42 cases linked to Tromsø and four to Sarpsborg made it the second largest shigellosis outbreak reported by 2013 in Norway. 110 laboratory-confirmed cases were recorded in a 1994 outbreak from imported iceberg lettuce.
It is unclear at which point in the process of cultivation, production and importation of the basil the S. sonnei contamination may have occurred, said Guzman-Herrador et al.
The basil pesto product tested negative for Shigella spp., but harboured high levels of Enterobacteriaceae and thermotolerant coliforms counts.
Detection of Shigella spp. in food is difficult and no reliable method is available, said the researchers.
The median age of the cases in Tromsø was 41 years (range: 19–84 years); twenty-four were female with the first case seeking medical attention and being tested on 5 October and the last on 21 October.
The cases with identical multiple-locus variable number tandem repeats analysis (MLVA) profiles were reported in 2011.
The Norwegian Veterinary Institute analysed the implicated food using a polymerase chain reaction (PCR)-based method by the Nordic Committee on Food Analysis.
The fresh basil was imported to Norway from Israel via the Netherlands.
The Norwegian importer received the basil on 25 September and delivered it to Tromsø on 27 September and to the restaurant in Sarpsborg on 27, 30 September and 4 October.
Shigellosis infections In the European Union (EU) are uncommon with 1.64 cases per 100,000 population in 2010.
They are less frequent than Campylobacter and Salmonella infections, which have respective incidences of 56.95 and 21.31 per 100,000 population.
In Norway, between 120 and 190 cases of shigellosis have been reported annually in the last ten years, an incidence between 2.5 to 4.0 per 100,000 population.
Only 10 to 20% of the cases are domestically acquired with imported fresh vegetables being identified as the vehicle of several outbreaks.
“A Shigella Sonnei outbreak traced to imported Basil – The importance of good typing tools and produce traceability systems, Norway, 2011.”
Authors: B R Guzman-Herrador, E Nilsen, K S Cudjoe, L Jensvoll, J M Kvamme, A Lindegård Aanstad, B A Lindstedt, K Nygård, G Severinsen, Ø Werner-Johansen, A L Wester, M Wiklund, L Vold