Series of Norovirus Outbreaks Caused by Consumption of Green Coral Lettuce, Denmark, April 2016

Luise Müller, Lasse Dam Rasmussen, Tenna Jensen, Anna Charlotte Schultz, Charlotte Kjelsø, Celine Barnadas, Kim Sigsgaard, Anne Ribert Larsen, Carl Widstrup Jensen, Simon Jeppesen, Katrine Uhrbrand, Nikolas Hove, Kåre Mølbak, Steen Ethelberg

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In early April 2016, an unusual high number of point-source outbreaks of gastrointestinal disease were reported to occur in Denmark. Outbreaks were individually investigated. Two analytical studies were performed. Patient stool samples collected and analysed; positive stool samples were sequenced over the polymerase and/or capsid gene areas. Implicated lettuce heads were collected and analysed for the presence of norovirus. Foods were traced-back and traced-forward and international alert systems applied. A total of 23 linked point-source outbreaks occurred over the course of one week. Fresh green coral lettuce (Lollo Bionda lettuce) had been consumed in all settings. In a cohort study including 234 participants a dish containing green lettuce was associated with illness. Norovirus of Genogroup I (GI) was detected in samples from 28 patients comprising eight of the outbreaks. Sequencing showed GI.P2-GI.2. GI norovirus was detected in one of 20 examined lettuce heads. All lettuce consumed was supplied by the same packer who in turn had bought the lettuce from a wholesaler in France. The two lots of lettuce came from two different growers in different parts of France. Green coral lettuce produced in France was found to have caused a large series of linked norovirus outbreaks in Denmark as established by a number of lines of evidence. A similar incidence occurred in 2010. Fresh lettuce increasingly appear to be a risk food for norovirus infections.
Original languageEnglish
JournalPLoS Currents
Publication statusPublished - 2016


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