BACKGROUND: Following a 4 day national off-road race, 26 individuals reported gastrointestinal illness (GE) to the Southern Nevada Health District (SNHD) via a web-based event forum in December 2016. Upon identification of the possible outbreak among 328 competitors, 110 volunteers, and 4,000 spectators an investigation was initiated to identify cases, evaluate risk factors, identify potential sources of infection and implement control measures.
METHODS: A web forum consisting of 26 GE related posts associated with the event aided in outbreak detection and provided insights into hypothesis generation and questionnaire development. An online survey enabled rapid identification of cases and collection of demographic, medical, and risk factor information. Links to the survey were posted on the web forum and two Facebook pages. Descriptive statistics and chi-square tests were performed using SAS 9.4. Microbiological testing of four stool specimens was conducted by SNHD (N=2) and the County of San Diego Health & Human Services Agency (N=2).
RESULTS: There were 122 survey respondents and 81 met the probable case definition and two met the laboratory-confirmed case definition. Residents from 8 U.S. states were affected, and most ill individuals were California residents (77%). Adults aged 30-49 years were the largest affected group (64%). Symptoms were predominantly diarrhea (90.4%) and vomiting (84.3%). Symptoms onset ranged from December 7th to December 13th with a peak on December 11th (53% of cases). The median incubation period was 48 hours. Symptoms lasted 24-72 hours. The investigation did not identify a common source of infection. Most ill participants (N=46, 87%) stayed at one hotel. The risk of being ill was higher (RR 3.1, 95% CI 1.5-6.2) in individuals who stayed at that particular hotel. The same hotel was also the focal point for registration events which occurred over a 7 hour period of time on December 8th. The implicated hotel was in the process of performing corrective measures after GE illness was reported by guests and before SNHD inspection. Of the four specimens collected for testing, two were positive for Norovirus genogroup I, one from Clark County and the other from San Diego County.
CONCLUSIONS: The use of the web forum, social media (facebook postings) and a web-based survey was instrumental in completing this outbreak investigation. Based on these findings and past investigations for similar popular events, SNHD recommends that organizers of such events raise awareness among participants of the risk of GE infections through educational collaboration with the local health department.