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The Dutch National Institute for Public Health and Environment (RIVM) reported today that 1,026 people are now confirmed to have been infected with Salmonella Thompson, in an outbreak of salmonellosis that has been linked to contaminated smoked salmon. This represents an increase of 76 cases in the past week.

RIVM notes that the number of new patients is significantly lower than the week before, suggesting that this major outbreak is finally waning.

According to a report in today’s issue of Eurosurveillance, Dutch authorities first became aware of the outbreak on August 2nd, when RIVM staff noticed an increase in the number of Salmonella Thompson cases. This particular Salmonella normally causes only four illnesses annually in the Netherlands.

The index patient developed symptoms of salmonellosis on June 20th. A few more scattered illnesses occurred over the next several weeks; however, the number of illnesses spiked noticeably only in early August. Nearly two-thirds (63%) of the confirmed outbreak victims were female, and 36% were hospitalized. The median age of case-patients was 44 years (range of 0 – 95 years).

While the official case total stands at 1,076, the true number of cases is undoubtedly far higher. RIVM advises that Salmonella is NOT a notifiable disease in the Netherlands. There is no requirement that doctors, hospitals or laboratories report Salmonella infections to  the authorities.

The outbreak was first linked to consumption of smoked salmon as a result of questionnaires administered to patients and to a cohort of healthy individuals (matched to the victims by age, gender and municipality), either by telephone or in person. The results of the questionnaires pointed to consumption of smoked fish – especially smoked salmon. Victims also were more likely to have consumed raw salads and also identified several supermarkets more often than did the healthy control individuals.

Traceback investigations carried out by the Dutch Food and Consumer Product Safety Authority (nVWA), which focussed on smoked salmon and on the highlighted supermarkets, determined that all of the identified supermarkets purchased at least some of their smoked salmon from the same producer –Foppen Paling en Zalm. Lab tests confirmed the presence of the outbreak strain in four of nine batches of smoked salmon from this producer.

A recall of smoked salmon was announced on the nVWA website on September 28th, and expanded a few days later to include a number of additional items – such as salads – that contained the recalled smoked salmon.

The implicated salmon originated in Norway, according to the Rapid Alert System for Food and Feed (RASFF), and was shipped to a Foppen production plant in Greece for processing before traveling in a northwesterly direction to the Netherlands. The smoked salmon was distributed in Belgium, Canada, Curacao, France, Germany, Luxembourg, Mexico, the Netherlands, Switzerland, the United Kingdom and the USA. Nevertheless, the Netherlands remains the only EU country to have noted an increased incidence of Salmonella Thompson infections.

The US Centers for Disease Control and Prevention (CDC) has been investigating a spike in salmonellosis due to this outbreak strain but, as of a few days ago, has not been able to establish a link between the illnesses and the consumption of smoked salmon – or of any other specific food item. This particular strain of Salmonella Thompson is not uncommon in the United States, making determination of a single outbreak source problematic.

For a complete list of recalled smoked salmon products, please consult my earlier post, “Smoked Salmon and Salmonella Thompson – Update #3.

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