other illness.
Taylor said the health district is working with the Forest Service to inform rafters about the outbreak and to educate them about how to prevent and deal with this issue while on a river trip.
He advises rafters to wash their hands frequently, and not to drink untreated water. Filtering river or creek water is not enough to remove the virus; water also should be treated with a chemical disinfectant.
The Forest Service cleans Middle Fork facilities daily and tests drinking water monthly, said Amy Baumer with the Salmon-Challis National Forest. So far, all water tests have been clean, she said.
Taylor said rafters should be diligent through the rest of the season because norovirus is hardy.
Norovirus is fairly stable in water and sunshine. Over the wintertime, it will be killed and we should start fresh next year, Taylor said.
Cynthia Sewell: 377-6428, Twitter: @CynthiaSewell