A study published in Eurosurveillance to investigate an outbreak of Shiga toxin-producing Escherichia coli (STEC) O157:H7 in the UK identified contaminated lettuce as the most likely source of the infection, and determined that heavy rainfall and flooding may have carried STEC from animal faeces to the lettuce crops. More heavy rainfall events are expected due to climate change in the future, leading to increased impacts on health and food security.
After a large outbreak of STEC 0157:H7 was detected in the UK in September 2022, a national-level investigation was initiated to identify the source of the outbreak and inform risk mitigation strategies. Whole genome sequencing (WGS) was used to identify outbreak cases, and strongly suggested a link to a single source.
Overall, 259 cases with illness onset dates between Aug. 5 and Oct. 12, 2022, were confirmed across the UK. A national outbreak was declared in early September. Epidemiological investigations identified a UK-grown, nationally distributed, short-shelf-life food item as the source.
Patients lived in all four nations of the UK. There was a higher proportion of females sick, with 142 compared to 117 male cases. The age groups most affected were 20- —to 29-year-olds and 30- —to 39-year-olds, according to the study published in the journal Eurosurveillance.
Seventy-seven confirmed cases attended hospital for their symptoms and 75 were admitted. None of them were diagnosed with hemolytic uremic syndrome (HUS) and no deaths were reported.
Analytical epidemiology and food chain analysis suggested lettuce was the likely vehicle of infection. Food supply chain tracing identified the grower as the likely implicated producer.
Loyalty card data revealed no strong link to one type of salad leaf. Supply chain information highlighted the distribution of suspected salad products to the UK and Ireland.
A lettuce grower was directly or indirectly linked to all the food service establishments or retailers of interest. This included ten retailers, seven processors, 25 suppliers, three food services, four manufacturers, and 14 wholesalers.
When authorities visited the grower, the harvest had ended. A hazard analysis and critical control points (HACCP) plan was in place, and legislative requirements were followed; no failures were identified in storage conditions or temperature control. The grower reported that produce at the farm was exposed to standing water following heavy precipitation and local flooding.
Fresh produce growers and manufacturers have been asked to consider extreme weather and flooding as hazards for pathogens such as STEC and determine proportionate controls, including pre-and post-harvesting checks, finished product testing, and irrigation water use.
Scientists used data on rainfall and temperature, information about land use, and information about sheep locations to better understand the events that led to the outbreak and the location of the lettuce grower. Analysis of meteorological data revealed over two months of rainfall recorded in an otherwise drought-like situation in the region where the grower was located.
They found lettuce incriminated in the outbreak may have been contaminated by heavy rainfall and flooding, transporting STEC from animal feces to crops in fields.
A limitation of the study was that no microbiological sampling was done from known animal reservoirs, products, or the environment.
Adverse weather conditions occurred within two weeks of the major peak of case symptom onset dates. This was consistent with the expected time taken for fresh lettuce to make its way from farm to fork, added to the known incubation period following exposure to STEC.
A small number of illnesses pre-dated the weather event, and a few predominantly secondary case reports in late September and October 2022 were noted.
Seven isolates of STEC O157:H7 closely related to the outbreak strain were previously reported, with three cases in 2019 and two each in 2020 and 2021.
“We hypothesize that both early and late primary cases, as well as the cases notified in previous years, could be explained by low-level environmental contamination due to the persistence of the organism in the environment,” said researchers.
