Pre-washed salad behind Swedish Salmonella outbreak

Researchers have shared details on one of the largest national Salmonella outbreaks in Sweden that was likely caused by produce.

The Salmonella Typhimurium outbreak involved more than 100 confirmed cases in Sweden, and other infections were recorded in Finland, the United Kingdom, Denmark, and Norway.

109 cases with symptom onset between Sept. 17 and Nov. 24, 2022, were reported from 20 of 21 Swedish regions. The median age of cases was 52 with a range from 4 to 87 years old and 68 were female.

A case-control study found cases associated with the consumption of rocket salad and bagged mixed salad, according to the study published in the journal Eurosurveillance.

Link to Rocket salad
Traceback, supported by authorities in Finland who identified the Swedish outbreak strain in a Finnish cluster during the same period, identified rocket salad, cultivated, pre-washed and pre-packed in Sweden as the likely source of the outbreak. No microbiological analyses of rocket salad were performed as the outbreak had stopped when the suspected source was identified.

At the end of September 2022, the Public Health Agency of Sweden (Folkhälsomyndigheten) observed an increase in domestic Salmonella cases, and a national outbreak was declared in early October.

In October 2022, 54 patients and 434 controls answered a questionnaire. Frequently reported symptoms among 52 patients who responded were diarrhea, fever, stomach pain, bloody diarrhea, and vomiting. Overall, 22 people were hospitalized.

Investigators received 69 receipts from 21 confirmed cases. They were dated in September and October. Only 65 receipts from one retailer were included in the investigation. Seventeen cases had receipts, including bagged rocket salad and/or mixed salad. These purchases were in September.

No food samples of suspected products from cases were available for laboratory analysis. As the outbreak was over when the investigation identified the producer, no attempts were made to sample products at this level.

Of the 18 patients in Finland, 17 were in September to October 2022 and one in January 2023. Two patients were part of a cluster in which six people became ill after dining at a restaurant in Western Finland. The Norwegian case had a sampling date in September 2022 and had recently traveled to Sweden. The Danish case had a sampling date in the same month with no additional information.

Salmonella survive despite washing
The seven UK patients had sampling dates between July 2021 and February 2022, and no travel to Nordic countries was reported. Patients were not likely linked to the Swedish outbreak as they were reported seven to 14 months before the Swedish and Finnish outbreaks. Researchers speculated that an environmental reservoir for the outbreak strain exists, such as surface waters or wildlife, such as gulls.

Traceback investigations showed that bagged rocket salad and mixed salad, including rocket salad sold at one retailer, were all delivered from one packaging company in Sweden. In September 2022, this company only bought rocket salad from three growers in southern Sweden.

These growers delivered 16 to 21 tons of rocket salad weekly to the packing company. At this company, all leafy greens underwent a washing process with potable water consisting of two washing steps and a final rinse before the salad was dried and packaged. All leafy greens were washed in the same tanks.

Finnish authorities identified a particular batch of rocket salad as the likely outbreak source. This batch included a rocket from two of the three Swedish growers delivering salad to the packaging firm. 

The packaging company told the retailer that Salmonella had not been found in samples from their routine produce analysis. Both growers had no problems with wildlife or flooding during the growing season. The latest water samples taken by growers did not show signs of contamination.

“Our investigation indicates that, regardless of pre-washing procedures in the production chain, bagged leafy greens such as rocket salad may contain Salmonella and cause outbreaks, posing a health risk to consumers. We emphasize the need for primary leafy greens producers to identify possible contamination points to prevent outbreaks,” said researchers.

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