Evaluation of Norovirus contamination in bivalve molluscs harvested from Northern Adriatic Sea, Italy

Abstract

Abstract Background Norovirus (NoV) is the leading cause of acute gastroenteritis in developed countries. Transmission is through direct contact, unsanitary food handling or ingestion of contaminated water or food. Shellfish bioaccumulate NoV and current post-harvest depuration is not effective for its removal. Materials and Methods A monitoring programme for NoV in bivalve shellfish harvested from Northern Adriatic Sea was initiated in 2016. From January 2016 to March 2019, 418 samples including 257 Manila clams, 73 mussels, 46 striped clams, and 42 oysters, were examined. Mollusc samples were tested for NoV genogroups I (NoV GI) and II (NoV GII) contamination by RealTime RT-PCR according to ISO 15216-2. Results Ninety-three out of 418 tested samples (22.2%) were contaminated by at least one NoV genogroup, the simultaneous presence of the two genogroups was detected in 19/418 of the cases. Positive samples were distributed among the tested species as follow: 29/73 (39.7%) mussels, 51/257 (19.5%) Manila clams, 8/42 (19.0%) oysters, 5/46 (10.9%) striped clam. In 2016 (n = 135), 2017 (n = 122), 2018 (n = 132) and in the first trimester of 2019 (n = 12), prevalence of NoV was 6.7%, 24.6%, 31.8% and 41.4%, respectively. NoV GII was largely predominant being detected, alone or in association with GI, in 98.9% of the contaminated samples. On the other hand, GI prevalence increased from 1.5% (2016) to 12.1% in 2018, maybe reflecting a higher circulation of this genogroup in production environments. The vast majority of positive samples (91.4%) were detected during the cold season (November to March). Conclusions Overall positive samples have increased over the years (from 6.7% in 2016 to 31.8% in 2018). NoV GII was the most frequently detected genogroup, but NoV GI prevalence significantly raised in 2018. The routine application of quantitative RT-PCR (ISO 15216-1) to determine the viral load in bivalve molluscs would expand knowledge on potential for foodborne transmission. Key messages Overall positive samples have increased over the years. The routine application of quantitative PCR to determine the viral load in bivalve molluscs would expand knowledge on potential for foodborne transmission.