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BACKGROUND: Annually, ~10,000 persons from the United States and abroad raft Idaho’s 104-mile Middle Fork of the Salmon River on 4–10-day trips without road access. During July–August 2013, rafters experienced a gastroenteritis outbreak, requiring air evacuations. We investigated to identify the agent, source, and risk factors for illness, and provided prevention recommendations.
METHODS: We solicited participants for a case-control study through the media, in person, and by e-mailing rafting permit holders. We provided an online questionnaire regarding symptoms, meals, drinking water, and environmental exposures. A case was defined as nausea, vomiting, or diarrhea within 25 days of rafting, in a person who had rafted during July 1–September 23. Control subjects were well rafters from the same period. Ill persons provided stool specimens and we collected environmental samples at locations along the river for laboratory-testing.
RESULTS: We identified 102 case-patients and 293 control subjects. Illness was associated with drinking filtered river water (odds ratio [OR]: 3.9; 95% confidence interval [CI]: 2.4–6.4). This association was stronger among case-patients ill <4 days (norovirus gastroenteritis-like), (n = 63; OR: 6.6; 95% CI: 3.3–12.9) than those ill ≥4 days (n = 38; OR: 2.2; 95% CI: 1.1–4.3). Norovirus (n = 3) and giardia (n = 8) were detected in stool specimens from 11 ill rafters. Norovirus was detected on water spigots and outhouses; Escherichia coli was detected in an unregulated water source.
CONCLUSIONS: This multiple-etiology outbreak apparently was associated with drinking filtered river water; examination of filtering methods used is warranted. Strict adherence among river rafters to disseminated guidelines for gastrointestinal illness prevention, including sanitation, food handling, and water treatment, is advised.