Analysis of frozen samples from certain production areas showed the presence of norovirus two weeks before consumption. Concentrations varied from 43 to 1,170 RNA copies/oyster.
Risk is elevated in the winter season and following sewage overflows and heavy rainfall.
dPCR on oyster samples
Researchers quantified norovirus levels by using digital PCR (dPCR) on samples from oysters implicated in human disease.
Norovirus reference materials are essential for quantification by real-time PCR but are not widely available for inclusion in standard curves.
In France, medical doctors who diagnose norovirus gastroenteritis in ≥2 persons who shared a common meal are required to declare a suspected foodborne illness outbreak.
Digital PCR is based on partitioning of the sample into thousands of individual PCRs that contain one or no copies of the nucleic acid target. After amplification, the number of target molecules is calculated, with no need for external reference standards.
Norovirus-specific primers and probes targeting the open reading frame 1-2 region used for the real-time reverse transcription PCR were used in a microfluidic-based dPCR to enable quantification in oyster samples associated with outbreaks.
Eight outbreaks were considered which occurred during winter in private houses except for one in a nursing home with median incubation times between 0.5 and two days.
Eight shellfish samples were collected from batches directly implicated and one sample was taken from leftovers in the nursing home’s refrigerator.
An additional 16 samples were collected from implicated production areas along different coasts of France, including frozen samples.
Viruses were eluted from oyster digestive tissues by the reference method and quantified using the QuantStudio 3D Digital PCR system (Thermo Fisher).
No norovirus was detected in one oyster sample; norovirus genogroups GI, GII, or both were found in nine, 11 and four samples, respectively.
Overall, norovirus concentrations ranged from 43 to 1,170 RNA copies/oyster; the highest concentrations were GI.
For the nursing home outbreak, in which a leftover sample from the implicated meal was obtained, norovirus GII was detected at a concentration of 82 RNA copies/oyster, whereas norovirus GI was detected at 185 RNA copies/oyster in the same batch from the oyster farm.
Researchers said although advances have been made in virus detection in shellfish, quantification of norovirus in oysters associated with outbreaks still presents a challenge.
More accurate quantification is essential for risk analysis and to understand the role played by shellfish in norovirus transmission, which is important to support regulation, they added.
“Identifying if oysters implicated in outbreaks are contaminated with norovirus GI or GII is important, because genetic susceptibility means that some consumers do not become infected with certain GI or GII strains; this affects the disease and favors the distribution of some norovirus strains.
“We believe that the development of technology such as next-generation sequencing will provide more detailed information on the full range of strains present in samples. Obtaining more accurate information on strain diversity and quantification will be valuable for molecular epidemiology studies and management.”
HEV foodborne transmission
Meanwhile, cases of hepatitis E were described after eating Corsican specialties made with raw pig liver known as ficatelli, traditionally eaten grilled or raw after curing, according to a different study.
A 2011 survey of French food products detected hepatitis E virus (HEV) RNA in 30% of figatelli samples.
Researchers retrieved partial sequences of HEV from samples from eight wild boars hunted during 2009-2013 and two domestic pigs collected at a slaughterhouse in 2013 and compared them with sequences available in GenBank.
All 10 sequences belonged to HEV genotype 3 and were distributed into three distinct clusters.
“Our results provide evidence suggesting a dynamic exchange of HEV between domestic and wild swine reservoirs and possibly resulting in transmission from those reservoirs to humans through ingestion of infected food products.”