More details have been shared about a major E. coli outbreak linked to salad and other past incidents in the United Kingdom.

Presentations at the European Scientific Conference on Applied Infectious Disease Epidemiology (ESCAIDE) covered a large E. coli outbreak in the UK and other epidemics caused by Shigella and Salmonella.

In late August and September 2022, there was an increase in presumptive Shiga-toxin-producing E. coli (STEC) O157 referrals to the reference laboratory of the UK Health Security Agency (UKHSA).

Large E. coli outbreak
More than 280 E. coli O157 cases were reported, including 259 in England, Wales, Scotland, and Northern Ireland. The majority of patients interviewed consumed salad products at food service sites. One grower of fresh salad leaves was directly or indirectly linked to all other growers and to the food service establishments.

Weather and land use data revealed two isolated and unusually heavy precipitation events in an otherwise drought-like situation at the grower’s location.

Another study revealed how, in the summer of 2022, the UK experienced a prolonged dry, hot spell followed by very heavy localized rainfall; this unusual weather preceded the large outbreak.

This led to a workshop in February 2023 with 33 participants from 13 organizations, including international experts, academics, UK public health and food safety agencies, the UK Met Office, the Animal and Plant Health Agency (APHA), and the UKHSA Centre for Climate and Health Security. 

Experts looked at risk assessment steps, data sources, evidence gaps, and policy considerations to understand the relationship between weather and gastrointestinal illness. Recommendations included incorporating analysis of local weather data, such as rainfall and temperature, and animal movement data for evidence generation and to inform risk analysis in outbreak investigations.

This fall, ESCAIDE from Nov. 22 to 24 was held in Barcelona and remotely. It was organized by the European Centre for Disease Prevention and Control (ECDC).

Shigella and Salmonella
In February 2023, 52 cases of gastrointestinal illness in customers of a takeaway in South Wales were reported to Public Health Wales, and 11 were hospitalized. Shigella flexneri was identified in 29 cases and for nine of those hospitalized.

In a case-control study, eating coleslaw and cabbage were the main associated factors for illness. Shigella was not detected in staff fecal samples. The cases involved people who ate from the takeaway two days in February. They ranged in age from 1 to 75, and 58 percent were female.

Coleslaw or cabbage was the most likely vehicle. A site visit by environmental health officers found hygiene and cross-contamination risks. The origin of contamination remained unknown, but researchers believe it occurred onsite, and a food handler was the most likely source.

Also, in February 2023, a Whole Genome Sequencing (WGS) cluster of six Salmonella Panama cases was discovered in England, with illness dates ranging from September 2020 to 2023. Routine Salmonella questionnaires identified a pub in the Midlands with primary or secondary links to four cases.

Four patients were male and were under 1 to 58 years old. In December 2021, environmental sampling in the kitchen identified Salmonella Panama in the drain of one of four ovens. Control measures were implemented, and resampling in January 2022 was satisfactory.

Three further cases were identified, and two had known exposure to the pub. In environmental sampling in February 2023, seven of 16 isolates from the kitchen, including the ovens, tested positive for Salmonella Panama or Salmonella Derby. Two cases of Salmonella Derby in 2019 and 2022 were similar to these samples. Oven replacement was carried out in April 2023.

Epidemiological and microbiological information identified an oven as a potential source of a prolonged multi-serotype Salmonella outbreak.

“Salmonella had persisted in the environment despite control measures being taken. Oven replacement is an expensive control measure, and while it appears to have been successful so far, the Salmonella clusters will continue to be monitored,” said scientists.

E. coli outbreaks

In July 2022, a genetically linked and geographically dispersed cluster of 12 cases of STEC O103 was detected by the UKHSA using WGS. Ten patients lived in England and two in Wales, aged 8 to 88. Five males and seven females were sick in May and June.

Analysis of food history questionnaires identified unpasteurized brie-style cheese and mixed salad leaves as potential vehicles. Twelve cases and 24 controls were included in the analysis. The brie-style cheese was significantly associated with illness. 

Sample results for cheese products and implicated dairy herd identified Stx genes and STEC, but not the outbreak strain. The premises were temporarily closed voluntarily during the investigation.

“The case-control study results provided key evidence for the brie-style cheese as the source of this outbreak in the absence of microbiological results; hygiene processes at the producer were reviewed and milk used for production was pasteurized,” said scientists.

In summer 2022, an increase in E. coli O157 cases was observed in North Wales. In August, WGS identified a cluster of three E. coli O157:H7 cases with no direct epidemiological links. Analysis of exposure histories revealed all three cases had visited the same open farm in England during their incubation period and within 48 hours of each other.

In November, WGS uncovered three E. coli O157 cases in the same household that were genomically indistinguishable from those in August. An asymptomatic child in the November cluster had visited the same open farm as the August cases.

Investigations at the farm revealed a single isolate from a pig pen identical to the six human cases in the cluster. 

“This incident further underscores the importance of effective control measures at open farms to prevent the transmission of infection to the visitors,” said researchers.