Scientists have revealed that lettuce contaminated by animal waste during floods was likely the cause of infection in a 2022 E. coli outbreak.
In September 2022, a major outbreak of STEC O157:H7 was identified in the United Kingdom. It was one of the largest outbreaks of E. coli since the early 1980s.
In late August and early September, the UK Health Security Agency’s (UKHSA) Gastrointestinal Bacteria Reference Unit (GBRU) reported a substantial increase in the submission of certain fecal samples and isolates suspected of being STEC.
A total of 259 cases were confirmed in the United Kingdom with an illness onset date between August 5 and October 12, 2022. A national outbreak was declared in early September. Epidemiological research has identified a British-grown, nationally distributed food product with a short shelf life as the source.
Patients lived in all four countries of Great Britain. There was a higher percentage of women 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 Eurosurveillance magazine.
Seventy-seven confirmed cases went to hospital due to their symptoms and 75 were admitted. None were diagnosed with hemolytic uremic syndrome (HUS) and no deaths were reported.
Link to lettuce grower
Analytical epidemiology and food chain analysis suggested that lettuce was the likely vehicle of contamination. Food supply chain tracing identified the grower as the likely producer involved.
Loyalty card data did not reveal a strong association with one type of lettuce leaf. Supply chain information revealed the distribution of suspected salad products to Great Britain and Ireland.
A lettuce grower was directly or indirectly connected to all catering companies or retailers involved. This included ten retailers, seven processors, 25 suppliers, three food services, four manufacturers and 14 wholesalers.
When the authorities visited the grower, the harvest was over. A Hazard Analysis and Critical Control Point (HACCP) plan was in place and regulatory requirements were followed; no errors were noted in storage conditions or temperature control. The grower reported that the produce on the farm was exposed to standing water due to heavy rainfall and localized flooding.
Growers and producers of fresh produce have been asked to consider extreme weather events and flooding as hazards for pathogens such as STEC and to establish proportionate controls, including pre- and post-harvest checks, testing of finished products and the use of irrigation water.
Weather impact
Scientists used data on rainfall and temperature, land use information and information on sheep locations to better understand the events that led to the outbreak and the location of the lettuce grower. Analysis of meteorological data showed that rainfall had been recorded for two months in an otherwise drought-like situation in the region where the grower was located.
They found that lettuce involved in the outbreak may have been contaminated by heavy rainfall and flooding, which transported STEC from animal feces to crops in the fields.
A limitation of the study was that no microbiological samples were taken from known animal reservoirs, products or the environment.
Adverse weather conditions occurred within two weeks of the main peak in disease symptom onset. This was in line with the expected time for fresh lettuce to get from farm to table, added to the known incubation period after exposure to STEC.
A small number of illnesses predated the weather events, with some predominantly secondary case reports noted in late September and October 2022.
Seven isolates of STEC O157:H7 closely related to the outbreak strain have previously been reported, with three cases in 2019 and two in 2020 and 2021.
“We hypothesize that both early and late primary cases, as well as those reported in previous years, can be explained by low environmental contamination due to the persistence of the organism in the environment,” researchers said.
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