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Related Experiment Video

Updated: Jan 12, 2026

EPA Method 1615. Measurement of Enterovirus and Norovirus Occurrence in Water by Culture and RT-qPCR. Part III. Virus Detection by RT-qPCR
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EPA Method 1615. Measurement of Enterovirus and Norovirus Occurrence in Water by Culture and RT-qPCR. Part III. Virus Detection by RT-qPCR

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Correlation Between Infectivity and qRT-PCR Values for Murine Norovirus Recovered from Frozen Berries.

Daniel Plante1, Julio Alexander Bran Barrera1, Maude Lord1

  • 1Microbiology Laboratory, Regulatory Operations and Enforcement Branch, Health Canada, 1001 St-Laurent Street West, Longueuil, QC, J4K 1C7, Canada.

Food and Environmental Virology
|October 31, 2025
PubMed
Summary

Quantitative reverse transcription PCR (qRT-PCR) detects non-infectious norovirus in frozen berries. This study found that high cycle threshold (Ct) values in qRT-PCR may not indicate viable virus, necessitating caution in public health risk assessments.

Keywords:
Frozen berriesInfectivity assayNorovirusqRT-PCR

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Area of Science:

  • Foodborne Pathogen Surveillance
  • Molecular Virology
  • Food Safety

Background:

  • Human norovirus (HuNoV) is a major cause of global gastroenteritis, with frozen berries frequently linked to outbreaks.
  • Current surveillance methods like quantitative reverse transcription PCR (qRT-PCR) cannot distinguish between infectious and non-infectious viral particles.
  • This limitation complicates accurate risk assessment for foodborne illnesses.

Purpose of the Study:

  • To determine the minimum viral load of infectious norovirus on frozen berries detectable by qRT-PCR.
  • To establish a correlation between qRT-PCR detection limits and infectious virus levels using murine norovirus (MNV) as a surrogate for HuNoV.
  • To evaluate the efficiency of the ISO 15216:2017 method for processing frozen berries.

Main Methods:

  • Artificial inoculation of frozen raspberries with serial dilutions of MNV.
  • Quantification of infectious virus using plaque assay and viral RNA detection via qRT-PCR following the ISO 15216:2017 method.
  • Comparison of detection limits between cell culture (plaque assay) and qRT-PCR, and assessment of extraction efficiency from raspberries and strawberries.

Main Results:

  • The limit of detection for infectious virus (cell culture) was 3.1 log PFU/25 g, while qRT-PCR detected viral RNA down to 1.0 log PFU/25 g (Ct ≈ 36.7).
  • A significant disparity (2-log difference) was observed between the detection limits, indicating that many qRT-PCR positive results may represent non-infectious viral genomes.
  • Viral recovery rates exceeded the ISO 15,216 threshold, and PCR inhibition was minimal; recovery from strawberries was similar to raspberries.

Conclusions:

  • A substantial proportion of norovirus genomes detected by qRT-PCR on frozen berries may not correspond to infectious particles.
  • High Ct values (e.g., >35) in surveillance studies might not indicate the presence of viable virus.
  • Contextualizing qRT-PCR results with epidemiological data is crucial for accurate public health decision-making regarding foodborne illness risks.