Shiga toxin-producing E. coli (STEC), also known as Verotoxigenic Escherichia coli (VTEC), are a group of bacteria which can cause gastrointestinal illness in humans. Previously, the organisms have been referred to as VTEC, however recently there has been a widespread move to standardise the nomenclature across Europe to STEC. While the two terms are interchangeable, for the purposes of this report, the term STEC will be used.
STEC are widespread in the environment and can colonise the gastrointestinal tract of farmed, wild, and domesticated animals and birds and can be shed in their faeces. Ruminants (cattle, sheep and goats) are considered to be the main reservoir of infection although STEC causes no clinical signs of infection in the animal. Transmission to humans can occur as a result of direct contact with STEC-contaminated faecal material, as a result of handling or petting animals or by exposure to faecally contaminated mud or vegetation often during recreational activities. Exposure can also occur from consumption of water or food which is contaminated.
Outcomes of STEC infection range from asymptomatic infection, to mild non-bloody diarrhoea, through to bloody diarrhoea, abdominal pain and occasionally fever. Further serious outcomes of infection can include haemolytic uraemic syndrome (HUS) which is a major cause of acute renal failure in children in Scotland.1
Although large foodborne outbreaks have occurred in Scotland,2,3 sporadic infection predominates. The potential for E. coli O157 to cause both secondary spread4 and large outbreaks is exacerbated by its low infectious dose2,3 and asymptomatic infection can also occur.5
Diagnostic laboratories investigate all diarrhoeal faeces for the presence of E. coli O157 and refer isolates to the Scottish E. coli O157/VTEC Reference Laboratory (SERL) for confirmation and further typing. STEC of serogroup O157 are the only STEC for which routine standard tests are performed in diagnostic laboratories. Under national guidelines, faeces from high-risk patients testing negative at the local laboratory are sent to SERL where more sensitive methods are used for detection and isolation of STEC.6
Health Protection Scotland undertakes enhanced surveillance of STEC in close collaboration with the SERL and NHS board health protection teams. Data are also integrated with other surveillance systems, in particular ObSurv, the system for the surveillance of all general outbreaks of infectious intestinal disease.7
Reported rates of Escherichia coli O157 (E. coli O157) infection in Scotland rose substantially in the mid-1990s and remain consistently high compared to other countries within the UK and Europe.
The number of non-O157 STEC detected at the SERL has risen over recent years and these are now identified in over one quarter of all laboratory-confirmed infections. Non-O157 STEC can be associated with significant morbidity including HUS.
HPS defines a case as a single person-infection episode with local laboratory or SERL laboratory confirmation of infection by one or more of the following:
- culture positive (isolates of E. coli O157 or other serogroups cultured from faeces);
- faecal PCR positive for Shiga toxin genes, and/or an E. coli O157 specific gene, but not confirmed by culture;
- serum positive (antibodies to O157 or other serotypes detected in blood serum).
Both symptomatic and asymptomatic cases are included.
HPS surveillance systems collect information about general outbreaks i.e. those affecting members of more than one household, or residents of institutions.7 Other cases are therefore either apparently sporadic or occur amongst members of a single household. For ease of comprehension, cases in general outbreaks are referred to as outbreak cases, while cases or clusters restricted to single households, whether or not they are secondary or primary cases, are referred to as sporadic cases.
Imported infections are defined as those cases from whose onset date, incubation period, and travel and other exposure histories, local investigators judge infection more likely to have been acquired outside the UK than from any other identifiable source.
This report presents analyses of the main variables for STEC infections reported to HPS in 2016.
To allow comparison with previous years, only faecal culture positive cases were included in the following analyses. Cases identified by serodiagnosis or PCR without culture confirmation were excluded. Data for E. coli O157 and non-O157 STEC are presented separately.
All data for 2016 will be provisional until 31 December 2017.
E. coli O157
In 2016, a total of 185 reports of E. coli O157 were made to HPS. The methods of identification of these are described in Table 1.
There were 181 faecal culture positive cases of E. coli O157 notified to HPS in 2016. This represented a slight increase on the 170 cases reported in 2015, but still less than the five-year (2011-2015) average of 218 cases.
The rate of faecal culture positive cases per 100,000 population for the whole of Scotland in 2016 was 3.4 compared to 3.2 in 2015 and 4.9 in 2014. Incidence rates varied across Scotland, as they have done historically. The overall rates decreased in six of the 14 NHS boards compared with the previous year. The rates for NHS boards with small populations should be interpreted with caution as the numbers disproportionately affect the incidence rates.
