Person-to-person transmission of norovirus resulting in an outbreak of acute gastroenteritis at a summer camp

Background: A significant proportion of norovirus (NV) gastroenteritis outbreaks described in the Spanish literature have been because of contaminated food or water. Aim: This study describes an outbreak of acute gastroenteritis because of NV in which there was person-toperson transmission. Materials and methods: A retrospective cohort study was carried out; we established the case definition for primary and secondary cases. An epidemiological survey was designed, including possible food exposures, and clinical and laboratory data. Water and stool samples were taken from affected individuals and food handlers. The presence of NV was detected using a reverse transcription-PCR assay. We analyzed the risk of gastroenteritis using relative risk and its 95% confidence interval as the measure of association, and estimated the basic reproductive number (R0). Results: The primary attack rate was 45.0%, with no significant differences between sexes. The secondary attack rate during the camp stay was 22.7%. The basic reproductive number for 5 days was R0=2.62. The most frequent symptoms were abdominal pain (85.7%) and vomiting (81.9%). Epidemiological analysis showed no association with food or drinking water. A total of 66.6% (8/12) of stool samples tested positive for NV (genogroup II). Conclusion: Control measures in general, and hand hygiene in particular, should be extended to the families once the children return home, to prevent secondary cases in NV outbreaks.