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Analysis shows source of E. coli outbreak was leafy greens

Analysis shows source of E. coli outbreak was leafy greens

Increased use of ingredient-based analyses is being promoted by researchers after the method was used to identify the source of infection in an E. coli outbreak. 

Public Health England was alerted to an outbreak of Shiga toxin-producing E. coli (STEC) O157 phage type (PT) 34 in July 2016 involving 56 cases in England and Wales. The source of infection was baby-mixed leaf salad, which was an ingredient in multiple dishes.

“A traditional analysis may have resulted in multiple menu items being associated with illness, thereby failing to identify the true source of infection,” said researchers.

Ingredient-based analyses are rarely used in outbreak investigations and are likely to be more resource-intensive than a traditional approach, according to the scientists.

A number of patients in the outbreak reported eating at a staff canteen and a garden café. The operations had a common salad supplier. The supplier, a wholesale distributor, sourced some of its baby mixed-leaf and rocket salad products from another supplier. That supplier provided salad items to 30 venues across the country, each associated with at least one case.

The researchers used control measures that included the supplier volunteering to suspend the distribution of salad leaves. Food distributors only provided limited information on when specific batches of product were bought and sold.

There were 76 different menu items served across the two venues containing salad ingredients supplied by the distributor. By measuring at ingredient level, the number of exposures to analyse was reduced to 22 salad ingredients.

Baby mixed-leaf salad supplied by the distributor was the only ingredient independently associated with being a case.

A retrospective case-control study was done using an online questionnaire to get information on menu items consumed at each venue. Among 203 responses, 24 cases were identified (13 confirmed, two probable and nine possible) and onsets ranged between June 7 and 25, 2016.

 There were no positive microbiological results from food samples as they were not collected until after the contamination had likely passed.

Use of an ingredient-based analysis may have resulted in more accurate exposure classification because respondents were more likely to accurately recall which main menu items they had eaten rather than a potentially non-memorable ingredient, said researchers. 

“Another example of an ingredient-based analysis used to identify a non-memorable ingredient as the source of infection is the outbreak of STEC O104 in Germany in 2011,” according to the research. 

“In that outbreak, all cases in the ingredient-based study had consumed sprouts but only 25 percent of cases in the previous case-control study reported eating sprouts. As sprouts were served as garnish or in side salads accompanying main dishes, consumption of this ‘concealed exposure’ was likely to be forgotten.”

A potential limitation of such analysis is that ingredients may not be used consistently in dishes, potentially resulting in exposure misclassification. 

For example, the chef at the garden café could not provide accurate information on which herbs were used in which dishes as this varied from day to day. This limitation did not impact the study as no herbs were supplied to either venue by the common supplier.

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