Shiga toxin-producing Escherichia coli (STEC) serotype O157 is a food-borne gastrointestinal pathogen of public health concern. Infections with STEC can present as sporadic cases or as outbreaks, and they can be symptomatic or asymptomatic. Symptoms can range from mild diarrhoea to abdominal cramps, vomiting and severe bloody diarrhoea, with ca 30% of cases requiring hospital admission. Overall, around 5% of cases (rising to 11% in children aged 1–4 years) develop haemolytic-uraemic syndrome (HUS), which is a severe multisystem condition that predominantly affects the kidneys and can be fatal [1].
In 2022, STEC was the third most commonly notified food-borne zoonotic pathogen in the European Union/European Economic Area (EU/EEA) countries [2]. Of the 29 EU/EEA countries reporting data for 2022, 25 reported 8,565 confirmed cases of STEC infection [3], with 71 food-borne STEC outbreaks reported by 14 countries [4].
The gastrointestinal tract of ruminants is the ecological niche of STEC, with cattle and sheep being the main animal reservoirs [5]. Transmission from animals to humans can occur via direct contact with colonised animals or their environment or by the consumption of food or water contaminated with the pathogen. Food items frequently associated with food-borne outbreaks of STEC O157 include raw or undercooked beef or lamb meat products, unpasteurised dairy products and fresh produce exposed to rainwater run-off, floodwater or irrigation water containing animal faeces [6].
Climate hazards have previously been described as having the potential to activate cascading risk pathways with a sequence of secondary, causally connected events [7]. For example, cascading risks associated with heavy precipitation followed by flooding of animal environments, may lead to contamination of crops and cause food-borne outbreaks of zoonotic diseases. Higher pathogen loads frequently detected in floodwater after rainstorms [8,9] and extreme weather events have been associated with outbreaks of gastrointestinal illness [10–12].
In late August and early September 2022, the United Kingdom (UK) Health Security Agency (UKHSA) Gastrointestinal Bacteria Reference Unit (GBRU) reported a substantial increase in the submission of containment level 3 faecal samples and isolates that were presumptive for STEC. The number of presumptive STEC isolates received by the reference laboratory in the first 6 days of September 2022 was 245, compared with 259 isolates received during the whole of September 2019, the most recent pre-pandemic year for which data were comparable. Between 5 and 7 September, the number of confirmed STEC O157 cases was 73 compared with an average of 16 cases of STEC O157 reported each week in the previous 4 weeks. This, coupled with a substantial increase in the number of enhanced surveillance questionnaires (ESQs) for STEC cases suggested a surge in cases warranting further investigation. On 7 September 2022, a national level outbreak was declared and a multi-agency incident management team (IMT) established.
The IMT aimed to investigate the source of the outbreak by undertaking traditional epidemiological analyses and traceback investigations. A separate sub-group of the IMT explored a novel triangulation methodology using meteorological, flood risk, land use and land-classification and sheep holding density data, to explain the contamination and independently determine the potential source of the STEC outbreak.