Age and sex
The age distribution of the 181 faecal culture positive cases ranged from under one to over 80 years of age, while 49% of cases were male and 51% were female. The mean age was 32 years. Children under 16 years of age accounted for 33% of cases and 15% of cases were aged over 65 years. As seen in previous years, children under five years of age had the highest rate of infection, with an overall rate of 10.2 per 100,000 people, while the next highest overall rate was 8.2 per 100,000 observed in the five-to-nine years age group. The age stratified rates per 100,000 population for males and females are shown in Figure 3.
The number of cases per four-week period is shown in Figure 4. As in most years, cases tend to peak in the summer months. Most infections occurred in the second and third quarters of the year, with 73% of cases occurring in this time period.
Reference Laboratory data
Isolates of E. coli O157 identified by local diagnostic laboratories are routinely sent to SERL for further typing. This includes phenotypic characterisation – phage typing, antimicrobial sensitivity testing as well as genotypic typing – PCR and MLVA. This additional typing facilitates the identification of outbreaks as well as monitoring the trends and emergence of new strain types.
Phage type (PT) results for culture positive E. coli O157 isolates are reported to HPS by SERL. As seen in previous years, PT 21/28 and PT8 accounted for the majority of cases. In 2016, as seen in 2015, these phage types accounted for over 60% of cases (Table 3).
The increase in PT21/28 reports in 2016 was due mainly to an outbreak of this phage type associated with the consumption of unpasteurised cheese where 26 cases were reported, 21 of whom were resident in Scotland.8
Of the 181 E. coli O157 culture positive isolates identified in 2016, 22 (12.2%) had no identifiable Shiga toxin-producing genes. This was an increase on the 11 (6.5%) Shiga toxin gene negative cases reported in 2015.
STEC E. coli O157 is the only serogroup of Shiga toxin-producing E. coli (STEC) routinely detected by diagnostic laboratories in Scotland. Identification of non-O157 STEC therefore requires submission of faecal samples to SERL for further investigation. In 2016, 63 isolates of non-O157 STEC were cultured and reported by SERL to HPS. This compares to 78 non-O157 isolates reported in 2015 (Figure 5). In addition, as seen in 2015, there were 19 reports of non-O157 STEC which were not confirmed by culture but were Shiga toxin gene positive . One serum positive non-O157 STEC case was reported in 2016.
The SERL also isolated a strain of Escherichia albertii carrying the stx2f gene variant. This was the first isolation of E. albertii in the UK. While the clinical significance of E. albertii is uncertain, it is known to cause gastrointestinal disease.9
Although the number decreased slightly in 2016, the general trend in recent years has been an increase in non-O157 isolates. This is thought to be largely due to a change in the referral pattern from laboratories sending samples to SERL resulting in an increase in identification of non-O157 cases. In total, 25 different non-O157 serogroups were identified in 2016, of which 15 were reported on one occasion only. STEC O26 was the most common serogroup reported as was the case in previous years and accounted for 12 (19%) of the non-O157 faecal culture positive cases. While five isolates were O-Unidentifiable, the next most common serogroups were O128 and O63, both with five cases reported.
At the start of January 2016, a new standard enhanced surveillance questionnaire was adopted by health protection teams across Scotland. This collects more detailed information than previously on both environmental and food exposures and enables comparison across NHS board areas. In 2016, information on 163 cases (90%) of faecal culture positive E. coli O157 and 54 cases (86%) of faecal culture positive non-O157 STEC was provided to HPS following case interviews by the local health protection team.
Information on hospitalisation was available for 217 cases of STEC. Of these, 35% of all cases of STEC were admitted to hospital for at least one night during their illness. In particular, for O157 this figure was 40% and for non-O157 this figure was 20%.
Information on clinical presentation was available for 217 cases, with 71% of cases of E. coli O157 having bloody diarrhoea, a further 22% reporting diarrhoea with no blood while 6% of confirmed cases reported experiencing no symptoms. Two cases, whilst not reporting diarrhoea of any kind, did report abdominal pain (one of who also additionally reported vomiting and fever). The clinical presentations of STEC cases are summarised in Table 5.
Sporadic or outbreak cases
During 2016, five general outbreaks of STEC were reported to ObSurv. This is similar to the number of STEC outbreaks reported in previous years, with five in 2015 and seven in 2014. All five outbreaks in 2016 were identified as serogroup O157 – two being phage type 21/28, two phage type 8 and one phage type 54. For three of the outbreaks the suspected mode of transmission was considered to be mainly foodborne, one was due to drinking untreated water and for one outbreak the mode of transmission was not confirmed.
