Shiga toxin-producing Escherichia coli (STEC) data: 2021
Updated 26 April 2024
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The main points of the 2021 annual report are:
Overall, 1,151 confirmed cases of STEC were reported in England during 2021, a decrease of 2% compared to 2020.
A total of 365 confirmed cases of Shiga toxin-producing Escherichia coli (STEC) O157 were reported in England in 2021, no change compared to 2020. A total of 1,234 confirmed cases of Shiga toxin-producing Escherichia coli (STEC) non-O157 were reported in England in 2021, an increase of 46% compared to 2020.
The lowest annual incidence of STEC O157 was in the East Midlands region (0.41 per 100,000 population) and the highest in the North East region (1.02 per 100,000 population). The lowest incidence of STEC non-O157 was in the Yorkshire and Humber region (0.62 per 100,000 population) and the highest in the London region (4.21 per 100,000 population).
Children aged 1 to 4 years had the highest annual incidence of infection due to STEC O157 (2.3 per 100,000 population, 95% confidence interval (CI) 1.78 to 3.02). Children aged under 1 years had the highest incidence of infection due to STEC non O157 (7.08 per 100,000 population, 95% confidence interval (CI) 7.01 to 7.21).
Around a third (36%, 127 out of 365) of confirmed STEC O157 cases in England were hospitalised and 2% (6 out of 365) were reported to have developed haemolytic uraemic syndrome (HUS). Where information is available, two-thirds (65%, n=274 out of 431) of confirmed STEC non-O157 cases in England were hospitalised; and 1% (14 out of 1,234) of all STEC non-O157 were reported to have developed HUS).
A further 448 specimens in England were positive for stx genes on PCR at the Gastrointestinal Bacteria Reference Unit (GBRU), indicating STEC, but an organism was not cultured therefore a STEC serogroup could not be ascertained.
The most commonly isolated non-O157 STEC serogroup was STEC O26 (n=145 out of 786, 18%).
Four outbreaks of STEC in England involving 52 cases, ranging from 10 to 19 cases, were investigated in 2021: 2 involved STEC O157 and suspected vehicles were identified and 2 involved STEC O26 outbreaks and no vehicles were identified.
Since 2018, the number of STEC O157 notifications has declined and the number of STEC non-O157 has increased two-fold (218%). Overall, there is an increase in STEC notifications, and the burden placed on public health and clinical services is also increasing, especially given the two-fold increase in hospitalisation reported in 2021 for non-O157 cases.
Background
Shiga toxin-producing Escherichia coli (STEC), also known as Vero cytotoxin-producing Escherichia coli (VTEC), are bacteria that can cause gastroenteritis. Symptoms vary from mild to bloody diarrhoea and, in severe cases, can cause haemolytic uraemic syndrome (HUS), a serious and life-threatening condition predominantly affecting the kidneys. A small proportion of patients, mainly children, develop HUS (1).
The main reservoir for STEC is cattle although it is also carried by other ruminants such as sheep, goats and deer. Transmission can occur through direct or indirect contact with animals or their environments, consumption of contaminated food or water, and person-to-person spread. STEC infections can present as sporadic cases or as outbreaks. Large national and multinational outbreaks have been associated with foodborne transmission (2 to 4).
The most common serogroup of STEC-causing illness in England is O157 (5), followed by O26 (6). Other serogroups (categorised collectively as non-O157), including O26, can also cause illness and have been implicated in outbreaks in England and elsewhere.
Frontline laboratories in England use culture methods to detect STEC O157 by its inability to ferment sorbitol on selective media (Cefixime-Tellurite Sorbitol MacConkey (CT-SMAC) agar). However, most non-O157 STEC ferment sorbitol and differentiation of non-O157 STEC from non-pathogenic E. coli in frontline laboratories is challenging. Therefore, detection of non-O157 STEC mainly relies on polymerase chain reaction (PCR) and selective agar such as CHROMagar™ STEC (CHROMagar). The implementation and roll out of gastrointestinal (GI) PCR assays at frontline hospital laboratories began in December 2013, and by December 2019 around 20% of frontline laboratories had adopted this approach. As a consequence, there has been a substantial increase in the detection of non-O157 STEC cases. However, PCR is not universally used for detection of non-O157 STEC, and therefore the true incidence in England remains unknown.
