[Gastroenteritis at sporting events with water and mud: an example to illustrate outbreak investigation methods].

OBJECTIVE To determine the size and causative pathogen of the outbreak and to identify risk factors for developing gastroenteritis among participants of the Survivalrun in Udenhout in September 2016. Design Retrospective cohort study. METHODS We sent an invitation to go to an online questionnaire to participants and volunteers of the Survivalrun by email. The link to the questionnaire was also shared on the Facebook page and website of the Survivalrun. We calculated attack rates (AR) and relative risks (RR) for several exposures to identify risk factors for developing diarrhoea and/or vomiting within 3 days after the run. In addition, stool samples of six participants were tested for common gastrointestinal pathogens. RESULTS A total of 444 people completed the questionnaire. Symptoms of gastroenteritis were reported by 163 study participants (37%). Five participants reported symptoms of gastroenteritis in the week before and three participants during the Survivalrun. Multivariate analysis identified the following risk factors for developing gastroenteritis: participation on the second day of the run(RR 2.4: 95% CI 1.1-5.3), ingesting water (RR 1.7: 95% CI 1.3-2.3) and ingesting mud (RR 1.3: 95% CI 1.1-1.6). Four out of six stool samples tested positive for norovirus (various types). CONCLUSION This outbreak investigation shows that pathogens, such as norovirus, can easily spread during sporting events where participants have to move through water and mud. Specific methods and knowledge of the circumstances are essential for a thorough outbreak investigation.