Scientists show how UK E. coli outbreak was traced to lettuce

Scientists have detailed the process of linking a major E. coli outbreak in the United Kingdom to lettuce.

In May 2024, public health agencies and laboratories identified an increase in stool specimens submissions and patients testing positive for Shiga toxin-producing E. coli (STEC). Whole genome sequencing (WGS) identified serotype O145:H28 stx2a/eae as the causative agent. It was a re-emergence of an STEC cluster investigated in 2023, where no source could be confirmed, said the UK Health Security Agency (UKHSA).

By early July 2024, 288 cases had been linked to the cluster. Most were adults and females, 129 people were hospitalized with 39 percent attending emergency care. England had more than 180 cases, while about 60 people were sick in Scotland, 30 in Wales, and a few in Northern Ireland.

According to a study published in the journal Epidemiology and Infection, symptom onset dates of primary cases ranged from April 29 to June 17. They had a median age of 29 and ranged from 1 to 89 years old. There were nine cases of hemolytic uremic syndrome (HUS) and two deaths.

Since 2020, STEC O145:H28 has been in the top five most common E. coli serotypes reported in England and Scotland.

The link to lettuce
Descriptive epidemiology and analytical studies identified consumption of nationally distributed pre-packed sandwiches as a common food exposure. The implicated food businesses recalled ready-to-eat sandwiches and wraps containing lettuce in mid-June.

Eleven of 15 cases interviewed with a trawling questionnaire reported eating pre-packaged sandwiches from different national retailers.

Food chain investigations identified the sandwich producer that supplied the retailers during May 2024. The producer had sourced lettuce from farms in England.

Domestically produced apollo lettuce was the most likely source of contamination. Samworth Brothers, This! and Greencore Group recalled a range of sandwich products.

Location of an animal reservoir or mechanisms of crop contamination are currently unclear. Possible routes include a failure in control measures protecting the crop from agricultural run-off, contamination of water or growing materials used in lettuce production, or contaminated seeds. 

Test results have been negative for STEC but general or indicator E. coli was identified in sandwich and lettuce products, indicating a possible contamination event.

A message posted in European surveillance systems revealed none of the 13 countries that replied were affected.

Ready-to-eat salad vegetables are vulnerable to contamination with pathogens at the pre-harvest level via flooding, rainwater run-off, or irrigation water containing animal feces. Current methods for washing and decontaminating fresh produce cannot guarantee that pathogens will be removed. Controls to minimize the risk of fecal contamination during growing, handling, and processing are important.

(To sign up for a free subscription to Food Safety News, click here.)