As has historically been the case in Scotland,2,3 the majority of cases in 2016 were apparently sporadic cases.
Of the 217 cases for which information was available, 15% of all STEC cases were considered to have acquired their infection outwith the UK. This figure was the same for both E. coli O157 and non-O157 cases.
Discussion and conclusions
The number of E. coli O157 cases reported in Scotland increased slightly in 2016 and the number of non-O157 cases reported decreased slightly. This was in keeping with the expected year-to-year variation.
Despite the variation in the number of cases seen annually, the consistently high rates of STEC infection reported in Scotland as compared to other UK countries reinforces the need for the continued and comprehensive application of the wide range of existing control measures embedded in food safety and other guidance in Scotland. In addition, it highlights the importance of a comprehensive multi-agency approach to tackling STEC in Scotland as set out in the VTEC Action Plan for Scotland.2 A multi-agency implementation group has been set up to deliver the recommendations within the Action Plan and is due to report in 2018.
In addition, a new web-page - https://www.nhsinform.scot/bugs-and-germs - has recently been launched, on the NHS Inform website, to provide the public with advice on how to avoid infections, including STEC, in Scotland’s outdoors.
HPS and SERL particularly wish to thank those patients and their families who have provided information; and also the following groups across Scotland: HP Teams in NHS boards; environmental health officers; consultant microbiologists and diagnostic laboratory staff; public analyst laboratory staff; and Scottish Agricultural College (now part of SRUC). We also thank Susan Brownlie and Genna Drennan at HPS and laboratory staff at SERL.
- Pollock KGJ, Locking ME, Cowden JM. Clinical surveillance of haemolytic uraemic syndrome surveillance in Scotland, 2010: emergence of highly virulent Escherichia coli O26. HPS Weekly Report. 2010;45:160-1. Available from: http://www.hps.scot.nhs.uk/ewr/redirect.aspx?id=47631. (accessed 9 August 2017).
- Scottish Government. VTEC/E. coli O157 Action Plan for Scotland 2013-2017. Edinburgh: Scottish Government; 2013. Available from: http://www.scotland.gov.uk/Publications/2013/11/8897/0. (accessed 9 August 2017).
- Scottish Executive Health Department/Food Standards Agency (Scotland). Report of the E. coli O157 Task Force. Edinburgh: The Stationery Office; 2001. Available from: http://webarchive.nationalarchives.gov.uk/20101210031745/http://www.food.gov.uk/news/newsarchive/2001/oct/ecolitask. (accessed 9 August 2017).
- Locking ME, Pollock KGJ, Allison LJ et al. Escherichia coli O157 infection and secondary spread, Scotland, 1999-2008. Emerg Inf Dis. 2011;17(3):524-7. Available from: http://wwwnc.cdc.gov/eid/article/17/3/10-0167_article. (accessed 9 August 2017).
- Health Protection Network. Guidance for the Public Health Management of Infection with Verotoxigenic Escherichia coli (VTEC). Health Protection Network Scottish Guidance 3 (2nd edition). Glasgow: Health Protection Scotland; 2013. Available from: http://www.hps.scot.nhs.uk/giz/guidelinedetail.aspx?id=39336. (accessed 9 August 2017).
- Scottish E. coli O157/VTEC Reference Laboratory (SERL). Annual Report April 2014 – March 2015. Available from: http://www.hps.scot.nhs.uk/reflab/SERL.aspx. (accessed 9 August 2017).
- Smith-Palmer A. General bacterial and protozoal outbreaks of infectious intestinal disease reported to HPS in 2015. HPS Weekly Report. 2016;50:269-270. Available from: http://www.hps.scot.nhs.uk/giz/wrdetail.aspx?id=70352. (accessed 9 August 2017).
- Incident Management Team. Outbreak of Escherichia coli O157 PT21/28. Summer 2016. Glasgow: Health Protection Scotland; 2017. Available from: http://www.hps.scot.nhs.uk/pubs/detail.aspx?id=3200. (accessed 9 August 2017).
- Ooko T, Seto K, Kawano K et al. Clinical significance of Escherichia albertii. Emerg Inf Dis. 2012;18(3):488-492. Available from: https://wwwnc.cdc.gov/eid/article/18/3/11-1401. (accessed 9 August 2017).