Norovirus gastroenteritis general outbreak associated with raw shellfish consumption in South Italy

BACKGROUND: Despite Noroviruses (NV, previously "Norwalk-like viruses") being a leading cause of acute gastroenteritis outbreaks, the impact of NV infection is at present unknown and little information is available about strains circulating in Italy. In April 2002 an outbreak of gastroenteritis occurred in the province of Bari (South-east Italy), involving several households. METHODS: A retrospective cohort study was performed in order to assess risk factors associated with illness. All households where a case occurred were included in the study. Faecal specimens were collected from ill individuals. NV-specific RT-PCR was performed. Eleven samples of mussels were collected from fish-markets involved in the outbreak. A nested PCR was used for mussel samples. RESULTS: One hundred and three cases, detected by means of active surveillance, met the case definition. Raw shellfish eating was the principal risk factor for the disease, as indicated by the analytic issues (Risk Ratio: 1.50; IC 95%: 1.18 – 1.89; p < 0.001). NVs were found by means of RT-PCR of all the stool specimens from the 24 patients tested. Eleven samples of shellfish from local markets were tested for the presence or NVs; six were positive by nested PCR and genotypes were related to that found in patients' stools. CONCLUSION: This is the first community outbreak caused by NVs related to sea-food consumption described in Italy. The study confirms that the present standards for human faecal contamination do not seem to be a reliable indicator of viral contaminants in mussels.