An investigation of a waterborne outbreak caused by microbiological contamination of the drinking water supply system Background: An outbreak of acute gastroenteritis occurred in March and April 2010 ...in the town of Menges where drinking tap water was contaminated, affecting an area with 3000 residents. The aim was to assess the extent of the outbreak, identify the etiological agents and test the hypothesis that drinking unboiled water from the distribution system was the vehicle for the outbreak, and if necessary to initiate appropriate control measures. Methods: A retrospective cohort study including 160 randomly selected households (20% of the affected residents) was conducted. A case was a resident of Menges developing either diarrhoea, vomiting or abdominal pain. Results: The overall attack rate was 31.3%. The epidemic curve showed a clear peak in the number of cases suggesting a common point source exposure. Residents who consumed unboiled tap water were 4.8 times (95% CI 0.7-32.7) more likely to become ill than the non-exposed. Drinking unboiled water, brushing teeth and eating raw fruit and vegetables washed with unboiled tap water was associated with gastroenteritis (RR 3.1 (CI 95% 1.5-6.5), 3.1 (CI 95% 1.2-8.1) and 2.3 (CI 95% 1.2-4.3)). There was a dose-response relationship between the volume of unboiled tap water drunk and the attack rate among the residents. Norovirus and Rotavirus were detected in the water samples, as well as in stool samples from the cases. Conclusion: The results suggest that the vehicle of transmission was contaminated drinking water. Residents of affected area were advised to temporarily boil tap water. Because of continuous problems with water from the distribution system, building a new one from other water sources was considered. PUBLICATION ABSTRACT
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