Norovirus on the Pacific Crest Trail: How Washington Tracked a Hikers' Outbreak Through Social Media, 2022

The Pacific Crest Trail runs 2,650 miles from California to Washington and draws hikers from around the world. On August 26, 2022, the Washington State Department of Health began hearing informally that many hikers on the trail's Washington section had acute gastroenteritis — sudden vomiting and diarrhea.

Finding the outbreak online

Investigators searched social media. A search on September 5 found 27 posts from Washington hikers reporting the illness in the previous month, 26 of them with a date when symptoms began — plus many more undated posts, suggesting illness through the whole 2022 hiking season.

During September 13–18, they posted an online survey (REDCap) in a Facebook group popular with Washington PCT hikers, and put it up with a QR code at trailheads where people had fallen ill.

Hikers with gastrointestinal symptoms by onset date: 26 from social media posts and 22 from the survey, Washington, July 20–October 4, 2022. CDC

What the survey showed

That clustering pointed to something in the environment along that stretch.

The virus. Only two respondents had had symptoms in the previous 14 days and were still in Washington; both gave stool samples. Both tested positive for norovirus at the Washington State Public Health Laboratory, and sequencing at the California Department of Public Health's CaliciNET laboratory identified both as genotype GII.10[P16].

Searching the trail

Investigators worked backward from where each hiker fell ill, subtracting the distance a person could plausibly walk during norovirus's incubation period, to estimate where the exposure happened. Matching that area against facilities on the trail picked out the places to sample on October 3–4, 2022:

• two sets of ventilated improved pit (VIP) latrines
• a cabin used as a rest stop
• a stream hikers drank from

The conclusion

No norovirus turned up in the environment. But the symptoms, the stool results, the environmental tests and the links between cases all pointed the same way: the outbreak was mainly caused by norovirus, and contaminated surfaces in the cabin and latrines probably amplified its spread. Health officials shared improved sanitation protocols, handwashing messages and guidance on reporting outbreaks with the authorities responsible for the trail.

The outbreak was probably much larger than the 27 survey respondents: the undated social media posts alone suggest as much.

Why it's hard in the backcountry

• Clean water and soap for handwashing are hard to come by.
• Shared surfaces such as cabins and restrooms can't be routinely disinfected.
• The alcohol-based hand sanitizers hikers commonly carry do not work against norovirus.

Preventing the next outbreak, the authors conclude, means raising awareness that handwashing matters and sanitizer does not stop norovirus, cleaning public facilities more often — and watching social media, which may help spot outbreaks early.

Sources

Based on Hamlet A, Begley K, Miko S, et al., "Notes from the Field: Gastrointestinal Illness Among Hikers on the Pacific Crest Trail — Washington, August–October 2022," MMWR volume 72, number 36, Centers for Disease Control and Prevention, with authors from CDC, the Washington State Department of Health and the California Department of Public Health; a work of the United States government in the public domain. The report says 26 of the 27 social media reports gave an onset date, and later describes all 27 as dated; the first figure, which matches its chart, is used here.