The Pennington Group

Report on the circumstances leading to the 1996 outbreak of infection with E.coli 0157 in Central Scotland, the implications for food safety and the lessons to be learned.

Chapter 3 E.coli O157

3.1 This Chapter is intended to aid understanding of E.coli O157 and the issues set out in this report. It is not intended as a scientific definition or explanation. Any attempt to describe formally the organism, its behaviour and effects will inevitably be complex and technical in nature. A full description of Verocytotoxin-producing Escherichia coli (or VTEC for short), of which E.coli O157 is one type, can be found in the early chapters of the 1995 ACMSF Report⁷.

3.2 E.coli is a bacterium, many strains (or types) of which live harmlessly in the guts of humans and animals. However, certain strains are pathogenic and can cause gastro-intestinal disease and complications - for example VTEC strains such as E.coli O157 produce toxins, or poisons, which can cause illness in humans.

3.3 The type of the bacterium which caused the Central Scotland and Tayside food poisoning outbreaks, and also previous outbreaks in West Lothian and in Grampian in 1994, is E.coli O157:H7. This particular type is often toxin-producing and is the predominant cause of VTEC infection in humans. It is highly virulent, requiring relatively few organisms to cause harm in humans and, because of that and the severity of the diseases it can cause, is potentially a much more dangerous form of footborne infection than, for example, Salmonella.

3.4 E.coli O157:H7 was first identified as a cause of human illness in 1982 in patients affected in 2 outbreaks of bloody diarrhoea in the USA, both associated with eating undercooked hamburgers. There have since been numerous and increasing reports world-wide of infection with the organism. This may be due in part to improved surveillance and methods of detection, but it is generally accepted that increases in the rate of infection with the organism are real rather than due simply to improved ascertainment.

Outbreaks in the UK/Scotland

3.5 Most outbreaks in the UK have affected fewer than 10 people and have been associated with the consumption of a variety of foods including minced beef (including burgers), milk, yoghurt, cheese and water. Prior to the Central Scotland outbreak, the largest recorded was that in West Lothian in 1994 referred in paragraph 3.3 above, associated with the consumption of contaminated pasteurised milk, when more than 100 people were affected and a child died.

Further details of recent outbreaks in Scotland are as follows:-

Outbreaks of E.coli O157 Infection in Scotland
Reported to SCIEH 1992-1996*

	Type of Outbreak	Location	Nos. Affected	Phage/VT Type	Mode of Spread

1992	Birthday Party	Borders	5	PT49/VT2	Paddling pool
	Hospital	Glasgow	5	PTI/VT1, 2	Acquired in hospital

1993	Birthday Party	Lanarkshire	5	PT2/VT2	Person to person

1994	Community	Fife etc	24	PT4/VT1, 2	Burger meat
	Community	Lothian	100	PT2/VT2	Milk
	Community	Highland	8	PT2/VT2	'Milk'
	Community	Lothian	16	PT28/VT2	'Burger meat'
	Community	Borders	4	PT2/VT2	Animal to human
	Community	Grampian	22	PT28/VT2	Cheese

1995	Community	Fife	5	PT2/VT2	Water & foodborne

1996	Community	Highland	3	PT2/VT2	Person to person
	Sports Club	Forth Valley	2	PT2/VT2	Not known
	Residential Home	Glasgow	8	PT2/VT2	Not known
	Butcher/Baker Shop	Lothian	N/A	N/A	Not known
	? 2 ý Outbreaks	Lothian	N/A	N/A	Not known
	Butcher Shop	Lanarkshire
		Forth Valley	496	PT2/VT2	Foodborne
		Lothian
		Glasgow

*Note: A new surveillance system was introduced in 1996. Figures are not therefore directly comparable.

3.6 The number of laboratory-confirmed cases of human infection with E.coli O157 in Scotland (reported to SCIEH) and in England & Wales (reported to the Public Health Laboratory Service's Communicable Disease Surveillance Centre), and the respective rates of infection per 100,000 population, since 1990 are:-

	Scotland		England & Wales
	No of Cases	Rate	No of Cases	Rate

1990	173	3.39	250	0.49
1991	202	3.96	361	0.71
1992	115	2.25	470	0.92
1993	119	2.32	385	0.75
1994	242	4.71	411	0.80
1995	247	4.8	792	1.52
1996	488*	9.5**	660*	1.26**

* Provisional
** Projected rates based on 1995 mid-year population estimates

The 1994 and 1996 figures for Scotland are, obviously, heavily influenced by the effects of the West Lothian and Central Scotland outbreaks.

3.7 Because of the general difficulties in identification of the organism, recent improvements in detection methods and differences in, for example, surveillance practices, it is difficult to draw conclusive comparisons year on year or between different parts of the UK. From the table in paragraph 3.6, however, it seems clear that the rate of infection with E.coli O157 in Scotland has increased over recent years. Significant regional variations in Scotland have also been identified. The underlying rate of infection in Scotland, per 100,000 of population, is also substantially (possibly around 4 times) higher than for England and Wales. The reasons for this are as yet unclear.