Amongst non-O157 STEC serogroups, variation exists in their associations with severe disease. This is likely explained by differences in the virulence factors produced by different strains. STEC can produce 2 Shiga toxins (Stx), Stx1 (subtypes 1a, 1c and 1d) and/or Stx2 (of which there are 7 subtypes stx2a to stx2g). Stx2, specifically subtype stx2a, is more likely to cause HUS in cases infected with STEC (1, 7). The increasing numbers of non-O157 STEC have led to the need to prioritise the public health actions due to insufficient resources to follow up all cases. Risk assessment, based on clinical symptoms, the risk group of the patient and the potential pathogenicity of the strain of STEC infecting the patient, is challenging. In response, the public health operational guidance for STEC for the public health management of O157 and non-O157 STEC cases (including an algorithm to assist in follow up decision making) were updated and published in January 2023.
National enhanced surveillance of STEC in England has been ongoing since 2009. This report summarises the epidemiological data on confirmed cases of STEC O157 and non-O157 STEC cases in England in 2021 and compares it to previous years.
COVID-19 pandemic
During 2020 and 2021 it is likely that the emergence of SARS-CoV-2 (COVID-19) and subsequent non-pharmaceutical interventions (NPIs) implemented to control COVID-19 transmission affected notifications of STEC infections to national surveillance in several ways. These include, but are not limited to, changes which may have impacted ascertainment (for example, healthcare-seeking behaviour, access to health care, availability, or capacity of testing and so on) as well as changes which likely impacted incidence (for example, closures of educational settings, limited foreign travel, closure of hospitality and attractions such as petting farms) which will have also varied over time. Therefore, trends presented in this report should be interpreted with caution.
Additional in-depth analyses examining the impact of the COVID-19 pandemic are underway which will be published separately.
Methods
The National Enhanced Surveillance System for STEC (NESSS) infection in England began in January 2009 in order to supplement our understanding of the epidemiology of STEC infection. The system collects a standard data set of clinical, epidemiological and microbiological data for all STEC cases, in order to improve outbreak recognition and facilitate public health investigations. The data is collected from enhanced surveillance questionnaires (ESQ) and reconciled with laboratory reports associated with cases.
STEC is notifiable under the Public Health (Control of Diseases) Act 1984 and the Health Protection (Notification) Regulations 2010. In England, local diagnostic laboratories report presumptive cases of STEC to UKHSA health protection teams (HPTs) and then refer samples to the Gastrointestinal Bacteria Reference Unit (GBRU) for confirmation and further testing. Depending on the virulence profile of the STEC strain and the outlined healthcare management process under the operational guidance algorithm, each HPT arranges for an ESQ to be completed to obtain a detailed history of exposures 7 days prior to onset of illness. The ESQ collects:
- demographic details
- risk status
- clinical conditions
- exposures including: travel, food and water consumption, environmental exposures and outbreak status
Completed ESQs are submitted to the National Gastrointestinal Infections team at UKHSA to be included in NESSS.
For cases resident in England data included in this report was validated and extracted from NESSS and cases meeting the case definitions (see tables 1 and 2) were included in analyses. Given the effect that NPIs had on notifications of STEC infections to national surveillance, data from 2020 has been excluded in the calculations of the data presented below – though 2020 data is included in the trend figures.
Data from the 2021 Office for National Statistics (ONS) mid-year population estimates was used to provide denominators for the calculation of incidence rates. All dates for the figures are based on the receipt date of a sample specimen at the GBRU.
In March 2020 the serological testing method for the diagnostic detection of STEC was discontinued, and so there were no probable cases with serological evidence of infection identified in 2021. With this, in 2021, there is only one microbiological case definition: a confirmed case is positive STEC culture confirmed by GBRU. Epidemiological case definitions are described in Table 1 below.
