Large Shigella flexneri outbreak linked to a takeaway, South Wales: a case-control study

In February 2023, 52 cases of gastrointestinal illness were reported in customers of Takeaway A, South Wales. Shigella flexneri serotype 2a was the causative organism. An outbreak investigation was conducted to determine the extent and vehicle of the outbreak.Following descriptive summary and environmental investigations, a case-control study was completed. Participants completed a telephone questionnaire on food, travel, and environmental exposures. A multivariable logistic regression model was built, including exposures with p-values < 0.2 and interactions identified on stratified analysis. Staff faecal samples were screened for Shigella sp.Thirty-one cases and 29 controls were included in the study. Eighty-seven per cent of cases and 76% of controls ate from Takeaway A on 10 February 2023. Coleslaw was the main factor associated with illness (aOR: 200, 95% CI: 12-3220) and an interaction with cabbage was identified (aOR: 886, 95% CI: 26-30034). Shigella sp. were not detected in any staff samples.Coleslaw was the most likely vehicle. Though the contamination route is unknown, a food handler is the most likely source. This large outbreak differs from recent European outbreaks, which primarily have been associated with sexual transmission. Although uncommon in the UK, S. flexneri should be considered as a cause of foodborne outbreaks.