An acute gastroenteritis (AG) outbreak occurred among participants in an obstacle race in France in the summer of 2015. An investigation in two phases was conducted to identify the source of infection and document the extent of the outbreak. First, a message on a social media website asked racers to report any symptoms by email to the Regional Health Agency of Provence-Alpes-Cte dAzur. Second, a retrospective cross-sectional study was conducted through an interactive questionnaire for all participants, followed by an analytical study of potential risks factors. Of 8,229 persons registered, 1,264 adults reported AG resolved within 48 hours. Of adults who reported AG, 866 met the case definition. Age group, departure time and ingestion of mud were associated with AG. Twenty stool specimens tested negative for bacteria. All four stool samples tested for viruses were positive for norovirus genogroup I and genotype 2. No indicator bac- teria for faecal contamination were found in drinking water but muddy water of ponds tested positive. The outbreak was possibly caused by human-to-human transmission of a norovirus introduced by one or more persons and transmitted through contaminated mud. Risks related to similar races should be assessed and recommendations be proposed to raise awareness among health authorities and organisers. On 22 June 2015 at 16:00, a hospital near Nice alerted the Regional Health Agency of Provence-Alpes-Cte dAzur (ARS PACA) of 22 patients with AG after participation in an obstacle race, in the south of France. On a social media website, many other participants reported also suffering from similar symptoms. An investigation was immediately conducted in order to identify the source of infection and document the extent of the outbreak. Methods The ARS PACA, responsible for the outbreak inves- tigation and implementation of control measures, requested support of the Regional Office of the French Public Health Agency (Cire Sud). Epidemiological investigation The investigation of the outbreak was conducted in two phases. The first phase started immediately, upon receipt of the outbreak alert, on 22 June 2015. ARS PACA and Cire Sud sent a message on a social media website informing the participants of the obstacle race that an investigation was conducted and asked them to report any recent or current gastrointestinal illness by email to a dedicated address of ARS. This information was also relayed by the organisers of the race and the local press. The second phase, aimed at collecting additional information, was conducted retrospectively through an interactive application, Voozanoo (a secure web-based platform for hosting personal health data) including sections on socio-demographic characteristics, presence of symptoms or not, and sections on potential exposures (catering, means of contact with mud). This short questionnaire (available from the authors upon request) was intended for all participants of the obstacle race, whether or not they had clinical signs of AG. On 30 June, the company that had organised the event sent the Internet link for this survey by email in a news- letter to all the participants of the obstacle race. Cross-sectional descriptive studies were conducted in two phases, and were followed by an analytical study of the potential risks factors (in the second phase). A case of AG was defined as a racer in the obstacle race that took place on 20 June, with self-reported gastrointestinal illness (vomiting and/or diarrhoea) associated or not with other symptoms within eight days from the race. A secondary case of AG was defined as a person who did not participate in the obstacle race as racer, with gastrointestinal illness following at least 24 hours after a contact with a case of AG among racers. We recommended all laboratories to test stool specimens from the racers for Salmonella, Shigella, Campylobacter and Yersinia, and to send stool specimens to the National Reference Laboratory (NRL) for Enteric viruses in Dijon in order to test for norovirus, sapovirus, rotavirus, adenovirus and astrovirus. These samples were tested by a laboratory in Lyon. Water samples were tested for Campylobacter and norovirus by the Laboratory for Hydrology of the French Agency for Food, Environmental and Occupational Health & Safety (ANSES,) in Nancy. The University Laboratory of Pharmacy in Lyon tested the specimens for species of Naegleria [10]. We provided recommendations on measures to prevent secondary spread during a press conference organised on 22 June, through a weekly report of the Cire Sud, on the ARS PACA website, and we posted prevention messages on a social media website. the outbreak investiga- tion was published on the social media website. Among the respondents, 1,300 AG cases were reported: 1,264 adults (attack rate: 16%; 1,264/7,804) and 36 secondary cases (not included in this analysis). Phase 2 was conducted from 30 June to 27 July and a total of 748 questionnaires were completed (for 745 adults and 3 children). Among the participants who filled out the questionnaire, more than half (404/748; 54%) did it the day the newsletter came out and one third (247/748; 33%) the day after. There were 375 AG cases and 373 non cases. Secondary cases reported by respondents rose to a total of 177 cases, not included in this analysis. The mean age of adult respondents was 33 years (range: 18-61 years). The most common symptoms besides vomiting and/or diarrhoea, were abdominal cramps (369/866; 43%) and fever (354/866; 41%). Those symptoms were over or being spontaneously resolved in 48 hours. In phase 2, 69% of participants notified being ill (513/745) and among them, 73% met the case definition (375/513); of these, 67% (251/375) presented fever and 43% (161/375) other digestive symptoms. The other symptoms were non-specific, such as chills (215/375; 57%), headaches (101/375; 27%), asthenia (322/375; 86%), and muscle ache (262/375; 70%). Microbiological investigations We obtained the results of 20 stool samples. Results of bacteriological testing were negative for all samples, except one (positive for Shigella sonnei). Among the four specimens sent to the NRL for enteric viruses, all were positive for norovirus genogroup I and genotype 2 (GI.2). Of the environmental water specimens (n=5) taken from five ponds on 23 June, all were found contaminated by aerobic microorganisms at 36 C (with counts ranging from 86 to 3,200,000 UFC/100 mL) and 22 C (with counts ranging from 2,500 to 1,400,000 UFC/100 mL), indicating bacterial contamination. All specimens were negative for Salmonella, Campylobacter, Enterovirus, V. cholerae and V. parahemolyticus. All environmental samples were positive for Naegleria spp. (with cells ranging from 400 to 280,000/L), but the pathogenic species N. fowleri was not detected. Norovirus geno- group I and II were not detected. A large AG outbreak occurred among the participants of obstacle race in the department of Alpes-Maritimes, France, on 20 June 2015. Of 7,804 adult participants, 1,264 were ill and 866 met the case definition of AG. This outbreak occurred from 20 to 25 June and the epidemiological curves during two different phases of the outbreak investigation were characteristic of a point-source outbreak. The epidemiological investigation enabled us to identify mud ingestion as the main risk factor of developing AG; no other source of infection was identified. Several arguments support a common source outbreak of human origin related to the ingestion of contaminated mud. 

{'answer_start': [471], 'text': ['questionnaire']}