Table 1. Epidemiological case definitions
| Classification | Definition |
|---|---|
| Primary | A symptomatic case with no history of close contact with a confirmed case in the 7 days prior to onset of illness |
| Co-primary | A symptomatic case with a date of onset within 3 days of another case where the exposure is believed to be through exposure to the same vehicle and not through social contact |
| Secondary | Case with a date of onset more than 4 days after the primary case or where transmission is believed to be through exposure to a primary case |
| Unsure | It is not possible to determine whether the case is primary or secondary with the information available. This may be because the patient was lost to follow-up, is asymptomatic or in an outbreak where it is not possible to identify the primary cases. |
| Travel-associated | Case who has reported any travel outside of the UK in the 7 days prior to their date of onset of illness |
| Asymptomatic | A person from whom STEC was identified through contact screening procedures but who is asymptomatic |
Data
Cases of STEC in England in 2021
In 2021, 1,151 confirmed cases of STEC were reported in England, a decrease of 2% compared to 2020. These comprised 365 culture-confirmed cases of STEC serogroup O157 and 786 cases where a serogroup other than O157 was isolated (non-O157). For a further 448 cases, samples were confirmed as STEC by testing positive by PCR for stx genes, but STEC was not cultured.
Fourteen confirmed cases were infected with multiple serogroups: O157 and O111, O117 and O107, O26 and O55, O157 and O26, O38 and O28, O91 and O146, O163 and O76, O21 and O112ab, O26 and O145, O157 and O111, O26 and O145, and O145 and O146.
The crude incidence rate of confirmed STEC O157 in England was 0.70 per 100,000 cases (95% CI 0.63 to 0.77), continuing the downward trend observed since 2015 (Figure 1). The rate in 2020 and 2021 were the lowest rates reported annually since 1996, when testing began in England for STEC O157 on all faecal specimens from patients with suspected gastrointestinal infection (8). However, it is likely that the COVID-19 pandemic contributed to the reduced rate 2021 so this trend should be interpreted with caution.
Figure 1. Incidence of Shiga toxin-producing Escherichia coli (STEC) O157 culture-confirmed cases by year, England, 2015 to 2021
Figure 2. Incidence of STEC O157 in England by region, 2021 only
The highest incidence of STEC O157 was in the North East (1.02 cases per 100,000 population, 95% CI 0.67 to 1.48) and the lowest was in the East Midlands (0.41 cases per 100,000 population, 95% CI 0.25 to 0.63) (Figure 2).
Age, gender and seasonality of STEC O157 cases
Of 365 confirmed STEC O157 cases in England, 189 (48%) were female. In 2021, children aged 1 to 4 years had the highest incidence of infection (2.34 per 100,000 population, CI 1.78 to 3.02).
Figure 3. Age-specific incidence rate of STEC O157 cases in England, 2021 only
As in previous years, STEC O157 infections in England in 2021 displayed a distinct seasonality with the peak of infection in the summer months, July or August (Figure 4).
Figure 4. Seasonal trend of laboratory-confirmed cases of STEC O157 in England, 2015 to 2021
Severity of illness
Of the 365 confirmed STEC O157 cases in England in 2021, ESQs were received for 357 (98%). Of those, symptoms were reported for 343 (96%) cases, the majority of which reported diarrhoea (92%, n=328) as well as bloody stools (62%, n=220). Other symptoms included abdominal pain (284, 80%), nausea (160, 45%), vomiting (176, 49%), and fever (166, 46%). Hospitalisation was reported by 36% (n=127) of cases. Duration of hospitalisation ranged from 1 to 7 days with a median stay of 2 days.
During 2021, HUS occurred in 6 confirmed cases (2%), half of which were under 5 years of age (n=3, 50%), however the median age was older at 9 years (range 1 to 25 years old). The incidence of STEC O157 HUS in children under 5 in 2021 was 0.09 per 100,000 cases per population (CI 0.02 to 0.29).
Transmission routes
In England, of the 357 confirmed cases for which ESQs were received during 2021, there were 328 primary or co-primary cases, 13 secondary cases and 16 asymptomatic contacts. A notable drop in travel-related cases was observed with only 15 cases (4%) reporting travel abroad during their incubation period (7 days prior to onset), compared to 28% and 31% in 2018 and 2019, respectively, and lower than during 2020 (8%). Of those 15, only 5 spent their entire incubation period abroad.
Frequently reported subtypes of STEC O157
As with previous years, phage type (PT) 8 and PT21/28 were the most commonly reported STEC O157 phage types in 2021 (Figure 5), with a combined proportion of 55%. These were followed by PT32 (9%), PT4 (8%), PT14 (7%) and PT54 (7%). The peak in PT34 in 2016 was due to a large outbreak linked to mixed-salad leaves, where 165 cases were reported between 31 May and 29 July 2016 (4).
