Research and analysis

Investigation into an outbreak of Shiga toxin-producing E. coli (STEC) O145 in Great Britain, May to June 2024

Published 27 June 2024

The UK Health Security Agency (UKHSA), Public Health Scotland, Public Health Wales and Public Health Agency Northern Ireland (PHA), in collaboration with the Food Standards Agency (FSA) and Food Standards Scotland (FSS) have been working together with local authorities to investigate an outbreak of Shiga toxin-producing Escherichia coli (STEC) O145 identified through the analysis of whole genome sequencing (WGS) data in May 2024.

A potential outbreak was first identified in England on 22 May 2024 through UKHSA’s routine surveillance, with a rapid ten-fold increase in the number of faecal samples from patients testing positive for non-O157 STEC toxin genes referred from the NHS to the national reference laboratory.

On 24 May, reference laboratory polymerase chain reaction (PCR) test results indicated the increase was likely driven by a strain of STEC which possessed the stx2a, eae+ virulence gene profile but was neither serotype O157 or O26 (defined as ‘probable cases’) and increased hospital emergency department attendances for gastrointestinal illness (1). Concurrent increases for similar surveillance indicators were reported in Wales and Scotland. A national incident was declared on 24 May.

Subsequent WGS analysis available on 28 May showed that the majority of these probable cases had illness caused by STEC serotype O145 with a specific genetic profile (‘genetic fingerprint’). These confirmed outbreak cases with a 5-single nucleotide polymorphism (SNP) termed ‘t5.206’, were distinct from other STEC O145 outbreaks recently investigated and from sporadic cases. This was a re-emergence of a STEC cluster investigated in 2023, where no source could be conclusively confirmed. No international cases of a similar genotype were reported on global databases and information was shared through established international communication platforms with other countries.

Between 25 May and 24 June there were 275 confirmed cases reported (273 primary cases and 2 secondary cases (Figure 1)). Cases were geographically dispersed in all regions of England (182), in Scotland (58), Wales (31) and Northern Ireland (4), with no notable geographic clustering. However, evidence suggests that cases resident in Northern Ireland likely acquired their infection in England. Primary cases were predominantly female (57%) and had a median age of 30 years (range: 1 to 89 years). The most affected age groups were 20 to 29 and 30 to 39 with 30% and 23% of cases respectively.

The STEC O145 t5:206 outbreak strain possesses a virulence gene profile (stx2a, eae+) which is associated with more severe disease and an increased likelihood of Haemolytic Uraemic Syndrome (HUS), a clinical syndrome associated with STEC which can lead to kidney failure and death.

Among confirmed t5:206 outbreak cases (Table 1):

• 81% have reported bloody diarrhoea
• 49% of cases have been admitted to hospital
• 10% attended A&E for their symptoms

Of the 122 hospitalised cases, 57% were female and had a median age of 35 (with a range of 6 to 85 years).

To date 7 cases of HUS have been confirmed to be associated with this outbreak. Of the 7 HUS reported cases:

• 5 were resident in England
• 2 were resident in Scotland
• their ages ranged from 12 to 59
• the majority were female (n=6)

These HUS cases all reported onset of illness in mid to late May. There have been 2 reported deaths in patients within 28 days of confirmation of infection with the STEC outbreak strain t5:206. Neither of these patients were diagnosed with HUS, both were adults and had underlying medical conditions. Based on the information available, one of these deaths is likely linked to STEC infection.

Early epidemiological analyses indicated that the cases were widely geographically dispersed across the UK with a rapid increase in confirmed cases consistent with a nationally distributed food product as the cause of the outbreak. The FSA and FSS therefore were involved from the start of the investigation. In the initial phase of the investigation, analysis of available information on reported cases, including routinely collected case questionnaire data (which captures clinical symptoms and environmental plus a wide range of food exposures (2), indicated a higher than expected proportion of cases reported consuming pre-packaged sandwich products during the 7 days prior to becoming unwell.

The hypothesis that sandwiches containing lettuce were the likely main cause of the outbreak was confirmed through multiple different epidemiological studies which showed a statistically significant association between illness and consumption of these products.

Food chain investigations were carried out by FSA and FSS, informed by the food histories of cases and alongside the epidemiological investigations carried out by the UK public health agencies. Lettuce was the main focus as the likely contaminated sandwich ingredient. The FSA and FSS initially identified one supplier of the potentially contaminated lettuce, investigations are still ongoing at 2 other lettuce suppliers at the grower stage of the supply chain.

