Less common but more deadly: E. coli and Listeria

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1 6 Less common but more deadly: E. coli and Listeria In this chapter we discuss two microbes, the verocytotoxin-producing Escherichia coli (or VTEC for short) and Listeria monocytogenes. Compared with Salmonella and Campylobacter, these two pathogens cause less than one tenth the number of cases of illness but when they do strike, they are much more likely to kill the patient. Fatalities are particularly high in the elderly and the very young. The problem with VTEC Most strains of E. coli are harmless and live in the lower intestinal tract of healthy animals and humans. E. coli are relatively easy to detect in the laboratory and so are often used as indicator organisms. The presence of E. coli in a food or water indicates that faecal contamination has occurred and poor hygiene has been practised. E. coli was discovered in 1885 by Dr Theodor Escherich and has since been studied extensively by researchers wishing to understand the basic principles of microbial biology. One of the strains, E. coli K-12, is very easy to grow in the laboratory and is considered so safe to handle that it is often used to teach microbiology in school laboratories. However, the early 1980s saw the emergence of a new pathogenic strain of E. coli with the ability to kill people. As a result, VTEC is often referred to as an emerging pathogen. The different strains of E. coli are distinguished by the chemical composition of their cell surface. These differences can be measured using antibodies as reagents. Antibodies are very specific and will react only with molecules (antigens) with specific structures. These reactions have been developed into immunological test kits that allow classification of the different strains into serotypes. There are more than 170 serotypes in E. coli based on the O antigen (corresponding to the sugars on the outer membrane of the cell), over 100 based on the K antigen (cell capsule) and over 50 based on the H antigen (flagella). One of the most common pathogenic strains of E. coli is referred to as E. coli O157:H7, meaning that it has the O antigen 157 and the H antigen 7. There are other strains of E. coli that also produce deadly toxins and these are sometimes referred to as shigatoxin producing E. coli or STEC, or

2 TheproblemwithVTEC 77 enterohaemorrhagic E. coli or EHEC. In this chapter and throughout this book, we refer to all toxin-producing E. coli collectively as verocytotoxin-producing Escherichia coli or VTEC. The name verocytotoxin comes from the method used in the laboratory to detect its presence. The method relies on culturing vero cells originating from African green monkey kidneys in laboratory dishes. As shown in the Microbial CV for VTEC, the pathogenic strains have a very low infective dose and are particularly dangerous for people aged over 65 years or under 5 years. In about 10 per cent of the cases, the initial enteric (gut-associated) symptoms (bloody diarrhoea and abdominal pain) are followed by haemolytic uraemic syndrome (HUS), a serious illness of the kidneys. Mortality is high for patients who develop the more serious symptoms. Figure 6.1 shows that the number of cases of VTEC in the UK increased alarmingly in the 1990s. Of the 995 cases reported in 2000, 377 were hospitalized and 22 died. After a dip in the early 2000s, numbers started rising again, reaching an average of cases per year. The upward trend in the 1990s followed by more stable but high figures in the 2000s is reflected in Europe with more than 5000 cases reported in 2005 alone. In the USA, it is estimated that about cases of VTEC occur annually. A very close relative of E. coli, Shigella, causes hundreds of millions of cases of bacterial dysentery (bloody diarrhoea) worldwide and a million deaths every year. However, 99 per cent of the infections occur in developing countries. The illness is usually associated with contaminated water supplies and poor sanitary facilities. Person-to-person transmission is also common and 60 per cent of all deaths from Listeria VTEC Number of cases Year Figure 6.1 NumberofconfirmedcasesofillnesscausedbyverotoxigenicEsherichia coli (VTEC)andListeria monocytogenesinenglandandwales.datafromhealthprotection Agency(HPA),UK(2011).