Figure 5. Number of STEC O157 cases by the 10 of the most common phage types, 2015 to 2021
Among 365 isolates from confirmed STEC O157 cases in England in 2021, most (n=238, 65%) had Stx2 only, 35% (n=127) had Stx1+2 and none (n=0) had Stx1 only. Of those isolates with Stx2 or Stx1+2, 54% (n=196) had stx2a, the subtype most likely to cause severe clinical presentation and HUS.
Non-O157 STEC cases in England
Historically, cases of non-O157 STEC have been under ascertained, with 89 cases of STEC non-O157 reported between 2009 and 2013, prior to PCR being implemented.
Following the increase in recent years in frontline laboratories using PCR, there has been a significant increase in the detection of non-O157 STEC in England. Due to limited information on testing coverage at the frontline laboratories it is not possible to calculate the true denominator, therefore an estimated incidence for non-O157 STEC is presented in this report.
During 2021 in England, 6,610 human faecal samples were received at GBRU for STEC testing and 1,234 were confirmed as non-O157 STEC cases, an increase of 46% compared to 2020. Of the 1,234 non-O157 cases, 786 culture positive cases of 83 different serogroups were confirmed. For 25 isolates, a serotype could not be identified as the genes encoding the somatic O antigen did not match any known sequence in the database.
In England, the 5 most common non-O157 serogroups isolated from the 786 non-O157 cases in 2021 were O26 (n=145, 18%) followed by O146 (n=129, 16%), O91 (n=97, 12%), O128ab (n=50, 6%) and O145 (n=39, 5%) (Table 2). All were also in the top 5 non-O157 serogroups reported in 2020 (see Escherichia coli O157: annual totals).
In 2021, ESQs were received for 35% (n=431 out of 1,234) of non-O157 STEC cases, of which 80% (n=346 out of 431) were for culture positive non-O157 STEC cases, and 20% of STEC cases confirmed only by PCR. Between 2016 and 2019, there was a decline of 24% in the number of ESQs received for STEC non-O157 cases. This was thought to likely be associated with increasing numbers necessitating prioritisation and the implementation of algorithms in the 2018 guidance. Since a low of 27% in 2019 the percentage of non-O157 cases for which an ESQ was received has increased to 35% in 2021. However, this increase should be interpreted with caution due to the impacts of the COVID-19 pandemic during this period, which likely resulted in fewer notifications and therefore a reduced burden of cases requiring follow up.
Figure 6. The number of confirmed STEC non-O157 cases and enhanced surveillance questionnaires received in England, 2021
Of 1,234 confirmed STEC non-O157 cases in England, 555 (49%) were female. In 2021, children aged under 1 years had the highest incidence of infection (5.87 per 100,000 population, CI 5.80 to 6.00). (Figure 7).
Figure 7. Age-sex distribution of non-O157 STEC cases in England, 2021
STEC non-O157 infections in England in 2021 displayed a distinct seasonality with the peak of infection in the summer and autumn months, July to October (Figure 8). This is later than the typically observed O157 seasonal trend which usually peaks in during the summer months.
Figure 8. Seasonal trend of laboratory-confirmed cases of non-O157 STEC in England, 2018 to 2021
Of the 431 ESQs received for confirmed cases, 420 (97%) were symptomatic. Among the symptomatic cases, 90% (n=379) reported diarrhoea, including bloody stools in 46% (n=194) cases. This was accompanied by abdominal pain (n=282, 67%), 42 vomiting (n=242, 58%), fever (n=214, 51%) and nausea (n=176, 35. In total 274 cases (65%) were hospitalised.
HUS occurred in 1% (14 out of 1,234) of confirmed STEC non-O157 cases. From these 14 cases, the most frequently isolated serogroups where O26 (64%, n=9) and O145 (14%, n=2). A total of 6% of confirmed STEC O26 cases developed HUS as compared to 2% in confirmed STEC O157 cases. HUS cases ranged from 7 months to 29 years of age and 57% (n=8) were between one and 4 years of age. There were 3 deaths reported among cases of non-O157 STEC.
