An international inter-laboratory study to compare digital PCR with ISO standardized qPCR assays for the detection of norovirus GI and GII in oyster tissue.

An optimized digital RT-PCR (RT-dPCR) assay for the detection of human norovirus GI and GII RNA was compared with ISO 15216-conform quantitative real-time RT-PCR (RT-qPCR) assays in an interlaboratory study (ILS) among eight laboratories. A duplex GI/GII RT-dPCR assay, based on the ISO 15216-oligonucleotides, was used on a Bio-Rad QX200 platform by six laboratories. Adapted assays for Qiagen Qiacuity or ThermoFisher QuantStudio 3D were used by one laboratory each. The ILS comprised quantification of norovirus RNA in the absence of matrix and in oyster tissue samples. On average, results of the RT-dPCR assays were very similar to those obtained by RT-qPCR assays. The coefficient of variation (CV%) of norovirus GI results was, however, much lower for RT-dPCR than for RT-qPCR in intra-laboratory replicates (eight runs) and between the eight laboratories. The CV% of norovirus GII results was in the same range for both detection formats. Had in-house prepared dsDNA standards been used, the CV% of norovirus GII could have been in favor of the RT-dPCR assay. The ratio between RT-dPCR and RT-qPCR results varied per laboratory, despite using the distributed RT-qPCR dsDNA standards. The study indicates that the RT-dPCR assay is likely to increase uniformity of quantitative results between laboratories.

Strategies to reduce norovirus (NoV) contamination from oysters under depuration conditions.

Depuration of oysters can effectively reduce levels of E. coli, however, may not be effective in safeguarding against viral contamination (EFSA, 2012). These trials assess the removal of Norovirus Genogroups I and II (NoV GI and GII) and F + RNA bacteriophage genogroup II (FRNAP-II) from oysters under depuration using molecular and viability assay methods. Our results show consistently better removal of NoV GII compared with Nov GI. We found approximately 46% removal of NoV GII at 18 °C after 2 days and 60% after 5 days compared with a maximum of 16% NoV GI removal. Twice the rate of NoV GII removal was achieved at 18 °C compared with 8 °C after 5 days. Results suggest better NoV removal when depuration water salinity is close to that prevailing in the harvesting area. Trials investigating algal feeding, light/dark and disturbance from pump vibration did not show any significant effect. We found that FRNAP-II was more readily removed than NoV. No significant difference was found between the rate of removal (as measured by RT-qPCR) and inactivation (as measured by bioassay) of FRNAP-II. This indicates that reduction in FRNAP-II may be primarily due to physical removal (or destruction) rather than in situ inactivation of the virus.

Whole Genome Sequencing of Hepatitis A Virus Using a PCR-Free Single-Molecule Nanopore Sequencing Approach.

Hepatitis A virus (HAV) is one of the most common causes of acute viral hepatitis in humans. Although HAV has a relatively small genome, there are several factors limiting whole genome sequencing such as PCR amplification artefacts and ambiguities in de novo assembly. The recently developed Oxford Nanopore technologies (ONT) allows single-molecule sequencing of long-size fragments of DNA or RNA using PCR-free strategies. We have sequenced the whole genome of HAV using a PCR-free approach by direct reverse-transcribed sequencing. We were able to sequence HAV cDNA and obtain reads over 7 kilobases in length containing almost the whole genome of the virus. The comparison of these raw long nanopore reads with the HAV reference wild type revealed a nucleotide sequence identity between 81.1 and 96.6%. By de novo assembly of all HAV reads we obtained a consensus sequence of 7362 bases, with a nucleotide sequence identity of 99.0% with the genome of the HAV strain pHM175/18f. When the assembly was performed using as reference the HAV strain pHM175/18f a consensus with a sequence similarity of 99.8 % was obtained. We have also used an ONT amplicon-based assay to sequence two fragments of the VP3 and VP1 regions which showed a sequence similarity of 100% with matching regions of the consensus sequence obtained using the direct cDNA sequencing approach. This study showed the applicability of ONT sequencing technologies to obtain the whole genome of HAV by direct cDNA nanopore sequencing, highlighting the utility of this PCR-free approach for HAV characterization and potentially other viruses of the Picornaviridae family.

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.