The root cause of a large E. coli outbreak in the United Kingdom in 2024 remains unknown with authorities warning a re-emergence is possible.
Almost 300 people fell ill with 196 cases in England, 62 in Scotland, 31 in Wales, and four in Northern Ireland. It was the largest reported E. coli outbreak in the UK since the start of routine whole genome sequencing (WGS) in 2015.
In May 2024, the UK Health Security Agency (UKHSA) detected a rise in Shiga toxin-producing E. coli (STEC) through routine disease surveillance. Similar patterns identified by Public Health Wales and Public Health Scotland suggested the emergence of a large national non-O157 STEC outbreak.
This led to an Incident Management Team (IMT) being formed to investigate. Experts met 13 times between May and November. They made seven recommendations including raising awareness of the risks posed by STEC with industry and local authorities and trying to find ways to speed up epidemiological work to enable more timely microbiological investigations.
Connection to 2023 outbreak
STEC O145:H28 was confirmed as the outbreak strain. This strain was responsible for 44 illnesses and 21 hospitalizations between June and October 2023, but there was insufficient evidence to identify the vehicle of infection or source of contamination despite an epidemiological link to cucumbers.
There were 293 confirmed and three probable cases reported across the UK between May and November 2024, but the majority of symptom onset dates were in mid-May. The median age of patients was 29 with a range of 1 to 89 and just over half were in the 20-to 39-year age group.
More than 200 sick people reported bloody stools, 129 were admitted to hospitals and 36 attended the Accident and Emergency department for their symptoms but were not admitted. Hospital stays ranged from 1 to 14 days.
Eleven people developed Hemolytic Uremic Syndrome (HUS), and two died. International communications concluded there were no cases of the same genetic profile outside the UK.
Amy Douglas, lead epidemiologist at UKHSA, said: “Outbreaks involving fresh produce or ready-to-eat foods are particularly challenging to investigate due to the speed with which they emerge and the high number of cases that we must assess.
“By the time we are made aware of an outbreak, often none of the implicated food products are available for testing because they have either been eaten or disposed of, making it difficult to confirm whether they were contaminated. STEC O145 is a growing threat which is now one of the most common types of STEC that we are detecting in our surveillance.”
Analysis of questionnaire data identified pre-packaged sandwiches, salad vegetables and eating out as exposures significantly associated with being an outbreak patient. Sandwiches were reported more often than expected with common types including bacon lettuce tomato and chicken salad, from major retailers.
Root cause unknown
Investigations by the Food Standards Agency (FSA), Food Standards Scotland (FSS), local authorities and industry found that UK grown Apollo leaf lettuce was the likely contaminated ingredient in the pre-packed sandwiches.
Food chain analysis led to three sandwich manufacturers supplied by four growers. In June 2024, Greencore, Samworth Brothers and THIS! undertook recalls because of possible contamination with E. coli after the pathogen was detected four times at low levels in samples from raw Apollo leaf and sandwiches. The seed associated with implicated crops was traced back to three suppliers.
Additional testing of raw material and final products were introduced by the sandwich makers until the end of the UK growing season in 2024.
Apollo leaf from one of the growers was the likely source of contamination. However, supply dates did not cover the full exposure period for reported cases.
A total of 88 food, water and environmental samples were collected from implicated premises between June and September 2024. One sample of irrigation water from another grower tested positive for Shiga toxin genes. This isolate was STEC O55:H12, which was not the outbreak strain.
Investigations failed to provide conclusive microbiological evidence of STEC contamination or identify the root cause of the incident. UKHSA said this means a re-emergence of the outbreak strain is possible. Limited options for non-O157 STEC testing for industry were also highlighted.
FSA held a session with 31 affected businesses, including growers, manufacturers, and retailers in January 2025 to improve industry awareness of challenges surrounding STEC.
The British Leafy Salads Association delivered training for growers and collaborated with local authorities to improve understanding of salad production risks, and the Chilled Food Association updated guidance for growers to reduce STEC risks.
Jacquelyn McCormick, head of incidents at the FSA, said: “This outbreak demonstrates the complexity of the supply chain and how critical it is for government organizations and the food industry to work together to respond rapidly to food safety concerns and protect public health. The learnings from this are being used to continually improve supply chain controls to ensure food is safe.”
