Factors associated with a prolonged Norovirus outbreak in a nursing home, Normandy, France, 2016

Introduction On September 26, 2016, the Normandy regional health agency was reported about an outbreak of acute gastroenteritis among 20 residents and 4 staff members in a nursing home. Onset of symptoms occurred between September 25 and 26, 2016. A foodborne outbreak was initially suspected to be the source of contamination of the cases as the initial environmental investigation found a sample of “mixed porc” meal served on September 24, 2016 positive for Norovirus type II. In total, the outbreak lasted 3 weeks. During the 3rd week of the outbreak, further epidemiological and environmental investigations were launched in order to identify factors associated with a prolonged transmission of Norovirus. Methods A case of gastroenteritis was defined as a resident or a staff member with symptoms of diarrhea and/or vomiting that occurred between October 17, 2016 and October 12, 2016. A retrospective cohort study was conducted among staff and residents in order to identify risk factors associated with a prolonged transmission of norovirus. A standardized questionnaire collected information on risk factors of norovirus infection during the outbreak period including sociodemographic characteristics, type of diet and place of lunch, score of dependency, type of accommodation, type of employment (staff) and type of symptoms. Risk factors of gastroenteritis were searched using a logistic regression ( P n  = 5) and further food samples were undertaken on the 3rd week of the outbreak. The premises were inspected and professional practices were reviewed. Results In total, 49/89 (55%) “resident” cases and 9/85 (11%) “staff” cases were identified between September 17, 2016 and October 22, 2016. The occurrence of a peak of cases on September 25 and 26 with a 20% attack rate motivated the reporting of the outbreak to the health agency. However, the retrospective epidemiological investigations showed the occurrence of previous unreported cases, including an index case in a staff with onset of symptoms on September 17, 2016. No further food sample other the “mixed pork” served on September 24, 2016 came back positive for Norovirus. The analyses of the supplier's batch however, tested negative for Norovirus. Three stool samples from residents were positive for Norovirus type II over the outbreak period. In multivariate analysis, the only risk factor for contamination was a high dependence score (GIR  Conclusion Norovirus outbreak are common in nursing home during the autumn–winter season. The introduction of the virus in the premises probably occurred through a sick staff who was not reported. The lack of reporting of index cases, in addition to a poor application of barrier measures, explains the persistence of gastroenteritis cases via cross-transmission, favored by the large dependency of residents. The training of professionals in timely recognition and reporting of gastroenteritis cases and proper application of control measures remains challenging in such setting.