Among all 1,234 isolates from confirmed cases of non-O157 STEC in England in 2021, 36% (n=448) possessed the Stx1 toxin alone, 36% (n=449) possessed Stx1 and Stx2 and 27% (n=337) possessed Stx2 toxin alone. The most common Stx subtypes detected through sequencing of the 786 isolates were stx1a only in 19% (n=152), stx1c only in 18% (n=141), stx1c stx2b in 17% (n=135), stx2a only in 1512 (n=97), stx2b only in 9% (n=68), stx1a stx2b in 9% (n=67), stx1a stx2a in 6% (n=48), and the following were around 1%: stx2c only (n=9), stx2f only (n=9), stx2d only (n=9), stx1c stx2d stx2b (n=7), stx2a stx2e (n=5), stx2e only (n=5), stx1a stx2c (n=4), and stx2g only (n=4) (Table 2). An additional 10 Stx subtype combinations were detected in the samples of 19 cases. Of the 1,234 isolates, 399 (32%) had the eae gene.
Table 2. Prevalence of Stx subtype combination reported amongst the top 9 non-O157 STEC serogroups in England, 2021
| Serogroup | Stx subtype | n | % |
|---|---|---|---|
| O26 (n=145) | stx2a | 53 | 37 |
| stx1a | 48 | 33 | |
| stx1a, stx2a | 43 | 30 | |
| O146 (n=129) | stx1c | 50 | 39 |
| stx1c, stx2b | 44 | 34 | |
| stx2b | 26 | 20 | |
| stx1c, stx2d, stx2b | 5 | 4 | |
| stx1c, stx2d | 1 | 1 | |
| stx2a, stx2b | 1 | 1 | |
| stx2c | 1 | 1 | |
| stx2d | 1 | 1 | |
| O91 (n=97) | stx1a, stx2b | 67 | 69 |
| stx2b | 12 | 12 | |
| stx1a | 11 | 11 | |
| stx1a, stx2b, stx2d | 3 | 3 | |
| stx1a, stx1c, stx2b | 1 | 1 | |
| stx1a, stx2a | 1 | 1 | |
| stx1c, stx2b | 1 | 1 | |
| stx2b, stx2d | 1 | 1 | |
| O128AB (n=50) | stx1c, stx2b | 32 | 67 |
| stx2b | 11 | 23 | |
| stx1c | 5 | 10 | |
| stx2f | 1 | 2 | |
| O145 (n=39) | stx2a | 34 | 87 |
| stx1a | 2 | 5 | |
| stx2f | 2 | 5 | |
| stx2c | 1 | 3 | |
| O103 (n=25) | stx1a | 25 | 100 |
| O76 (n=25) | stx1c | 25 | 100 |
| O113 (n=19) | stx1c, stx2b | 17 | 89 |
| stx2a, stx2d | 1 | 5 | |
| stx2b | 1 | 5 | |
| O117 (n=19) | stx1a | 17 | 89 |
| stx1c | 2 | 11 |
Increase of non-O157 STEC
In England, a shift in the burden of disease from notifications of STEC O157 cases to non-O157 STECs has been observed in recent years, with the trend continuing in 2021 (Figure 8). While the notifications of STEC O157 have declined the number of STEC non-O157 has increased two-fold (218%) since 2018, resulting in an overall increase in STEC notifications and the burden placed on public health and clinical services, especially as there was a two-fold increase in hospitalisation reported in 2021 for non-O157 cases.
Figure 9. Proportion of laboratory-confirmed STEC cases in England by serogroup, 2018 to 2021
STEC O26 cases in England
Questionnaires were received for 92% (n=132) of all 145 confirmed STEC O26 cases. Of the 128 (96%) cases that were symptomatic, 95% (n=122) cases had diarrhoea, 74% (n=95) reported abdominal pain, 74% (n=38) vomiting, 39% (n=50) reported bloody stools and 41% (n=52) nausea. Overall, in 2021 a lower proportion of STEC O26 cases (33%, n=42) were hospitalised compared to STEC O157 cases (36%, n=127), while a higher proportion of HUS was reported O26 cases with 6% (n=9) compared to 2% in O157 cases (n=6).
Of 145 STEC O26 cases in England, 88 (61%) were female. Children aged 1 to 4 years of age comprised 24% (n=35) of cases.
