A Shigella sonnei outbreak traced to imported basil--the importance of good typing tools and produce traceability systems, Norway, 2011

On 9 October 2011, the University Hospital of North Norway alerted the Norwegian Institute of Public Health (NIPH) about an increase in Shigella sonnei infections in Tromsø. The isolates had an identical ‘multilocus variable-number tandem repeat analysis’ (MLVA) profile. Most cases had consumed food provided by delicatessen X. On 14 October, new S. sonnei cases with the same MLVA-profile were reported from Sarpsborg, south-eastern Norway. An outbreak investigation was started to identify the source and prevent further cases. All laboratory-confirmed cases from both clusters were attempted to be interviewed. In addition, a cohort study was performed among the attendees of a banquet in Tromsø where food from delicatessen X had been served and where some people had reported being ill. A trace-back investigation was initiated. In total, 46 cases were confirmed (Tromsø= 42; Sarpsborg= 4). Having eaten basil pesto sauce or fish soup at the banquet in Tromsø were independent risk factors for disease. Basil pesto was the only common food item that had been consumed by confirmed cases occurring in Tromsø and Sarpsborg. The basil had been imported and delivered to both municipalities by the same supplier. No basil from the specific batch was left on the Norwegian market when it was identified as the likely source. As a result of the multidisciplinary investigation, which helped to identify the source, the Norwegian Food Safety Authority, together with NIPH, planned to develop recommendations for food providers on how to handle fresh plant produce prior to consumption.