Series of Norovirus Outbreaks Caused by Consumption of Green Coral Lettuce, Denmark, April 2016.
BACKGROUND
In early April 2016, an unusual high number of point-source outbreaks of gastrointestinal disease were reported to occur in Denmark.


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Department of Infectious Disease Epidemiology, Statens Serum Institut, Copenhagen, Denmark.
Luise
L
Müller
Microbiological Diagnostics and Virology, Statens Serum Institut, Copenhagen, Denmark.
Lasse Dam
LD
Rasmussen
Division of Food and Feed Safety, Danish Veterinary and Food Administration, Ministry of Environment and Food, Glostrup, Denmark.
Tenna
T
Jensen
Division of Microbiology and Production, National Food Institute, Technical University of Denmark, Søborg, Denmark.
Anna Charlotte
AC
Schultz
Department of Infectious Disease Epidemiology, Statens Serum Institut, Copenhagen, Denmark.
Charlotte
C
Kjelsø
European Programme in Public Health Microbiology, European Centre for Disease Prevention and Control, Stockholm, Sweden; Department of Microbiological Diagnostics & Virology, Statens Serum Institut, Copenhagen, Denmark.
Celine
C
Barnadas
Food Control Office Copenhagen, Danish Veterinary and Food Administration, Copenhagen, Denmark.
Kim
K
Sigsgaard
Food Control Office Zealand/Funen, Danish Veterinary and Food Administration, Denmark Food Control Office Zealand/Funen, The Danish Veterinary and Food Administration, Denmark.
Anne Ribert
AR
Larsen
Food Control Office Copenhagen, Danish Veterinary and Food Administration, Copenhagen, Denmark.
Carl
C
Widstrup Jensen
Food Control Office Copenhagen, Danish Veterinary and Food Administration, Copenhagen, Denmark.
Simon
S
Jeppesen
Division of Microbiology and Production, National Food Institute, Technical University of Denmark, Søborg, Denmark.
Katrine
K
Uhrbrand
Division of Food and Feed Safety, Danish Veterinary and Food Administration, Ministry of Environment and Food, Glostrup, Denmark.
Nikolas
N
Hove
Department of Infectious Disease Epidemiology, Statens Serum Institut, Copenhagen, Denmark.
Kåre
K
Mølbak
Department of Infectious Disease Epidemiology, Statens Serum Institut, Copenhagen, Denmark.
Steen
S
Ethelberg