Harderwijk-based Foppen was found to be the source of 1,149 laboratory-confirmed cases between August and December 2012 of which four elderly people (76–91 years) died.
The smoked salmon in the production line had been continuously contaminated through reusable dishes in the production process, which turned out to be porous and became loaded with bacteria.
The cleaning and disinfecting regime was not sufficient to kill bacteria on the inside of the plates, found the report in Eurosurveillance.
In September 2012, the Dutch Food and Consumer Product Safety Authority (NVWA) visited the fish producer and collected samples from different batches of smoked salmon products.
Salmonella Thompson was detected by the NVWA in four of nine sampled batches and all smoked salmon was recalled.
Greek processing plant
A trace-back analysis by the fish producer showed that the positive batches were produced on certain production lines in the Greek processing plant, where it is processed before being transported to the Netherlands for further distribution.
The Greek authorities (EFET: Hellenic Food Authority) were informed about the problem in the production facility of the Dutch fish producer.
EFET temporarily closed the production site (3–11 October). After analysis of the production process in Greece, the Dutch producer concluded, that the continuing contamination of smoked salmon must have been caused by cross contamination from dishes on which the salmon was transported within the processing lines.
Dishes were introduced in the production process in Greece in February 2012 and were immediately replaced by single-use dishes after being identified as the source of the contamination.
Additional research conducted by the Netherlands Organisation for Applied Scientific Research (TNO) showed that the inner layer of the dishes was highly porous and absorbing the Salmonella.
Closure and recall action
The temporary closure of the processing site and recall of the smoked salmon stopped the outbreak.
An estimated four to six million Dutch residents were possibly exposed to the contaminated smoked salmon and an estimated 23,000 persons would have had acute gastroenteritis with S. Thompson during this outbreak.
For the 1,079 outbreak cases for which sex was known, 696 (65%) were female.
Age information was available for all cases and the median age was 45 years (range: 0–95 years).
For comparison, the median age of the other 1,624 cases of salmonellosis reported to the RIVM in 2012 was 29 years, and 53% were female.
Five outbreak cases were younger than six months and most likely not eating solid foods yet, three of them had family members with a confirmed infection.
Source identification challenge
Although a case–control study was started immediately after detection of the outbreak, it took several weeks before the cause could be identified.
The main reason for the delay was the low number of cases in the beginning of the outbreak and the low number of completed questionnaires, from cases and controls, available for analysis.
A number of other possible sources were suggested before the analysis led to salmon. This was most likely caused by the relatively low number of cases reporting consumption of smoked fish (62/108, 57%).
This can be due to recall bias, but also because the salmon was incorporated in other products, for example pre-sale ready-to-eat salads or as part of a menu in the catering industry.
“Large outbreak of Salmonella Thompson related to smoked salmon in the Netherlands, August to December 2012”
Authors: I Friesema, A de Jong, A Hofhuis, M Heck, H van den Kerkhof, R de Jonge, D Hameryck, K Nagel, G van Vilsteren, P van Beek, D Notermans, W van Pelt