More than 30 people were part of an E. coli outbreak in the United Kingdom linked to salad in sandwiches in 2019, according to a study.
It was the first UK-wide foodborne outbreak of Shiga toxin-producing E. coli (STEC) O26.
Across the UK, 32 cases of STEC O26:H11 were identified from early October to mid-November, including 21 males. Six people needed hospital treatment. The two people reported sick in November were based on sample dates.
There was an association with eating pre-packed sandwiches purchased at outlets belonging to an unnamed national food chain franchise, found the study published in Epidemiology and Infection.
The common ingredient in the majority of sandwiches was a mixed salad of Apollo and iceberg lettuce and spinach leaves.
Initial outbreak signal
In October 2019, routine microbiological surveillance at the Scottish E. coli O157/STEC Reference Laboratory (SERL) identified the outbreak.
Overall, 14 people were sick in Scotland, 16 in England and two in Wales. Ages ranged from 3 to 77 years old with a median of 27.
Of 26 cases with available information, 22 had bloody diarrhea and six were hospitalized but none of them developed hemolytic uremic syndrome (HUS) or died.
In total, 26 of 28 cases reported eating out at various food outlets within seven days of symptom onset. Of these, 17 purchased food from one outlet, three bought sandwiches from another site and three ate at a different outlet. The remaining three patients ate at three different food outlets.
Microbiological food and environmental testing were negative for STEC O26:H11 but STEC O36:H19 was isolated from a mixed salad sample from premises owned by the food outlet.
“Although this does not constitute direct evidence that this salad was the source of the outbreak as it is a different strain, it does highlight a potential pathway of human exposure to STEC from the mixed leaf salad,” said researchers.
The product was no longer available for recall because of the time delay between sampling and test results. However, follow-up investigations by the business investigated the source of STEC contamination and HACCP-based food safety processes were reviewed.
Complexity of supply chain
The salad mix supplier was investigated and environmental health officers said they were satisfied with procedures in place. Between early September and late October, the firm was supplied by 11 different growers, 10 farms were located in the UK and one was in France.
On-farm sampling was not done for a number of reasons, including the large number of farms in the supply chain, the absence of evidence for ongoing contamination, and elapsed time since production of the implicated batch of salad leaves.
Epidemiological analysis provided evidence that one of the mixed leaf salad components was the contaminated vehicle. The implicated salad was a component of sandwich fillings or garnish.
The link between cases was established because the majority of infected individuals reported buying takeaway products from the same national food chain franchise and other outlets linked to the salad mix supplier.
Researchers said epidemiological analysis should be sufficient to direct timely and targeted on-farm investigations as there are challenges in detecting STEC in produce with a short shelf-life.
“A shift in focus from testing the microbiological quality of the produce to investigating the processes and practices through the supply chain and sampling the farm environment is recommended. Such an approach is essential to identify the root cause of outbreaks linked to salad and raw vegetables, and to establish an evidence-base for improving guidance and policy,” they said.
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