Cause

The most common serogroup of Vero cytotoxin-producing E. coli (VTEC) in the United Kingdom is E. coli O157.  The following statements apply to VTEC O157. So far there have been few isolates of other VTEC in England and Wales and their true incidence and risk factors for infection are unclear.  This may be due to the isolation methods used in the laboratory favouring the isolation of O157 over other VTECs. More evidence supporting other VTECs will come to light as laboratory practices change over time.

Reservoir

The principal reservoir for VTEC O157 in the UK is cattle. The organism has been isolated from the faeces of many livestock and wildlife species, however their role in sustaining the population structure of  VTEC O157 in the UK is unclear. The disease is a zoonosis.

Transmission

Primary infections are acquired by:

• Consumption of contaminated food:
  • Foods derived from infected animals:
    • mainly beef products especially undercooked dishes made from mince such as burgers, meat balls, meat loaf etc;
    • unpasteurised milk and soft cheeses etc made from unpasteurised milk;
  • Salad vegetables, herbs and fruit that have been contaminated through: being irrigated by contaminated water; the use of  inappropriate fertilizers; contact with faeces from local livestock or wildlife; 
  • Ready to eat foods such as cold cooked meats, salad vegetables and cut fruit that have been contaminated through cross-contamination from infected raw meat or those that were prepared by infected food handlers;
• Consumption of contaminated water from untreated sources or from supplies that have had treatment failures;
• Contact with infected animals or their faeces, particularly on farms, including open farms;
• Recreational exposure to water from rivers, streams, ponds and lakes that has been contaminated with agricultural run off or faeces from wildlife.

Secondary infections are acquired by person-to-person spread by direct contact (faecal-oral). This is particularly important in households, nurseries, primary schools and residential care institutions (see below ).

Other relevant factors

VTEC may give rise to a haemorrhagic colitis and about 2-7% of cases progress to the haemolytic uraemic syndrome, of which the case fatality rate is about 5% [1] Antibiotic treatment may be harmful [3].
Diagnosis is by isolation of the causative organism but evidence of infection can be obtained by examination of a serum or saliva (useful for younger children) sample in the absence of a faecal sample.  For HUS cases where the bacteria have cleared from the intestine, serological diagnosis may be the only option.

Control of human source

Statutorily notifiable if thought to be food poisoning. The informal reporting of haemolytic uraemic syndrome (HUS), particularly with a diarrhoeal prodrome, is to be encouraged.

Cases

Standard enteric precautions should be observed. Hospital admission if haemorrhagic complications occur. Isolation of bacteria may be possible only during the acute diarrhoeal phase although some patients still harbour the organism when asymptomatic.

Contacts

Contacts in risk groups A to D  should be screened microbiologically, initially to identify those who are excreting VTEC and subsequently for microbiological clearance (below). Authorities must satisfy themselves of the adequacy of hygiene and toilet facility arrangements. Hand washing by children must be supervised in nurseries and infant schools.

Risk categories

1. Any person of doubtful personal hygiene or with unsatisfactory toilet, hand-washing or hand drying facilities at home, work or school.
2. Children who attend pre-school groups or nursery.
3. People whose work involves preparing or serving unwrapped foods not subjected to further heating.
4. Clinical and social care staff who have direct contact with highly susceptible patients or persons in whom a gastrointestinal infection would have particularly serious consequences.

Exclusions

Guidelines for the control of VTEC infection suggest that patients are excluded for 48 hours after the first normal stool for cases not in risk groups. Cases and contacts in risk groups A to D until microbiological clearance is obtained.

Microbiological clearance

Risk groups A to D only - two negative faecal specimens taken at intervals of not less than 48 hours. The ease of spread means that it may be wise to ensure that all cases and contacts in high-risk groups in a given household or similar setting are no longer excreting before being allowed to return to work, school, etc. Such risks depend in part on the risk of transmission in the household and can ultimately only be assessed locally.

Epidemiology

The incidence of VTEC O157 infections is variable throughout the UK with the highest rate in Scotland The majority of cases of VTEC O157 infection in the UK are apparently sporadic or occur with households.

In England and Wales, strain typing has been performed by GIRU for 195 outbreaks recognised between 1992 and 2008, while EpiServices are officially responsible for surveillance of cases and outbreaks. Outbreaks of VTEC are usually small with an average of 8 cases, however, larger outbreaks can occur. The largest outbreak in England occurred in Cumbria in 1999, and was associated with pasteurised milk. It affected 114 people, 88 of whom were laboratory-confirmed. An outbreak in south Wales in 2005  comprised 118 microbiologically confirmed cases (of 157 suspected cases). Outbreaks have been documented since 1982, and a report was published on the 18 general outbreaks that occurred between 1992 and 1994 [2] with reports on many individual outbreaks published in addition, frequently by the local authority of the area in which the outbreak occurred.

In several countries outbreaks of bloody diarrhoea and HUS have been associated with VTEC belonging to serogroups other than O157. The vehicles of infection have included milk and salami. In England and Wales, sporadic cases and household incidents have been associated with non-O157 VTEC, but outbreaks have not been described. This is partly due to the lack of methods for the detection of all VTEC that are appropriate for use in diagnostic laboratories.  A limited diagnostic service is provided for VTEC of all serogroups by the LGP (see Laboratory ).

 

HPA work on VTEC

The HPA has given a high priority to work on VTEC and particularly E. coli O157 and has initiated a number of service developments. The HPA undertakes microbiological and epidemiological surveillance of VTEC O157 infections. In addition to providing full identification and typing for all VTEC the LGP also subtypes E. coli O157 using phage typing and DNA-based methods including pulsed field gel electrophoresis and variable number tandem repeat typing. There is also a serodiagnostic service for E. coli O157 based on serum antibodies which is particularly helpful in providing evidence of infection when faecal culture is negative. A simple non-invasive test for antibodies to E. coli O157 in saliva has been developed and the HPA is encouraging the submission of saliva samples in addition to sera. Other research projects are in progress which are aimed at providing a better understanding of the virulence mechanisms of VTEC and leading to improved typing methods for application in epidemiological investigations.

Case control studies on sporadic infections of VTEC O157 have been conducted in Wales, Scotland  and England. Such studies aim to identify risk factors for infection. A three-year study of childhood HUS was carried out in association with the British Paediatric Surveillance Unit, the British Association of Paediatric Nephrologists, the Scottish Centre for Infection and Environmental Health and the Royal Hospitals Aberdeen NHS Trust.

Contact Details

For information and advice on the epidemiology of VTEC infections, including E. coli O157, please contact the VTEC surveillance team: vtec@hpa.org.uk

References

1. Lynn R, O'Brien S, Taylor CM, Adak BA, Chart H, Cheasty T, et al.  Childhood Hemolytic Uremic Syndrome, United Kingdom and Ireland. Emerging Infectious Diseases 2005; 11:590-6.
2. Wall PG, McDonnell RJ, Adak GK, Cheasty T, Smith HR, Rowe B. General outbreaks of Vero cytotoxin producing Escherichia coli O157 in England and Wales from 1992 to 1994. Commun Dis Rep CDR Rev 1996; 6: R26-32.
3. Wong CS, Jelacic S, Habeeb RL, Watkins SL,Tarr, PL. The Risk of the Hemolytic-Uremic Syndrome after Antibiotic Treatment of Escherichia coli O157:H7 Infections. N Engl J Med 2000; 342:1930-6.

 

View epidemiological data for other gastrointestinal infections

• Surveillance and VTEC Questionnaire