Yersinia enterocolitica Outbreak Associated with Ready-to-Eat Salad Mix, Norway, 2011

Emily MacDonald

, Berit Tafjord Heier, Karin Nygård, Torunn Stalheim, Kofitsyo S. Cudjoe, Taran Skjerdal, Astrid Louise Wester, Bjørn-Arne Lindstedt, Trine-Lise Stavnes, and Line Vold

Author affiliations: Norwegian Institute of Public Health, Oslo, Norway (E. MacDonald, B.T. Heier, K. Nygård, A.L. Wester, B.-A. Lindstedt, T.-L. Stavnes, L. Vold); Norwegian Food Safety Authority, Oslo (T. Stalheim); and Norwegian Veterinary Institute, Oslo (K.S. Cudjoe, T. Skjerdal)

Abstract

In 2011, an outbreak of illness caused by Yersinia enterocolitica O:9 in Norway was linked to ready-to-eat salad mix, an unusual vehicle for this pathogen. The outbreak illustrates the need to characterize isolates of this organism, and reinforces the need for international traceback mechanisms for fresh produce.

Yersiniosis, a notifiable disease in Norway, is the fourth most common cause of acute bacterial enteritis registered by the Norwegian Surveillance System for Communicable Diseases. Approximately 30 domestic cases are reported annually (2010 incidence rate 0.5 cases/100,000 population). In Norway, >98% of cases of Yersinia enterocolitica infection are caused by serotype O:3, which is also the dominant serotype in Europe, Japan, and parts of North America (1). Infection by Y. enterocolitica is often associated with ingestion of pork because pigs commonly harbor the pathogenic serotypes O:3 and O:9 (1). Recent foodborne outbreaks have been associated with pork products (2,3) and pasteurized milk (4).
In Norway, fecal specimens from patients who have acute gastroenteritis are routinely tested for the presence of Y. enterocolitica. Presumptive Y. enterocolitica O:3 and O:9 isolates are sent by primary laboratories to the National Reference Laboratory (NRL) at the Norwegian Institute of Public Health, where they are routinely verified, serotyped against a range of O antisera, biotyped if relevant, and tested for Yersinia virulence plasmid (pYV). If the strains are pathogenic Y. enterocolitica, they are typed by use of multiple-locus variable-number tandem repeat analysis (MLVA), using multicolor capillary electrophoresis (5). In March 2011, a multidisciplinary investigation was initiated after the NRL received 5 isolates of Y. enterocolitica O:9 from humans in disparate areas of the country. All had an identical MLVA profile, which had not been previously seen in Norway. An international request for information produced no reports of similar outbreaks