A cluster of YE O9 infections was reported from a military base in northern Norway in May 2014.
By the end of the month, 133 cases were identified, of which 117 were linked to four military bases and 16 were civilians.
Civilian cases ranged in age from 24 to 95 years and just over half were female.
Imported salad mix
The most likely source was salad mix containing imported radicchio rosso, due to its long shelf life.
It is also stored at +1 °C before being supplied to the market. These conditions allow growth as YE is able to grow down to –2 °C.
The four military bases and cafeterias visited by 14/16 civilian cases received iceberg lettuce or radicchio rosso from the same supplier.
The distributor uses only Norwegian produce during the summer, but imports 70–80% during the winter months.
Radicchio rosso imported on 6 April 2014 came from the previous harvesting season, while the import on 16 April 2014 was from a new harvesting season.
Washed and salad mixes were assembled at a processing factory in Norway that belongs to a subsidiary company to the distributor.
An inspection of the site where the salad mixes were produced for the distributor found significant lapses in hygiene, including not changing water in rinsing tanks on a regular basis.
“The geographically widespread occurrence of the yersiniosis cases and the prolonged duration indicated that the source of the product was widely distributed and available for a sustained period of time, which does not immediately suggest fresh produce as a source.
“However, the traceback investigations’ results for both the civilian and military cases strongly indicate that almost all cases were exposed to salad products supplied by the same distributor.
“If the implicated radicchio rosso was from the previous harvesting season and was imported before the changeover to Norwegian produce, it may have been stored for a long period of time, facilitating microbiological growth.
“Given the uncommon serotype and novel MLVA profile, it is suspected that the contamination of an imported salad product occurred outside of Norway, but potential lapses in processing after importation may have contributed to the spread.”
However, there were no reports of similar yersiniosis outbreaks in other European countries.
A case was defined as someone with laboratory-confirmed YE O9 infection with the outbreak multilocus variable number tandem repeat analysis (MLVA) profile with onset of symptoms between 1 March and 15 June 2014. The profile had not been seen in Norway before the outbreak.
Fifteen food samples were taken from mess halls, cafeterias and private homes at the time of inspection on 27 May 2014 but all tested negative.
All seven positive food handlers worked in camp T2 which the researchers said might explain the concentration of cases in that camp.
“Concerning the military cases in Norway, infected food handlers may have propagated the outbreak by contaminating food that was served in the camp’s kitchen but it is unlikely that a food handler introduced the infection as the outbreak commenced simultaneously in several locations.”
Yersinia enterocolitica (YE) infection is the fourth most commonly reported cause of bacterial diarrhoeal disease in Norway. Since 2008, between 40 and 60 cases have been reported annually.
Transmission is most frequently through eating contaminated food, particularly raw or undercooked pork but there was an outbreak of YE O9 due to imported ready-to-eat salad mix in Norway in 2011.
Yersiniosis is notifiable to the Norwegian Institute of Public Health (NIPH) via the Norwegian Surveillance System for Communicable Diseases (MSIS).
More than 80% of yersiniosis cases are due to serotype O3, which is also the dominant cause of yersiniosis in Canada, Europe, Japan, and parts of the US.
Incubation period is generally under 10 days, but most often between three and seven days.
“National outbreak of Yersinia Enterocolitica infections in military and civilian populations associated with consumption of mixed salad, Norway, 2014”
Authors: E MacDonald, M Einöder-Moreno, K Borgen, L Thorstensen Brandal, L Diab, Ø Fossli, B Guzman Herrador, AA Hassan, GS Johannessen , EJ Johansen, R Jørgensen Kimo, T Lier, BL Paulsen, R Popescu, C Tokle Schytte, K Sæbø Pettersen, L Vold , Ø Ørmen , AL Wester, M Wiklund, K Nygård