A Shigella outbreak in England in 2021 has been linked to spring onions imported from Egypt.

In November 2021, the UK Health Security Agency (UKHSA) detected an outbreak of 17 people infected with the same strain of Shigella sonnei. Analysis of whole genome sequencing (WGS) data revealed it was closely related to strains of Shigella sonnei isolated from people returning to the UK from Egypt.

The incident occurred at the same time as an outbreak in Denmark also linked to spring onions from Egypt. However, the pathogen-causing illness was different. 

In the UK, no outbreak cases were reported to travel and all 17 ate at a restaurant or food outlet in the week prior to symptom onset, of which 11 dined at branches of the same national franchise. All cases were adults and 14 were female, according to a study published in the Journal of Food Protection.

Finding the source
Ingredient-level analyses of the meals consumed by patients identified spring onions as the common ingredient. Food chain investigations revealed spring onions served at the implicated restaurants could be traced back to a single Egyptian producer. 

In mid-November 2021, the UKHSA Health Protection Team in South West England started investigating a cluster of five cases of Shigella sonnei. This revealed sick people reported eating out at the same national restaurant. Cases were then identified in two other English regions and the incident was escalated to a national response later in November.

There were 17 confirmed cases of Shigella sonnei with sample dates between Dec. 5 and 24, 2021, and two probable cases, with onset dates from Nov. 2 to Dec. 16. Ages ranged from 18 to 63 with a median of 29 years old. Thirteen confirmed cases lived in South West England, one was from the South East and three lived in London.

Eleven cases consumed food from one of three branches of a national food chain. Six patients had no known links to the restaurant, however, four of them reported additional restaurant exposures of which two recalled consumption of spring onions. Two cases did not report eating out or takeaway in the week prior to the onset of symptoms.

No microbiological evidence
In total, 65 samples including 14 environmental swabs and 51 foods were examined from two branches of the restaurant chain. All tests were negative for Shigella species, including three samples of spring onions.

Spring onions were sourced from the UK in October but this changed to an Egyptian grower in November. A food chain investigation showed that three of the implicated restaurants purchased spring onions from two different supply chains that included the same Egyptian grower.

The outbreak strain showed reduced susceptibility to fluoroquinolones and third-generation cephalosporins, the recommended first and second-line treatment options for shigellosis in the UK, respectively.

The Statens Serum Institute (SSI) in Denmark reported an outbreak of enteroinvasive E. coli (EIEC) linked to spring onions from the same grower in Egypt around the same time. They reported that at harvest time, there was evidence of flooding and concluded human fecal contamination in river water had contaminated the crop.

“It is plausible that this event contributed to the temporally linked outbreak described in this study, despite the two outbreaks being caused by different pathogens,” said researchers.