VTEC/E. coli O157 Action Plan for Scotland, 2013-2017

IV. BACKGROUND

Escherichia coli O157 and non-O157 VTEC

E. coli O157 is the serogroup of verotoxigenic E. coli (VTEC) most commonly detected in humans in Scotland and the UK. Health Protection Scotland (HPS) surveillance data show that in recent years, verotoxigenic strains of non-O157 E. coli (non-O157 VTEC) have also caused significant morbidity in Scotland. This Action Plan therefore focuses on E.coli O157 and non-O157 VTEC. For simplicity, these are referred to as VTEC, unless stated otherwise.

Clinical features

Incubation period and infectious dose

The incubation period for VTEC infection is usually three to four days, seldom less than one or more than eight, but occasionally as long as 14. The infectious dose is low, probably well under 1000 organisms.

Clinical presentation and sequelae

VTEC infection can be asymptomatic, or may cause a spectrum of illness from mild non-bloody diarrhoea, through bloody diarrhoea and haemorrhagic colitis, to haemolytic uraemic syndrome (HUS), or death. HPS data show that HUS occurs in 9% of cases in Scotland, although this proportion is falling; it is more likely to develop in children under 16 years old and adults over 60 years old but can occur in any age group as evidenced by the outbreak of VTEC O104 infection in mainland Europe in 2011^[3]. The median period for cases continuing to excrete VTEC is 13-29 days; the maximum excretion period reported to HPS surveillance exceeds three months. Pre-symptomatic excretion has also been identified.

Transmission

The sources and routes of transmission of E. coli O157 and non-O157 serogroups of VTEC appear generally similar, but novel vehicles of transmission should not be ruled out for any serogroup.

Animals and the environment

VTEC can colonise the gastro-intestinal tract of farm and wild animals, especially cattle and sheep, usually without causing illness. Any food, environment or water (including surface or ground water) contaminated by the excreta of an animal or human is a potential source of infection. Occupational, as well as recreational, exposures have been associated with infection. HPS surveillance and research data show that 17% of all E. coli O157 outbreaks (1996-2012^[4]), involved farm premises (71% of which involved private farms, their residents and visitors), and that contact with farm animal faeces was the risk factor most strongly associated with sporadic E. coli O157 infection.

Food

The surface of meat can become contaminated during slaughter and processing. Unpasteurised or inadequately pasteurised milk, or raw vegetables, may also be contaminated. Minced or ground beef products pose a particular risk if they are not cooked properly. Although meat and dairy products have caused large VTEC outbreaks in the UK, other food vehicles identified worldwide include salad leaves, white radish and other sprouted seeds, and raw vegetables. Less commonly reported food vehicles have included unpasteurised apple juice, fermented sausage and hazelnuts. HPS data show that in 28% of all E. coli O157 outbreaks (1996-2012^[4]), the main mode of transmission was foodborne.

Drinking Water

Sources of drinking water can become contaminated by VTEC. Although rare in the developed world, public water treatment failures can cause large outbreaks. Private water supplies (PWS) in Scotland, which are generally in more rural areas, are more likely to be contaminated with E. coli O157 than public supplies. Visitors to rural areas may be more susceptible to infection than local residents. HPS data show that 13% of all E. coli O157 outbreaks (1996-2012⁴) involved PWS, with E. coli O157 isolated from 17/18 implicated PWS.

Person-to-person

HPS enhanced surveillance shows that person-to-person (or secondary) spread of infection with VTEC accounts for about 11% of cases. Secondary spread does not require direct contact and can be spread via swimming and paddling pools, environments and surfaces, such as towels.

Epidemiology

E. coli O157 is the only serogroup routinely detected by culture in UK diagnostic laboratories. Reported incidence varies between and within UK countries, and is consistently higher in Scotland (Figure 1) with an average annual total for the last five years 2008-2102, of 235 cases^[2]. HPS data show around 15% of infections reported in Scotland are likely to have been acquired outside the UK.

Figure 1: E. coli O157: Culture positive cases, rates per 100,000 population, 1984-2012

Almost 50% of cases in Scotland are in children under 16 years of age. Rates of infection are highest in children under 5 years, at 15 cases per 100,000 population, compared with a population average of 4.5 per 100,000. On average 43% of all cases are hospitalised. Amongst patients with HUS, 85% are under 16 years. Whilst exact cause of death is often unknown, less than 2% of patients with E. coli O157 in Scotland died following infection. Reports generally peak in the third quarter of the year. On average 80% of cases are not part of general outbreaks (i.e. they are apparently sporadic cases or single household clusters). Although the mechanism is complex, geographic differences in infection rates appear to be related to cattle and human population densities^[5].

Microbiology

Most strains of E. coli O157 are non-sorbitol fermenting. Diagnostic laboratories therefore use the absence of sorbitol fermentation to detect E. coli O157 (98% of which is VTEC in Scotland and therefore requires further laboratory investigation). Only three cases of sorbitol fermenting (SF) VTEC O157 were identified in the UK prior to 2006. In 2006, the first outbreak of SF VTEC O157 was identified in the UK.

It is important to maintain vigilance for newly emerging strains; routine diagnostic techniques to detect these may become more widely available in the future. The significant characteristic of non-O157 and SF VTEC is that they are difficult to identify. Once identified, their public health management is the same as E. coli O157.

Public health management of VTEC infection in Scotland

Many professionals and organisations have a role in the public health management of VTEC, including GPs, hospital staff, NHS Boards' Health Protection teams (HPTs), Local Authorities (LAs), Health Protection Scotland (HPS), microbiologists, veterinary surgeons and others. Guidance on the public health management of VTEC is available elsewhere^[6].