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Volume 16, Number 11—November 2010
Letter

Enteric Viruses in Ready-to-Eat Packaged Leafy Greens

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To the Editor: Fresh produce increasingly has been implicated in viral disease outbreaks (1). In some instances, lettuce was contaminated before wholesale distribution (1). Enteric viruses can be introduced in the field if produce is exposed to human waste. Processed and packaged produce can be contaminated if equipment or wash water is not effectively sanitized. Fewer than 10 infectious viral particles are sufficient to cause disease (2), and these organisms are resistant to disinfectants at concentrations that reduce bacterial levels (3). Contamination of fresh produce could pose a health risk to humans because fresh produce is eaten raw. High levels of viral contamination can result in large outbreaks, but intermittent contamination of fresh produce accounts for some sporadic cases of norovirus and rotavirus gastroenteritis.

During April 27–November 23, 2009, we performed viral testing on 328 samples of packaged leafy greens (representing 12–14 different lots from 3–6 companies per week; no samples were taken on weeks with a statutory holiday) for norovirus or rotavirus RNA. Packaged leafy greens were purchased from retail stores in southern Ontario, Canada. Shipments maintained an average temperature of 3.8°C during transit to the testing laboratory. Each 25-g sample was spiked with 106 PFU of feline calicivirus (FCV) as a sample process control (4). Virus was concentrated by using an adsorption-elution-ultrafiltration filtration protocol (4).

Recovery of FCV was quantified from an RNA standard curve. FCV process control recovery was <0.01% for 55 (17%) samples. Recovery of >0.01% of the FCV was observed for the remaining 273 (83%) samples. Two samples from which FCV was not recovered were positive for norovirus (CE-V-09–0138) and rotavirus (CE-V-09–0129); they were considered true positive results.

Of these 275 samples, 148 (54%) were positive for norovirus by real-time reverse transcription–PCR (RT-PCR) (5), and 1 (0.4%) was positive for rotavirus group A by RT-PCR (6). To confirm detection of norovirus RNA, we amplified a second norovirus target by RT-PCR of region C (5). Only 40 samples (15% of total) produced a band of the expected size for this second norovirus amplicon. Of these 40 amplicons, only 16 (6% of total) could be sequenced to confirm norovirus RNA. The rotavirus-positive sample was confirmed by sequencing.

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For some sample dates, multiple lots were positive; for others, no positive samples were identified (Figure). Multiple detections on the same date were not caused by cross-contamination; partial capsid sequencing showed different genetic types on dates when multiple samples were positive (Figure). Results were positive from 5 different brands, and no organic samples were confirmed positive for enteric virus contamination. Of the 16 norovirus strains confirmed, 13 belonged to genogroup I (GI) and 3 to genogroup II (GII) (Figure). All were strain types known to be human pathogens. The group A rotavirus was not subtyped; group A rotaviruses can be human or animal pathogens.

Most noroviruses detected belonged to GI. Previous reports indicate that GI norovirus are more frequently identified in foodborne or waterborne outbreaks; GII.4 noroviruses are more common in large outbreaks spread person to person (7). Identification of GI norovirus is consistent with occasional contamination of produce or wash water. Disinfectants and sanitation agents are used in wash water at low concentrations, at which they have limited efficacy against norovirus (3).

Washing and disinfecting produce before eating it can reduce the risk for infection by reducing the viral load by 10- to 1,000-fold (8). The median level of confirmed contamination in this study was ≈500 RNA copies for norovirus (range 1.4 copies to 9 × 106 copies).

A limitation of our findings is the inability to determine the association between molecular detection results and infectious virus. No outbreaks were related to the sequences detected here. There is no routine cell culture system for the laboratory growth of human norovirus. Genomic RNA can persist after the virus has been inactivated (9). The new ViroNet Canada network, which went online in April 2010, will monitor strains detected in leafy greens and other food products together with strains from community outbreaks to identify outbreaks linked to contaminated foods.

Our comprehensive surveillance study identified norovirus and rotavirus contamination of packaged leafy greens. We detected noroviruses on 6% and rotavirus on 0.4% of lots tested from retail markets in southern Ontario. Packages with confirmed positive samples were both imported into Canada and had been conventionally grown. Noroviruses have a low infectious dose (2), and detection of viral RNA is associated with human health risk in oysters, another commodity that is eaten raw (10). Our results suggest a possible risk for foodborne transmission of norovirus and rotavirus from packaged leafy greens.

