Monitoring and genotyping of Norovirus in edible shellfish in Northern Italian seas (2018-2020)

Abstract

Abstract Background Norovirus is an enteric virus with foodborne transmission. Bivalve shellfish are a main source of infections and outbreaks, which in the last years are showing an increasing trend. Therefore, in Italy a voluntary based monitoring plan to control the safety of bivalve shellfish was set up at provincial level. This study describes the occurrence and distribution of Norovirus in different areas of Northern Italy seas. Methods Between the years 2018-2020 622 samples (n = 173 oysters, n = 139 mussels, n = 300 clams, n = 10 other bivalve shellfish) were tested by OneStep RT Real Time PCR for Norovirus GI and GII and quantified according to the ISO 15216:2017. Positive samples were further analyzed to determine genotype by sequencing of the ORF1/ORF2 junction of the Norovirus genome. Results A total of 111 (18%) samples were positive for Norovirus (mussels 22%; oysters 18%; clams 16%, other molluscs 20%), positive samples were found mainly in the winter season. The mean contamination was 103 ge/µL and ranged 100-105 ge/µL. Thirty-two samples were typeable (GI n = 10; GII n = 22) with GI samples belonging to four genotypes and GII samples belonging to five genotypes. GII.4, particularly Sydney 2012 subtype (found in three oysters and three clams samples), was the most prevalent, followed by GII.3. Often more than one genotype was found in samples collected simultaneously in the same sampling site. The virus was found with relatively high occurrence both in samples collected close to the coast (38%) and in those harvested from farms located 3-6 nautical miles offshore (73%). Conclusions The overall prevalence of Norovirus in edible shellfish, particular those to be eaten raw, is moderately high (18%). No difference in contamination was highlighted among the different species or different sampling sites. One of the most prevalent variants found (i.e. GII.4 Sydney 2012) is a leading cause of infections worldwide. Key messages Given the moderately high proportion of positive samples (18%), oysters (which are often eaten raw), regardless of the origin, represent the main risk of infection. The presence of genotypes frequently involved in human infections strengthens the need for ongoing monitoring, which should be extended to all provinces involved in shellfish farming.