Binding-Based RT-qPCR Assay to Assess Binding Patterns of Noroviruses to Shellfish

Research Progress on Biological Accumulation, Detection and Inactivation Technologies of Norovirus in Oysters

Noroviruses (NoVs) are major foodborne pathogens that cause acute gastroenteritis. Oysters are significant carriers of this pathogen, and disease transmission from the consumption of NoVs-infected oysters occurs worldwide. The review discusses the mechanism of NoVs bioaccumulation in oysters, particularly the binding of histo-blood group antigen-like (HBGA-like) molecules to NoVs in oysters. The review explores the factors that influence NoVs bioaccumulation in oysters, including temperature, precipitation and water contamination. The review also discusses the detection methods of NoVs in live oysters and analyzes the inactivation effects of high hydrostatic pressure, irradiation treatment and plasma treatment on NoVs. These non-thermal processing treatments can remove NoVs efficiently while retaining the original flavor of oysters. However, further research is needed to reduce the cost of these technologies to achieve large-scale commercial applications. The review aims to provide novel insights to reduce the bioaccumulation of NoVs in oysters and serve as a reference for the development of new, rapid and effective methods for detecting and inactivating NoVs in live oysters.

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.

Contamination, bioaccumulation mechanism, detection, and control of human norovirus in bivalve shellfish: A review

Human norovirus (HuNoV) is a major foodborne pathogen that causes acute viral gastroenteritis, and bivalve shellfish are one of the main carriers of HuNoV transmission. A comprehensive understanding of bivalve shellfish-related HuNoV outbreaks focusing on contamination factors, bioaccumulation mechanisms, and pre- and post-harvest interventions is essential for the development of effective strategies to prevent contamination of shellfish. This review comprehensively surveys the current knowledge on global contamination and non-thermal treatment of HuNoV in bivalve shellfish. HuNoV contamination in bivalve shellfish is significantly related to the season and water. While evaluating the water quality of shellfish-inhabited waters is a key intervention, the development of non-heat treatment technology to effectively inactivate the HuNoV in bivalve shellfish while maintaining the flavor and nutrition of the shellfish is also an important direction for further research. Additionally, this review explores the bioaccumulation mechanisms of HuNoV in bivalve shellfish, especially the mechanism underlying the binding of histo-blood group antigen-like molecules and HuNoV. The detection methods for infectious HuNoV are also discussed. The establishment of effective methods to rapidly detect infectious HuNoV and development of biological components to inactivate or prevent HuNoV contamination in shellfish also need to be studied further.

Effect of natural ageing and heat treatments on GII.4 norovirus binding to Histo-Blood Group Antigens

Human noroviruses (HuNoVs) are the leading cause of viral foodborne outbreaks worldwide. To date, no available methods can be routinely used to detect infectious HuNoVs in foodstuffs. HuNoVs recognize Histo-Blood Group Antigens (HBGAs) through the binding pocket (BP) of capsid protein VP1, which promotes infection in the host cell. In this context, the suitability of human HBGA-binding assays to evaluate the BP integrity of HuNoVs was studied on GII.4 virus-like particles (VLPs) and GII.4 HuNoVs during natural ageing at 20 °C and heat treatments. Our results demonstrate that this approach may reduce the over-estimation of potential infectious HuNoVs resulting from solely using the genome detection, even though some limitations have been identified. The specificity of HBGA-binding to the BP is clearly dependent on the HGBA type (as previously evidenced) and the ionic strength of the media without disturbing such interactions. This study also provides new arguments regarding the ability of VLPs to mimic HuNoV behavior during inactivation treatments. The BP stability of VLPs was at least 4.3 fold lower than that of HuNoVs at 20 °C, whereas capsids of both particles were disrupted at 72 °C. Thus, VLPs are relevant surrogates of HuNoVs for inactivation treatments inducing significant changes in the capsid structure.