Figure 10. Age-sex distribution of STEC O26 cases in England, 2021
STEC O145 cases in England
Questionnaires were received for 95% (n=37) of all 39 confirmed STEC O145 cases. Of the 36 (97%) cases that were symptomatic, 97% (n=35) cases had diarrhoea, 81% (n=29) reported abdominal pain, 39% (n=X14 vomiting, 58% (n=21) bloody stools and 58% (n=21) nausea. Overall, in 2021 a higher proportion of STEC O145 cases (47%, n=17) were hospitalised compared to STEC O157 cases (36%, n=127), while a higher proportion of HUS was reported in O145 cases with 5% (n=2) compared to 2% in O157 cases (n=6).
Of 39 STEC O145 cases in England, 23 (59%) were female. Children aged one to 4 years of age comprised 13% (n=5) of cases.
Figure 11. Age-sex distribution of STEC O145 cases in England, 2021
Outbreaks
STEC outbreaks investigated in England during 2021
Four STEC outbreaks affecting 52 people in England were investigated in 2021. (Typically, an exceedance is investigated when 5 or more cases fall within the same 5 SNP single linkage cluster for clinically severe serotypes, or an outbreak may be declared if a number of cases are linked to a setting. However, an investigation may be undertaken after considering a number of factors, including the number of persons affected, temporal and geographic distribution, clinical severity, and the microbiological characteristics of the STEC strain).
Two outbreaks were caused by O157 and thus accounted for 5% of confirmed STEC O157 cases. The other 2 outbreaks were caused by O26 and accounted for 3% of non-O157 cases and 22% of O26 cases (Table 4).
There was one death and no HUS cases associated with outbreaks in 2021.
Despite epidemiological investigations, it was not possible to determine the vehicle and/or source of infection for all outbreaks, although for 2 outbreaks suspected vehicles were identified (Table 4).
Table 4. STEC O157 and non-O157 outbreaks in England, 2021
| Agent | Total number affected | Total laboratory confirmed | Hospitalised | Source | Setting |
|---|---|---|---|---|---|
| STEC O157stx2c | 10 | 10 | 2 | Foodborne (suspected, composite food product – pasta pot) | National |
| STEC O157stx1a stx2c | 10 [note 1] | 10 | 5 | Foodborne (suspected, food of non-animal origin – watermelon) | National |
| STEC O26 stx1a stx2a | 13 [note 2] | 13 | 4 | Foodborne, (vehicle not identified) | National |
| STEC O26 stx1a | 19 [note 3] | 15 | 10 | Foodborne, (vehicle not identified) | National |
Notes to Table 4
[note 1] 10 confirmed cases in England, 1 in Scotland and 6 in Wales.
[note 2] 13 confirmed cases in England, 2 in Northern Ireland, 3 in Scotland and 2 in Wales.
[note 3] 11 confirmed cases in England, 1 in Northern Ireland, 1 in Scotland and 1 in Wales.
In May, UKHSA investigated an outbreak of STEC O157 stx2c eae positive. There were 10 confirmed cases of which, all 10 completed an ESQ: 30% of cases reported bloody diarrhoea (n=3), 20% were admitted to hospital (n=2) but none were reported to have developed HUS and one case (with report of immunocompromised) associated with this cluster died. Descriptive epidemiological investigations identified a composite product, a multi-ingredient pasta pot, as the likely vehicle of infection.
In July 2021, UKHSA along with Public Health Scotland (PHS) and Public Health Wales (PHW) investigated an outbreak of STEC O157 stx1a stx2c eae positive. There were 17 confirmed cases, of which 10 were resident in England, 6 in Wales and 1 in Scotland. The median age was 21 years. Clinical information was available for all 17 cases and full enhanced surveillance questionnaires were available for 16 cases. Overall, 76% of cases reported blood (n=13) in stools and 6 were hospitalised (35%) but none developed HUS. No deaths were reported. Epidemiological investigations identified watermelons and the likely vehicle of infection (9).
UKHSA, The Public Health Agency Northern Ireland (PHA NI), PHS and PHW, as well as colleagues from Health Service Executive (HSE) Ireland, investigated an outbreak of STEC O26 stx1a stx2a eae positive in August 2021. There were 25 confirmed cases of which 17 cases resided in England, 2 in Northern Ireland, 3 in Scotland, 2 in Wales and 1 in Ireland. Questionnaires were received for 21 (84%) cases, of which 18 reported on clinical presentation. Sixteen cases reported bloody diarrhoea (89%) and 10 reported hospitalisation for their symptoms (56%), however there were no reports of HUS or deaths. Although epidemiological investigations were undertaken no vehicle of infection was identified.