3 78 Lesscommonbutmoredeadly:E. coliandlisteria Microbial CV Verocytotoxin-producing Escherichia coli (VTEC) Verocytotoxin-producing Escherichia coli, Scanning Electron Micrograph, magnification 6836X. Photo credit: CDC. Main address: Intestines of healthy cattle. Other addresses: Sheep, goats, water buffalo, rabbits and rodents; wild birds; slugs, worms, beetles, houseflies, fruitflies. Family background: More than 320 types of Escherichia coli, distinguished mainly by the chemical composition of their cell surface. Most strains are harmless but a few like E. coli O157:H7 and O104:H4 can kill people. Appearance: Gram negative, straight rods, about 0.5 μm wide and 2 μm long. Flagella (tails) help the organism to move around. Life skills: Ideal growth temperature is 37 C but grows happily at 25 C and more slowly at lower temperatures. Can grow with or without oxygen. Sensitive to salt but relatively tolerant of acids and dry conditions. Like Salmonella, Campylobacter and Listeria, VTEC is very sensitive to heat. Can spread from person to person, or from animal to person (as on children s petting farms) as well as via food and water. Unusual qualities: Pathogenic strains produce potent toxins (poisons) with severe consequences for the kidney, blood and the central nervous system. E. coli O157:H7 is much more tolerant to acidic conditions than its non-pathogenic cousins. Mode of transport to human victim: Animal faeces, soil, manure, sewage, untreated water Beef products including those produced organically e.g. undercooked burgers, steak tartare, raw milk, poorly prepared fermented sausages Green leafy vegetables such as spinach, lettuce and cabbage; herbs such as coriander; celery; sprouts from alfalfa, cress, fenugreek, mung beans and radish Apples, unpasteurised apple juice, mayonnaise Disease-causing skills: Excellent. It takes just 10 to 100 cells to give you bloody diarrhoea and abdominal pain for up to 2 weeks. Symptoms start within 3 to 4 days of eating the contaminated food. Around 10% of cases develop Haemolytic Uraemic Syndrome (HUS), an illness of the kidneys. Kidney failure and death can follow, especially in the elderly and children under the age of 5. The number of cases of illness is highest in the late summer months. Achievements: Relatively rare by comparison with Campylobacter or Salmonella but much more deadly.

4 LessonsnotlearnedfromVTECoutbreaksintheUK 79 shigellosis occur in children under the age of 5 years. In developed countries, foodborne outbreaks caused by Shigella are relatively rare. VTEC outbreaks Illness caused by VTEC is largely sporadic but several high-profile outbreaks have occurred and some examples of these are given in Box 6.1. Two large outbreaks that occurred in the UK are discussed in more detail below. Lessons not learned from VTEC outbreaks in the UK That men do not learn very much from the lessons of history is the most important of all the lessons that history has to teach Aldous Huxley, 1959 The largest VTEC outbreak in the UK occurred in Scotland in More than 500 people were made ill and 21 died. The majority of the people with serious Box 6.1 VTEC:Examples of outbreaks Fast food restaurant chain, USA, 1992/3 732 cases, 195 hospitalizations, 55 cases of Haemolytic Uraemic Syndrome (HUS), four deaths Food source:beefburgers Reasons for outbreak:contaminated ground beef used to make burgers, inadequate cooking School lunches, Japan, cases, mostly schoolchildren, 11 deaths Food source:radish sprouts Reasons for outbreak:contaminated seeds used by single supplier to produce radish sprouts Multiple outlets, USA, cases, 103 hospitalizations, 31 cases of HUS, five deaths Food source:spinach Reasons for outbreak:raw spinach contaminated with agricultural run-off from cattle Multiple restaurants and catering establishments, Germany and France, cases, over 800 cases of HUS, 48 deaths Food source:sprouts made from fenugreek seeds imported from Egypt Reason for outbreak:contaminated fenugreek seeds used to grow sprouts for raw consumption

5 80 Lesscommonbutmoredeadly:E. coliandlisteria disease and all those who died were over the age of 60 years. The size and severity of the outbreak shocked the nation. A public inquiry chaired by Professor Hugh Pennington of the University of Aberdeen investigated the reasons for the outbreak and made 32 recommendations for improvement of food-safety practices. It was thought at the time that the lessons learned would prevent future problems. The butcher responsible for the Scottish outbreak supplied both raw and cooked meat products to individuals and institutions. A church lunch attended by about 100 people, a birthday party held in a pub and meals taken at a nursing home were all involved in the outbreak. Investigations revealed that VTEC was present in the gravy and on the faulty boiler used to cook the steak for the church lunch, on the cooked ham supplied for the birthday party and on several joints of roast beef, pork and ham sold to individuals. The outbreak strain was found all over the butcher s premises, including a vacuum packing machine and on raw beef sirloin, beef sausage, stewing steak wrapped in plastic and legs of pork. The Pennington inquiry identified several shortcomings with the procedures used by the butcher, the most important of which was the failure to prevent crosscontamination. In addition, the temperature reached by the meat products during cooking was too low to kill VTEC. Once cooked, the products were not cooled quickly enough, refrigeration was inadequate and there was evidence of poor cleaning practices. The proprietor had failed to implement a proper Hazard Analysis Critical Control Point (HACCP) plan (more on HACCP in Chapter 9). The butcher was prosecuted but there were also systemic failures in local government inspection procedures and enforcement of hygiene. As a result of one of the recommendations, licensing of butchers shops was introduced in the UK in May The licensing rules required butchers selling both unwrapped meat and ready-to-eat foods to be licensed on an annual basis by their local authority. To obtain a licence, butchers had to demonstrate a functioning HACCP plan, enhanced hygiene training of staff and good temperature control within their premises. The licensing scheme in England and Scotland was evaluated in 2002 and was shown to have improved hygiene standards in butchers shops. However, the scheme was dropped at the end of 2005 because it was superseded by new European Union hygiene regulations that came into force in January These required all food businesses, including butchers shops, to operate HACCP-based food safety procedures. The new European Union regulations were thought to provide an equivalent level of public health protection as the butchers licensing scheme. Some of the other recommendations of the Pennington group were also heeded. Central funding was made available to local authorities and several codes of practice and guidance notes were revised and improved. There was a sense of optimism that new measures would ensure that a similar outbreak would not recur. But not all the lessons of the Scottish outbreak were learned. Nine years later, another large VTEC outbreak occurred in Wales, this time striking young children

