Surveillance of Municipal drinking-water supply after a Norovirus outbreak in Italy

Abstract

BACKGROUND Norovirus (NoV), in the family Caliciviridae, is the major causative agent of non-bacterial acute gastroenteritis, whose possible routes of transmission include drinking water. During March 2011 an outbreak of gastroenteritis was identified by the local public health service in S. Stefano di Quisquina, Sicily, Italy. Over two weeks, 156 cases were identified and NoV genotype GII.4 v2010 was detected in stool samples from eleven out of twelve patients. The finding of identical genomic sequences between patients suggested a common infection source and an epidemiological investigation indicated a possible correlation to municipal drinking water consumption. One of the wells supplying the public water network was thought to be a possible source of contamination. The use of municipal drinking water was restricted and alternative water supplies were provided to the population (drinking bottled water). The submersible water pump of the municipal well was replaced and chlorine concentration of the water supply was increased. To monitor the efficiency of control measures, regular sampling from the public water system and testing for NoV were performed during a period of nine months (March – November). METHODS Water samples (n=48) were tested for NoV by different molecular methods. The NoV genome fragments amplified from the samples were sequenced directly and analysed using CLUSTALW and MEGA 5.0 for genotyping. RESULTS Eight water samples were positive for NoV over a period of three months until May 2011, no NoV-positive sample was found thereafter and until November 2011. Over the surveillance period, a variety of NoV genotypes was detected in water, including GI.1, GI.4, GII.7, GII.21 and GII.4. However, the two GII.4 strains identified in water and stool samples were not identical, belonging to different GII.4 variants (2004 vs 2010). Furthermore, not all samples that tested positive for NoV by Real Time RT-PCR were confirmed by One-Step RT-PCR. CONCLUSION A water origin of the NoV outbreak of March 2011 remains tentative, since the NoV strains detected in environmental samples were different from those involved in the outbreak cases. Indeed, the contamination of municipal water might have followed the outbreak. However, epidemiological and molecular investigations guided outbreak control measures and suggested interventions to prevent future network contaminations. These measures resulted in a quick decrease of cases. The investigations also resulted in formulation of recommendations to local authorities pertaining to outbreak management in order to prevent similar outbreaks in the future.