Molecular characterisation of noroviruses detected in mussels (Mytilus galloprovincialis) from harvesting areas in Slovenia

Noroviruses are a leading cause of viral gastroenteritis in humans and are responsible for many outbreaks worldwide. Mussels are one of the most important foodstuffs connected with norovirus outbreaks, also resulting in multinational dimensions. Two hundred and thirty-eight (238) samples of mussels (Mytilus galloprovincialis) were collected in periods between the years 2006-2008 and 2010-2012 to study the prevalence of noroviruses (NoVs) from harvesting areas along the Adriatic coast of Slovenia. Between 2006 and 2008, 9.1% to 24.6% of mussel samples tested by specific GI and/or GII real-time RT-PCR methods were found to be positive for NoVs while between 2010 and 2012 the percentage of NoV positive samples varied from 12.5% to 22.2%. At the nucleotide level within the RdRp gene fragment the genetic diversity of NoVs detected in mussels ranged between 78.8-81.0% nucleotide identity among GII strains (92.1-99.6% within the GII.P4 genotype), 100% nucleotide identity among GI and 58.4-60.2% among GI and GII strains. Nine of the NoV strains detected from mussels were genotyped as GII.4, while two samples were within GI.P2 and one was a positive sample within genotype GII.P21. This study confirmed that mussels are a potential source of the NoV infection. The detected NoVs share the same topology on the phylogenetic tree within the NoV strains detected in water samples and human patients, not only from Slovenia but also from many different countries worldwide. We can assume that mussels in harvesting areas are not only contaminated from the surrounding area but also by contaminated water and sewage from large transport ships, which are regularly present in the area.