Separate norovirus outbreaks linked to one source of imported frozen raspberries by molecular analysis, Denmark, 2010–2011

Norovirus outbreaks occur frequently in Denmark and it can be difficult to establish whether apparently independent outbreaks have the same origin. Here we report on six outbreaks linked to frozen raspberries, investigated separately over a period of 3 months. Norovirus from stools were sequence-typed; including extended sequencing of 1138 bp encompassing the hypervariable P2 region of the capsid gene.

Norovirus was detected in 27 stool samples. Genotyping showed genotype GI.Pb_GI.6 (polymerase/capsid) with 100% identical sequences. Samples from five outbreaks were furthermore identical over the variable capsid P2 region. In one outbreak at a hospital canteen, frozen raspberries was associated with illness by cohort investigation (relative risk 6·1, 95% confidence interval 3·2–11).

Bags of raspberries suspected to be the source were positive for genogroup I and II noroviruses, one typable virus was genotype GI.6 (capsid). These molecular investigations showed that the apparently independent outbreaks were the result of one contamination event of frozen raspberries. The contaminated raspberries originated from a single producer in Serbia and were originally not considered to belong to the same batch.

The outbreaks led to consultations and mutual visits between producers, investigators and authorities. Further, Danish legislation was changed to make heat-treatment of frozen raspberries compulsory in professional catering establishments.