Use of F-Specific RNA Bacteriophage to Estimate Infectious Norovirus Levels in Oysters

Contamination of bivalve shellfish, particularly oysters, with norovirus is recognised as a significant food safety risk. Methods for quantification of norovirus in oysters using the quantitative real-time reverse transcription polymerase chain reaction (RT-qPCR) are well established, and various studies using RT-qPCR have detected norovirus in a considerable proportion of oyster samples, both in the UK and elsewhere. However, RT-qPCR detects viral genome, and by its nature is unable to discriminate between positive results caused by infectious viruses and those caused by non-infectious remnants including damaged virus particles and naked RNA. As a result, a number of alternative or complementary approaches to RT-qPCR testing have been proposed, including the use of infectious viral indicator organisms, most frequently F-specific RNA bacteriophage (F-RNA phage). In this study, we investigated the relationships between F-RNA phage and norovirus in digestive tissues from two sets of oyster samples, one randomly collected at retail (630 samples), and one linked to suspected norovirus illness outbreaks (nine samples). A positive association and correlation between PCR-detectable levels of genogroup II F-RNA bacteriophage (associated with human faecal contamination) and norovirus was found in both sets of samples, with more samples positive for genogroup II phage, at generally higher levels than norovirus. Levels of both viruses were higher in outbreak-related than retail samples. Infectious F-RNA phage was detected in 47.8% of all retail samples, and for a subset of 224 samples where characterisation of phage was carried out, infectious GII phage was detected in 30.4%. Infectious GII phage was detected in all outbreak-related samples. Determination of infectivity ratios by comparing levels of PCR-detectable (copies/g) and infectious GII phage (pfu/g) revealed that in the majority of cases less than 10% of virus detected by RT-qPCR was infectious. Application of these ratios to estimate infectious norovirus levels indicated that while 77.8% of outbreak-related samples contained > 5 estimated infectious norovirus/g, only 13.7% of retail samples did. Use of a combination of levels of PCR-detectable norovirus and infectious F-RNA phage showed that while only 7.0% of retail samples contained both > 100 copies/g norovirus and > 10 pfu/g F-RNA phage, these combined levels were present in 77.8% of outbreak-related samples, and 75.9% of retail samples with > 5 estimated infectious norovirus/g. We therefore suggest that combining RT-qPCR testing with a test for infectious F-RNA phage has the potential to better estimate health risks, and to better predict the presence of infectious norovirus than RT-qPCR testing alone.

Assessment of the Applicability of Capsid-Integrity Assays for Detecting Infectious Norovirus Inactivated by Heat or UV Irradiation

Human noroviruses are the leading cause of viral gastroenteritis. In the absence of a practical culture technique for routine analysis of infectious noroviruses, several methods have been developed to discriminate between infectious and non-infectious viruses by removing non-viable viruses prior to analysis by RT-qPCR. In this study, two such methods (RNase and porcine gastric mucin) which were designed to remove viruses with compromised capsids (and therefore assumed to be non-viable), were assessed for their ability to quantify viable F-specific RNA bacteriophage (FRNAP) and human norovirus following inactivation by UV-C or heat. It was found that while both methods could remove a proportion of non-viable viruses, a large proportion of non-viable virus remained to be detected by RT-qPCR, leading to overestimations of the viable population. A model was then developed to determine the proportion of RT-qPCR detectable RNA from non-viable viruses that must be removed by such methods to reduce overestimation to acceptable levels. In most cases, nearly all non-viable virus must be removed to reduce the log overestimation of viability to within levels that might be considered acceptable (e.g. below 0.5 log₁₀). This model could be applied when developing alternative pre-treatment methods to determine how well they should perform to be comparable to established infectivity assays.

Detection of Human Enteric Viruses in French Polynesian Wastewaters, Environmental Waters and Giant Clams

