The lack of virus control in oysters could lead to a norovirus outbreak

Norovirus, a genus in the family Caliciviridae, is a leading cause of viral gastroenteritis in humans and responsible for many outbreaks worldwide. Filter-feeding shellfish are important vehicles for transmission of foodborne pathogens, including enteric viruses such as norovirus, when grown in sewage-polluted water. In this study, we investigated a norovirus outbreak linked with consumption of oysters and mussels. In January 2019, a gastroenteritis outbreak was identified involving eight patients with symptoms of gastroenteritis. Norovirus was diagnosed in stool samples using immunochromatographic test RIDA (R) QUICK Norovirus, and confirmed with real-time PCR. Of four shellfish (oysters, mussels) samples analyzed using real-time PCR, three were norovirus GI-positive and GII-positive, while one sample was only GII-positive. Six stool samples were collected, two of which were norovirus GI-positive and GII-positive, while three were GII-positive only. Following phylogenetic characterization of the human stool viruses, five out of eight belonged to the GII.6 cluster. Shellfish collected during this outbreak investigation contained the same GII.6 sequence. This is the first norovirus outbreak connected with shellfish consumption in Croatia. Understanding the transmission routes and vehicles of norovirus outbreaks is of great public health importance, and these results imply the co-circulation of GII.6 norovirus in people and oysters in Croatia.