How Infection Occurs

3.8 VTEC, including E.coli O157, exists in a wide range of animals (wild, farmyard and domestic) and even birds. As indicated in our interim report, however, it is generally accepted that the main reservoir of E.coli O157 is in the rumens and intestines of cattle and, possibly, sheep.

3.9 The organism can be excreted and may therefore exist in animal manure or slurry, which could be a source of environmental or water contamination, or direct contamination of food such as vegetables. (Most of the evidence for this is, however, circumstantial.) It seems likely that there can be animal to animal infection/re-infection. There is good evidence that it is transferred to animal carcasses through contamination from faecal matter during the slaughter process. Many early outbreaks were associated with the consumption of hamburgers. There have also been documented cases attributed to meat, meat products and other foods such as milk, cheese and apple juice. In a recent large Japanese outbreak, radishes were identified as a possible source of the infection. The vehicle for most cases of infection, however, remains unknown.

3.10 The organism survives well in frozen storage and freezing cannot be relied upon to kill it. It is killed by heating but can survive if food is not properly cooked (as in the case of the hamburgers mentioned above). If appropriate hygiene measures are not taken, there can also be cross-contamination between raw meat carrying the organism and cooked or ready to eat foods. E.coli O157 appears to be relatively tolerant to acidic conditions (compared, for example, to Salmonella).

3.11 Human infection may occur as a result of direct contact with animals carrying the organism, from contamination from their faeces, or through consumption of contaminated food or water. It may also spread directly from person to person as a result of poor hygiene practices which allow faecal-oral spread. The latter is, obviously, a particular potential problem in institutions such as nursing homes, day-care centres or hospitals and in places where pre-school children meet and underlines the need for good personal hygiene and meticulous attention to procedures designed to prevent cross-infection. Cases may be related to outbreaks or may be sporadic (ie isolated and apparently unrelated to other cases). The role of asymptomatic food handlers in outbreaks is unclear but may be important in light of the low infectious dose.

3.12 The incubation period for infection with E.coli O157 (that is, the period between infection with the organism and the onset of symptoms caused by infection) is normally between 1 and 10 days, but can be longer as shown in the recent outbreaks in Scotland.

The Effects of Infection

3.13 As the Central Scotland and Tayside outbreaks have shown, infection with E.coli O157 is potentially very serious for vulnerable groups - particularly the elderly and the very young. There is no specific treatment available for infection. The complications which infection can cause include haemorrhagic colitis (bloody diarrhoea), haemolytic uraemic syndrome (HUS)⁸ and thrombotic thrombocytopaenic purpura (TTP)⁸. The latter two complications are much less common but can be very serious causing kidney and other problems and, in the most severe cases, even death. Infection with E.coli O157 and associated HUS is the most common cause of acute renal failure in children in the UK. Morbidity for the vulnerable groups is particularly high compared to other forms of foodborne illness.

3.14 Importantly however in some cases the illness or symptoms caused by infection with E.coli O157 can be relatively mild. Indeed, cases of infection can be misdiagnosed so that not all those affected may be identified. In addition, some cases are asymptomatic, showing no symptoms at all, although excreting the organism. These cases may represent a potential source of person to person (secondary) infection. It is important that the potential for infection from a variety of sources, the implications of asymptomatic carriage of the organism and the potential dangers which E.coli O157 presents for public health (even from a very small infective dose) are all borne in mind in the consideration of preventive and outbreak control measures.

Detection and Identification

3.15 Despite improvements in surveillance and testing techniques, the organism remains more difficult to detect and identify accurately than other important foodborne bacterial pathogens. VTEC does not generally cause illness in animals other than, at worst, transient diarrhoea in very young animals. There is, therefore, no reason for farmers to seek to identify the presence of the organism in their animals. In any event it appears to be excreted only intermittently.

3.16 The very few organisms that are required to cause harm in humans can, under present rules and practices, easily escape detection and pass along the food chain, whether from animal faeces, carcasses, meat, equipment or humans. It has been difficult to identify in foods and although techniques have improved over the years, rates of detection are still unsatisfactory. This is due in part to the low levels of the organism which appear to occur in food. The most sensitive techniques for identifying the organism (particularly, but not only, in food) are complex and sophisticated, requiring specialised equipment and expertise that is not generally available.

3.17 The costs and complexities of identification have so far prevented the routine, universal testing for E.coli O157 in food safety surveillance. Cost is also reported as the main reason why most laboratories do not follow the ACMSF advice (in its 1995 report) to examine routinely all diarrhoeal stool specimens for E.coli O157. Such tests are normally restricted to patients with bloody diarrhoea and possible symptoms of haemolytic uraemic syndrome.

3.18 Strains of E.coli O157 can be further distinguished by subtyping. Phage typing recognises more than 80 types: types 2 and 28 are the commonest in Scotland. Phage types can be further sub-divided by DNA typing methods. Pulsed-field gel electrophoresis (PFGE) is one such method which is widely used as a means of further discriminating among sub-types.

7 ACMSF: Report on Verocytotoxin-Producing Escherichia coli 1995 (HMSO 1995 ISBN 0 11 321909 1)

8 See glossary.