6 TacklingtheVTECproblem 81 rather than the elderly. One boy, aged 5 years, died. Once again, Professor Pennington was asked to chair a new inquiry. The circumstances of the Welsh outbreak were uncannily similar to those leading to the Scottish one. Professor Pennington was not amused. The Welsh outbreak was caused by cooked meats supplied to 44 schools by a catering butcher who also owned the abattoir (slaughterhouse) supplying his business. There were 157 cases (mainly schoolchildren) of which 31 were hospitalized and one 5-year-old boy died. The same strain of E. coli O157 isolated from patients was found in cooked meats recovered from several schools, in raw meat from the butcher s premises, and in cow faeces taken from a farm supplying cattle to the butcher. The business owner was charged with seven food hygiene offences, sentenced to 12 months in prison and banned from managing any food businesses in the future. As in the Scottish outbreak, the proprietor had repeatedly failed to separate raw and cooked meats, ensure that cleaning was carried out properly and implement a proper HACCP plan. As in Scotland, many of the systems intended to assure food safety including local government inspection procedures, the commissioning of school catering contracts and enforcement of abattoir hygiene were ineffective. The second Pennington inquiry produced 24 recommendations. The coroner at the inquest into the death of the 5-year-old boy who died recommended stronger enforcement of food hygiene laws. It remains to be seen if there will be a third Pennington inquiry. VTEC in fresh produce A massive outbreak of bloody diarrhoea and HUS caused by a rare strain (O104:H4) of VTEC struck Europe in mid More than 4000 people were made ill, over 800 developed HUS and 48 people died. Unlike previous outbreaks in the UK and elsewhere affecting mainly the elderly or young children, most of the patients were women over the age of 20 years. The symptoms of illness were unusually severe and a high proportion of patients developed HUS. About half of the patients with HUS went on to develop neurological complications, including life-threatening seizures. The outbreak strain was eventually linked with raw sprouts made from fenugreek seeds imported from Egypt. Several national public health authorities issued revised guidance to consumers that sprouted seeds such as fenugreek and alfalfa as well as mung beans should not be eaten raw and should be thoroughly cooked until steaming hot throughout. Tackling the VTEC problem Healthy cattle are carriers of VTEC and shed large numbers of the microbe in their faeces. Some animals excrete very large numbers of VTEC and are known as

7 82 Lesscommonbutmoredeadly:E. coliandlisteria super-shedders or super-spreaders. Like Typhoid Mary, who was a healthy human spreader of Salmonella (see Box 5.1), VTEC is spread by some animals more than others. Since high numbers may be present in faeces, VTEC transfers easily to carcasses during slaughter or to milk during milking. VTEC survives readily in soil, sawdust, walls and roof of animal housing for up to 10 months. Although there are measures that can be taken on the farm to reduce the level of contamination with VTEC, the widespread nature of E. coli means that complete elimination of the microbe from raw agricultural products is impossible to guarantee. Therefore, it must be assumed that any raw beef ingredient or food crop produced with the aid of manure or irrigation water may be contaminated with E. coli at the point of delivery to the kitchen or food processor. Relevant Control Points (discussed in more detail in Chapter 8) must be in place to reduce the hazards during processing and to prevent recontamination of processed products. The problem with Listeria Listeria monocytogenes was first isolated and named in the 1920s but like VTEC, it was not recognized as a serious foodborne pathogen until the 1980s. Several notable outbreaks in the early 1980s alerted microbiologists, healthcare professionals and the general public to the growing problem of listeriosis. To this day, the number of cases occurring worldwide is relatively small (about 200 in the UK and 1500 each in Europe and the USA) by comparison with the hundreds of thousands of illnesses caused by Salmonella and Campylobacter. But with fatality rates as high as 40 per cent, listeriosis continues to be a major cause for concern (Table 6.1). Table 6.1 Someexamplesoflargelisteriosisoutbreaks.DatafromBellandKyriakides(2005),Health Canada,UKHealthProtectionAgency(HPA)andUSCentersforDiseaseControl(CDC) Year Source Number of cases Number of deaths Mortality rate (per cent) Location Coleslaw Canada VacherinMontD Or cheese Switzerland 1985 Mexican-stylecheese California, USA Pâtémadebysingle Belgianmanufacturer UK 1992 Porktongueinaspic France 2008 Delicatessenmeat Canada 2011 Cantaloupemelons USA