Occurrence and persistence of enteric viruses, arsenic and biotoxins in Pacific oysters farmed in an Italian production site

Monitoring and Genotyping of Norovirus in Bivalve Molluscan Shellfish from Northern Italian Seas (2018-2020)

Norovirus (NoV) is an enteric virus with foodborne transmission. Bivalve shellfish are a main source of infections and outbreaks. In Italy a voluntary based monitoring plan to check the safety of bivalve shellfish was set up at provincial level. This study describes the occurrence and distribution of NoV in the Northern Adriatic Sea and in the Ligurian Sea. From October 2018 to September 2020, 807 bivalve shellfish samples (n = 205 oysters, n = 182 mussels, n = 348 clams, n = 72 other bivalve shellfish) were tested by One-Step Retrotranscription Real-time polymerase chain reaction for NoV GI and GII and quantified according to the ISO 15216-2:2013 and ISO 15216-1:2017. Positive samples were further analyzed to determine genotype by sequencing of the ORF1/ORF2 junction of the viral genome. A total of 126 samples were positive for NoV, mussels, and oysters had the highest probability of being positive and positive samples were found mainly in the colder season. Of these samples, 46% were NoV GII, 13% NoV GI, and 40% carried both genogroups. Thirty-seven samples were typeable (GI n = 12 and GII n = 25) with GI samples belonging to four genotypes and GII samples belonging to five genotypes. GII.3 genotype was the most prevalent, followed by GII.4, particularly Sydney 2012 subtype, a leading cause of infections worldwide, was found in three oysters' and three clams' samples. The phylogenetic analysis revealed a high heterogeneity among the species that are scattered in several clusters. Considering the low infectious dose the overall presence of NoV in edible shellfish, particular those to be eaten raw or undercooked, is moderately high. The presence of genotypes frequently involved in human infections strengthens the need for ongoing monitoring, which should be extended at national level.

Shellfish sanitation monitoring in La Spezia gulf: Chemometric evaluation of data from 2015 to 2021

Shellfish sanitary controls are very important to guarantee consumer health because bivalve molluscs (BVM) are filter-feeders so they can accumulate pathogens, environmental contaminants and biotoxins produced by some algae, causing infections and food poisoning in humans after ingestion. The purpose of this work was to analyse with chemometric methods the historical data relating to routine analyses carried out by the competent authority (Liguria Local Health Unit, National Health Service) on the BVM reared in a shellfish farm located in the Gulf of La Spezia (Italy). Chemometric analysis was aimed at identifying any correlations between the variables, as well as any seasonal trends and similarities between the stations, in order to be able to provide further material for a more accurate risk assessment and to improve the monitoring organization for example by reducing sampling stations and/or sampling frequency. The dataset used included 31 variables classified as biotoxicological, microbiological and chemical variables, measured twice a week, monthly or half yearly respectively, for a total of 6 years (from 2015 to 2021), on samples of Mytilus galloprovincialis coming from 7 monitoring stations. The results obtained by the application of principal component analysis have shown positive alga-biotoxin correlations, as well as seasonal trends linked to algae growth, with a greater algal biomass and their toxins during the spring months. In addition, periods characterised by low rainfall were found to affect algal development, promoting especially species such as Dinophysis spp. Considering the microbiological and biotoxicological variables, significant differences between the monitoring stations were not found. However, stations could be distinguished on the basis of the nature of the predominant chemical pollutants.

Temperature Dependent Depuration of Norovirus GII and Tulane Virus from Oysters (Crassostrea gigas)

Raw oysters are considered a culinary delicacy but are frequently the culprit in food-borne norovirus (NoV) infections. As commercial depuration procedures are currently unable to efficiently eliminate NoV from oysters, an optimisation of the process should be considered. This study addresses the ability of elevated water temperatures to enhance the elimination of NoV and Tulane virus (TuV) from Pacific oysters (Crassostrea gigas). Both viruses were experimentally bioaccumulated in oysters, which were thereafter depurated at 12 °C and 17 °C for 4 weeks. Infectious TuV and viral RNA were monitored weekly for 28 days by TCID₅₀ and (PMAxx-) RT-qPCR, respectively. TuV RNA was more persistent than NoV and decreased by < 0.5 log₁₀ after 14 days, while NoV reductions were already > 1.0 log₁₀ at this time. For RT-qPCR there was no detectable benefit of elevated water temperatures or PMAxx for either virus (p > 0.05). TuV TCID₅₀ decreased steadily, and reductions were significantly different between the two temperatures (p < 0.001). This was most evident on days 14 and 21 when reductions at 17 °C were 1.3-1.7 log₁₀ higher than at 12 °C. After 3 weeks, reductions > 3.0 log₁₀ were observed at 17 °C, while at 12 °C reductions did not exceed 1.9 log₁₀. The length of depuration also had an influence on virus numbers. TuV reductions increased from < 1.0 log₁₀ after seven days to > 4.0 log₁₀ after 4 weeks. This implies that an extension of the depuration period to more than seven days, possibly in combination with elevated water temperatures, may be beneficial for the inactivation and removal of viral pathogens.

Assessment of the Impact on Human Health of the Presence of Norovirus in Bivalve Molluscs: What Data Do We Miss?

In the latest One Health ECDC EFSA technical report, Norovirus in fish and fishery products have been listed as the agent/food pair causing the highest number of strong-evidence outbreaks in the EU in 2019. This review aims to identify data gaps that must be filled in order to increase knowledge on Norovirus in bivalve molluscs, perform a risk assessment and rank the key mitigation strategies for this biological hazard, which is relevant to public health. Virologic determinations are not included in any of the food safety and process hygiene microbiologic criteria reflected in the current European regulations. In addition, the Escherichia coli-based indices of acceptable faecal contamination for primary production, as well as the food safety criteria, do not appear sufficient to indicate the extent of Norovirus contamination. The qualitative risk assessment data collected in this review suggests that bivalve molluscs present a high risk to human health for Norovirus only when consumed raw or when insufficiently cooked. On the contrary, the risk can be considered negligible when they are cooked at a high temperature, while information is still scarce for non-thermal treatments.