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Kirsten MattisonComments to Author , Jennifer Harlow, Vanessa Morton, Angela Cook, Frank Pollari, Sabah Bidawid, Jeffrey M. Farber, and Jeffrey M. Farber

Author affiliations: Author affiliations: Health Canada, Ottawa, Ontario, Canada (K. Mattison, J. Harlow, V. Morton, S. Bidawid, J.M. Farber); University of Ottawa, Ottawa (K. Mattison, V. Morton, J.M. Farber); Public Health Agency of Canada, Guelph, Ontario, Canada (A. Cook, F. Pollari)

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References

  1. Ethelberg  S, Lisby  M, Bottiger  B, Schultz  AC, Villif  A, Jensen  T, Outbreaks of gastroenteritis linked to lettuce, Denmark, January 2010. Euro Surveill. 2010;15:19484.PubMed
  2. Teunis  PF, Moe  CL, Liu  P, Miller  SE, Lindesmith  L, Baric  RS, Norwalk virus: how infectious is it? J Med Virol. 2008;80:146876. DOIPubMed
  3. Baert  L, Vandekinderen  I, Devlieghere  F, Van Coillie  E, Debevere  J, Uyttendaele  M. Efficacy of sodium hypochlorite and peroxyacetic acid to reduce murine norovirus 1, B40–8, Listeria monocytogenes, and Escherichia coli O157:H7 on shredded iceberg lettuce and in residual wash water. J Food Prot. 2009;72:104754.PubMed
  4. Mattison  K, Brassard  J, Gagne  MJ, Ward  P, Houde  A, Lessard  L, The feline calicivirus as a sample process control for the detection of food and waterborne RNA viruses. Int J Food Microbiol. 2009;132:737. DOIPubMed
  5. Mattison  K, Grudeski  E, Auk  B, Charest  H, Drews  SJ, Fritzinger  A, Multicenter comparison of two norovirus ORF2-based genotyping protocols. J Clin Microbiol. 2009;47:392732. DOIPubMed
  6. Jean  J, Blais  B, Darveau  A, Fliss  I. Simultaneous detection and identification of hepatitis A virus and rotavirus by multiplex nucleic acid sequence-based amplification (NASBA) and microtiter plate hybridization system. J Virol Methods. 2002;105:12332. DOIPubMed
  7. Verhoef  L, Vennema  H, van Pelt  W, Lees  D, Boshuizen  H, Henshilwood  K, Use of norovirus genotype profiles to differentiate origins of foodborne outbreaks. Emerg Infect Dis. 2010;16:61724.PubMed
  8. Mara  D, Sleigh  A. Estimation of norovirus infection risks to consumers of wastewater-irrigated food crops eaten raw. J Water Health. 2010;8:3943. DOIPubMed
  9. Baert  L, Wobus  CE, Van Coillie  E, Thackray  LB, Debevere  J, Uyttendaele  M. Detection of murine norovirus 1 by using plaque assay, transfection assay, and real-time reverse transcription-PCR before and after heat exposure. Appl Environ Microbiol. 2008;74:5436. DOIPubMed
  10. Lowther  JA, Avant  JM, Gizynski  K, Rangdale  RE, Lees  DN. Comparison between quantitative real-time reverse transcription PCR results for norovirus in oysters and self-reported gastroenteric illness in restaurant customers. J Food Prot. 2010;73:30511.PubMed

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DOI: 10.3201/eid1611.100877

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Table of Contents – Volume 16, Number 11—November 2010

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Kirsten Mattison, Bureau of Microbial Hazards, 251 Sir FG Banting Drwy, PL 2204E, Ottawa, Ontario K1A 0K9, Canada

Craig Hedberg, University of Minnesota School of Public Health, MMC 807, 420 Delaware St SE, Minneapolis, MN 55440, USA

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Page created: December 20, 2011
Page updated: December 20, 2011
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The conclusions, findings, and opinions expressed by authors contributing to this journal do not necessarily reflect the official position of the U.S. Department of Health and Human Services, the Public Health Service, the Centers for Disease Control and Prevention, or the authors' affiliated institutions. Use of trade names is for identification only and does not imply endorsement by any of the groups named above.
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