In November 2021, UKHSA, PHA NI, PHS and PHW investigated a second O26 outbreak – STEC O26 stx1a eae positive. There were 19 cases associated with the outbreak, 15 of which were laboratory confirmed at the 5-SNP level. Of the confirmed cases, 11 were resident in England, one in Northern Ireland, one in Scotland and one in Wales. The median age was 41 years. ESQs were completed for 12 of the 15 confirmed cases (80%). Overall, 58% of cases reported blood (n=7) in stools and 3 were hospitalised (25%). There were no reports of HUS or deaths in association with this outbreak. Although epidemiological investigations were undertaken no vehicle of infection was identified.
These outbreaks continued to highlight the importance of stx1a in causing severe disease and is supported by increasing evidence in the literature that the presence of stx1a is also a risk factor for severe symptoms (5).
Conclusions
While the number of STEC O157-confirmed cases in England continued to drop in 2021 the number of STEC non-O157-confirmed cases continued to rise, resulting in an overall increase of STEC case notifications to national surveillance. There is a reduction in O157 outbreaks investigation compared to previous years and a concurrent increase in non-O157 outbreaks. While the reason for this decline in O157 in recent years is unclear it is likely that the impact of the COVID-19 pandemic contributed the 32% drop in cases observed in 2021 compared to 2019. The reasons for this decreased incidence are considered multifactorial but the concurrent drop in reported foreign travel by STEC O157 cases from approximately 30% in previous years, excluding 2020, to only 4% in 2021 indicates limited travel abroad is likely a key factor.
This report presents changes in epidemiology of STEC O157 infections in 2021 – the main peak in reporting in the summer months aligned with previous years with a second later peak as seen in 2020. The age-sex distribution differed from previous years; the highest incidence was observed in the 1 to 4 years, in line with what has been historically observed, while an overall higher incidence in males was reported in contrast with data reported in 2019 and before. While the percentage of STEC O157 cases who developed HUS remained similar at 2%, the percentage of cases hospitalised for their illness increased from 29% in 2019 to 36% in 2021, suggesting better ascertainment of more clinically severe cases during 2021 and which is supported by studies on the impact of COVID-19 on GI pathogen surveillance (9).
The detection of non-O157 STEC infections increased compared to 2020, continuing the trend observed in pre-COVID-19 years. The most common non-O157 serogroup detected in 2021 was STEC O26 and cases infected with this serogroup had a higher proportion (6%) of HUS than O157 cases (2%).
Overall trends during 2021 should be interpreted with caution due to the wide-ranging impacts of the COVID-19 pandemic. It is likely that the interventions implemented to control COVID-19 led to a reduction in STEC transmission, of both O157 and non-O157. However, given the decreasing trend in pre-COVID-19 years, the reduction in STEC cases in 2021 cannot wholly be attributed to these control measures alone.
