Abstract
Public Health England was alerted to a national outbreak of Shiga toxin-producing Escherichia coli O157 PT34 in July 2016. Early investigations suggested that the likely source was a salad item consumed outside of the home. A number of cases reported consuming meals at a staff canteen (Venue A) and a garden café (Venue B). Both venues shared a common salad supplier. An investigation was undertaken to measure associations between salad items and illness using an 'ingredient-based analysis'. A retrospective case–control study was conducted using an online questionnaire to collect information on menu items consumed at each venue. Chefs at both venues were interviewed to identify ingredients contained within each menu item. Both venues were pooled together for multivariable analysis measuring associations at the ingredient level. Among 203 responses, 24 cases were identified (13 confirmed, two probable and nine possible). Case onsets ranged between 7 and 25 June 2016. Multivariable analysis identified strong evidence that only baby mixed-leaf salad from the common supplier was a vehicle of infection (adjusted odds ratio = 13.1; 95% confidence interval: 1.6–106.5). Identifying the specific salad ingredient associated with illness was made possible by using an ingredient-based analysis. We recommend the increased use of ingredient-based analyses.
Highlights
In June 2016, eight cases of Shiga toxin-producing Escherichia coli (STEC) serotype O157 phage type (PT) 34 were notified in a 48-hour period in South West England
Public Health England was alerted to a national outbreak of Shiga toxin-producing Escherichia coli O157 PT34 in July 2016
Nine days after the initial alert, 56 cases of STEC O157 PT34 had been reported across England and Wales, representing a substantial increase compared with expected levels
Summary
In June 2016, eight cases of Shiga toxin-producing Escherichia coli (STEC) serotype O157 phage type (PT) 34 were notified in a 48-hour period in South West England. Nine days after the initial alert, 56 cases of STEC O157 PT34 had been reported across England and Wales, representing a substantial increase compared with expected levels (on average 2.4 cases of STEC O157 PT34 were reported in England and Wales during spring and summer between 1994 and June 2016). Subsequent whole genome sequencing on case isolates revealed that the cases were genetically highly related at the 5-SNP level, confirming that they were outbreak cases. Initial investigations included enhanced trawling questionnaires and a case–case study. Results of these investigations suggested that consumption of salad items and eating outside of the home were associated with illness [1]
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