Further investigations carried out by FSA and FSS with the identified sandwich product producers and salad growers indicated that these food business operators (FBOs) had detailed and robust hazard analysis and critical control points (HACCP) plans and biosecurity in place. Enhanced testing at these FBOs was carried out in response to the investigation. While all test results were negative for STEC, general or ‘indicator’ E. coli was identified in sandwich and lettuce products, indicating a possible contamination event had occurred. As a result of the epidemiological and food chain investigation outcomes, the FBOs elected to undertake a voluntary withdrawal and recall of all potentially associated ready to eat products (sandwiches, wraps and salads) – first on 14 June and then, following further findings, on the 15 and 16 June (3, 4, 5) . Investigations into the root cause of the outbreak are still ongoing.

There are challenges in the investigation of any foodborne disease outbreak of STEC as previously reported (6). In the case of short shelf life and highly perishable products such as salad products, microbiological confirmation of the outbreak strain (the same WGS profile) in the foods implicated in the outbreak is very difficult as the foods have usually already been consumed and there is no residual product to test. As performed in this investigation, environmental and food testing may be useful for the detection of E. coli as an indicator of a potential contamination event, as well as detection of specific toxin genes present in STEC which causes food poisoning.

Rapid response based on early surveillance indicators prior to an outbreak being confirmed, and subsequent rapid confirmation of the outbreak (via routine implementation of WGS for characterisation of STEC and other major bacterial pathogen causes of foodborne disease) is crucial for the protection of public health. Early collaboration and communication with UK government agencies and international counterparts identified that the outbreak was limited to the UK, supporting the epidemiological and food chain investigations findings that a UK produced salad product was the cause of the outbreak.

STEC is a zoonotic pathogen (meaning a bacteria transmissible between humans and animals) and is naturally present in the intestines of animals, especially ruminant animals such as cattle and sheep. It is not always possible to determine the reasons for or causes of contamination of food products with zoonotic pathogens that can cause food poisoning outbreaks. Ready-to-eat salad vegetables can be contaminated with pathogens at the pre-harvest level (via flooding, rainwater run-off or irrigation water containing animal faeces) or post-harvest during washing and packaging.

As of 24 June, newly reported confirmed cases have now markedly declined (Figure 1) and the surveillance indicators for probable cases have now reduced back down to expected levels for this time of year. The majority of the recently reported probable cases are not part of the O145 t5:206 outbreak, but rather are small numbers of sporadic cases. A small number of cases may be reported in the short term due to the time lag between:

• when people become ill
• when they visit their GP or hospital
• when a sample is taken for testing
• when this sample is referred to the national reference laboratories for further typing
• the availability of WGS results

Based on available data, the ongoing public health risk is minimal.

Public health agencies are continuing to monitor surveillance and WGS data for confirmed cases and follow up cases to identify any common links. The FSA and FSS continue to work with the relevant local authorities, salad growers, sandwich suppliers and manufacturers to identify the root cause of the outbreak so that actions can be taken to prevent a re-occurrence.

Figure 1: Confirmed cases by week sample collection and country of residence (n = 267)

Note:

• sample date is unavailable for 8 cases
• the 4 confirmed cases in NI probably contracted their infection while in England
• primary cases are those individuals with illness who are assessed to have contracted their infection from exposure to a common source or food, while secondary cases have contracted infection through person-to-person transmission through contact with a primary case or the environment that has been contaminated by a primary case — secondary cases were excluded from epidemiological studies

Table 1: Clinical information for confirmed cases including reported symptoms (n=249), UK

Clinical information	Number of cases	%
Diarrhoea	242	97%
Blood in stool	201	81%
Nausea	141	57%
Vomiting	97	39%
Abdominal pain	228	92%
Fever	75	30%
Other symptoms	41	16%
Attended A&E	101	41%
Admitted to hospital	122	49%
Haemolytic uraemic syndrome	7	3%

References

1. UKHSA. Emergency department: weekly bulletins for 2024: week 20

2. UKHSA. Shiga toxin-producing Escherichia coli: questionnaire

3. FSA. Greencore Group recalls sandwiches, wraps and salads because of possible contamination with E. coli

4. FSA. Samworth Brothers Manton Wood recalls sandwiches and wraps because of possible contamination with E. coli

5. FSA. THIS! recalls Chicken and Bacon wrap because of possible contamination with E. coli

6. UKHSA. Health Protection Report volume 18 issue 2: news