UKHSA STEC publications in 2021
Mulchandani R, Brehmer C, Butt S, Vishram B, Harrison M, Marchant E, Ferris S, Jorgensen F, Smith R, Godbole G, Jenkins C, Dallman TJ, Verlander NQ, Phin N, Todkill D, Gharbia S, Hawker J. ‘Outbreak of Shiga toxin-producing Escherichia coli O157 linked with consumption of a fast-food product containing imported cucumbers, United Kingdom, August 2020’ International Journal of Infectious Disease 2021: volume S1,201 to 9,712, issue 2, page 00312-X. doi: 10.1016/j.ijid.2021.04.001. Online ahead of print
Sawyer C, Vishram B, Jenkins C, Jorgenson F, Byrne L, Mikhail AFW, Dallman TJ, Carroll K, Ahyow L, Vahora Q, Godbole G, Balasegaram S. ‘Epidemiological investigation of recurrent outbreaks of haemolytic uraemic syndrome caused by Shiga toxin-producing Escherichia coli serotype O55:H7 in England, 2014 to 2018’ Epidemiology and Infection 19 April 2021: pages 1 to 29. doi: 10.1017/S0950268821000844. Online ahead of print
Dallman TJ, Greig DR, Gharbia SE, Jenkins C. ‘Phylogenetic structure of Shiga toxin-producing Escherichia coli O157:H7 from sub-lineage to SNPs’ Microbial Genomics 2021: volume 7, issue 3. doi: 10.1099/mgen.0.000544. Epub 15 March 2021.PMID: 33720818
Greig DR, Jenkins C, Gharbia SE, Dallman TJ. ‘Analysis of a small outbreak of Shiga toxin-producing Escherichia coli O157:H7 using long-read sequencing’ Microbial Genomics 2021: volume 7, issue 3. doi: 10.1099/mgen.0.000545. Epub 8 March 2021
Carroll KJ, Jenkins C, Harvey-Vince L, Mohan K, Balasegaram S. ‘Shiga toxin-producing Escherichia coli diagnosed by Stx PCR: assessing the public health risk of non-O157 strains’ European Journal of Public Health 2021: ckaa232. doi: 10.1093/eurpub/ckaa232. Online ahead of print
Dallman TJ, Greig DR, Gharbia SE, Jenkins C. ‘Whole genome sequencing for the national surveillance of Shiga toxin-producing Escherichia coli (STEC) serotype O26:H11 in England’ Microbial Genomics 2021: doi: 10.1099/mgen.0.000551
Butt S, Smith-Palmer A, Shand A, McDonald E, Allison L, Maund J, Fernandes A, Vishram B, Greig DR, Jenkins C, Elson R, Outbreak Control Team. ‘Evidence of ongoing transmission of Shiga toxin-producing Escherichia coli O157:H7 following a foodborne outbreak’ Epidemiological Infections 2021: 149:e147. doi: 10.1017/S0950268821001278
Vishram B, Jenkins C, Greig DR, Godbole G, Carroll K, Balasegaram S, Byrne L. ‘The emerging importance of Shiga toxin-producing Escherichia coli other than serogroup O157 in England’ Journal of Medical Microbiology 2021: volume 70, issue 7. doi: 10.1099/jmm.0.001375
Butt S, Allison L, Vishram B, Greig DR, Aird H, McDonald E, Drennan G, Jenkins C, Byrne L, Licence K, Smith-Palmer A and others on behalf of the Incident Management Team. ‘Epidemiological investigations identified an outbreak of Shiga toxin-producing Escherichia coli serotype O26:H11 associated with pre-packed sandwiches’ Epidemiological Infection 2021: 149:e178 doi: 10.1017/S0950268821001576
Kaindama L, Jenkins C, Aird H, Jorgensen F, Stoker K, Byrne L. ‘A cluster of Shiga toxin-producing Escherichia coli O157:H7 highlights raw pet food as an emerging potential source of infection in humans’ Epidemiological Infections 6 May 2021: 149:e124. doi: 10.1017/S0950268821001072
Chan YW, Hoban A, Moore H, Greig DR, Painset A, Jorgensen F, Chattaway MA, Jenkins C, Balasegaram S, McCormick J, Larkin L, Incident Management Teams. ‘Two outbreaks of foodborne gastrointestinal infection linked to consumption of imported melons, United Kingdom, March to August 2021’ Journal of Food Protection January 2023: volume 86, issue 1: 100027. doi: 10.1016/j.jfp.2022.100027. Epub 13 December 2022. PMID: 36916586
Data caveats
This report was produced using laboratory data for England only, therefore the number of Shiga toxin-producing Escherichia coli laboratory reports published in previous reports which include data from other UK countries will be higher than those included in this report.
Acknowledgements
We are grateful to:
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the microbiologists, local authorities and local health protection and environmental health specialists who contribute data and reports to the National Enhanced Surveillance System for STEC (NESSS)
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the epidemiologists and information officers who have worked on NESSS
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staff in the Gastrointestinal Bacterial Reference Unit (GBRU) for providing the Reference Laboratory Services and laboratory surveillance functions and expertise
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UKHSA Regional and Collaborating Public Health Laboratories and Food Water and Environmental Microbiology Services for providing a surveillance function for GI pathogens and testing of food and environmental samples during outbreak investigations
References
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Contact
Prepared by: Gastrointestinal Infections and Food Safety (One Health) Division, UKHSA.
For queries relating to this document, please contact VTEC@ukhsa.gov.uk