Lack of wastewater treatment efficiency causes receiving seawaters and bivalve molluscan shellfish to become contaminated, which can lead to public health issues. Six wastewater samples, five seawater samples and three batches of giant clams from Tahiti (French Polynesia) were investigated for the presence of enteric viruses, but also if present, for the diversity, infectivity and integrity of human adenoviruses (HAdV). Enteroviruses (EV), sapoviruses (SaV) and human polyomaviruses (HPyV) were detected in all wastewater samples. In decreasing frequency, noroviruses (NoV) GII and HAdV, rotaviruses (RoV), astroviruses (AsV), NoV GI and finally hepatitis E viruses (HEV) were also observed. Nine types of infectious HAdV were identified. HPyV and EV were found in 80% of seawater samples, NoV GII in 60%, HAdV and SaV in 40% and AsV and RoV in 20%. NoV GI and HEV were not detected in seawater. Intact and infectious HAdV-41 were detected in one of the two seawater samples that gave a positive qPCR result. Hepatitis A viruses were never detected in any water types. Analysis of transcriptomic data from giant clams revealed homologues of fucosyltransferases (FUT genes) involved in ligand biosynthesis that strongly bind to certain NoV strains, supporting the giant clams ability to bioaccumulate NoV. This was confirmed by the presence of NoV GII in one of the three batches of giant clams placed in a contaminated marine area. Overall, all sample types were positive for at least one type of virus, some of which were infectious and therefore likely to cause public health concerns.

Effect of the Shellfish Proteinase K Digestion Method on Norovirus Capsid Integrity

Norovirus outbreaks are associated with the consumption of contaminated shellfish, and so efficient methods to recover and detect infectious norovirus in shellfish are important. The Proteinase K digestion method used to recover norovirus from shellfish, as described in the ISO 15216, would be a good candidate but its impact on the virus capsid integrity and thus infectivity was never examined. The aim of this study was to assess the impact of the Proteinase K digestion method, and of the heat treatment component of the method alone, on norovirus (genogroups I and II) and MS2 bacteriophage capsid integrity. A slightly modified version of the ISO method was used. RT-qPCR was used for virus detection following digestion of accessible viral RNA using RNases. MS2 phage infectivity was measured using a plaque assay. The effect of shellfish digestive glands (DG) on recovery was evaluated. In the presence of shellfish DG, a reduction in MS2 phage infectivity of about 1 log₁₀ was observed after the Proteinase K digestion method and after heat treatment component alone. For norovirus GII and MS2 phage, there was no significant loss of genome following the Proteinase K digestion method but there was a significant 0.24 log₁₀ loss of norovirus GI. In the absence of shellfish DG, the reduction in MS2 phage infectivity was about 2 log₁₀, with the addition of RNases resulting in a significant loss of genome for all tested viruses following complete Proteinase K digestion method and the heat treatment alone. While some protective effect from the shellfish DG on viruses was observed, the impact on capsid integrity and infectivity suggests that this method, while suitable for norovirus genome detection, may not completely preserve virus infectivity.

Temperature-Dependent Persistence of Human Norovirus Within Oysters (Crassostrea virginica)

This study characterizes the persistence of human norovirus in Eastern oysters (Crassostrea virginica) held at different seawater temperatures. Oysters were contaminated with human norovirus GI.1 (Norwalk strain 8FIIa) by exposing them to virus-contaminated water at 15 °C, and subsequently holding them at 7, 15, and 25 °C for up to 6 weeks. Viral RNA was extracted from oyster tissue and hemocytes and quantitated by RT-qPCR. Norovirus was detected in hemocytes and oysters held at 7 and 15 °C for 6 weeks and in hemocytes and oysters held at 25 °C for up to 2 and 4 weeks, respectively. Results confirm that NoV is quite persistent within oysters and demonstrate that cooler water temperatures extend norovirus clearance times. This study suggests a need for substantial relay times to remove norovirus from contaminated shellfish and suggests that regulatory authorities should consider the effects of water temperature after a suspected episodic norovirus-contamination event.

Tulane Virus as a Potential Surrogate To Mimic Norovirus Behavior in Oysters

Oyster contamination by noroviruses is an important health and economic problem. The present study aimed to compare the behaviors of Norwalk virus (the prototype genogroup I norovirus) and two culturable viruses: Tulane virus and mengovirus. After bioaccumulation, tissue distributions were quite similar for Norwalk virus and Tulane virus, with the majority of viral particles detected in digestive tissues, while mengovirus was detected in large amounts in the gills and mantle as well as in digestive tissues. The levels of persistence of all three viruses over 8 days were comparable, but clear differences were observed over longer periods, with Norwalk and Tulane viruses displaying rather similar half-lives, unlike mengovirus, which was cleared more rapidly. These results indicate that Tulane virus may be a good surrogate for studying norovirus behavior in oysters, and they confirm the prolonged persistence of Norwalk virus in oyster tissues.