Fact Sheet N 125
ESCHERICHIA COLI O157:H7
1. What is Escherichia coli O157:H7 ?
Escherichia coli is a bacterium that is a common inhabitant of the gut of warm blooded animals, including man. Most strains of E.coli are harmless, however, some strains, such as E. coli O157:H7, can cause severe foodborne disease and are referred to as enterohaemorrhagic E .coli (EHEC). This pathogen produces toxins, known as verotoxins or Shiga-like toxins because of their similarity to the toxins produced by Shigella dysenteriae. The organism can grow from around 7-10C to 50C, with an optimum temperature of 37C. Some EHEC can grow in acidic foods, down to a pH of 4.4, and in foods with a minimum water activity (Aw) of 0.95. It is destroyed by thorough cooking of foods until all parts reach a temperature of 70C or higher. The designation "O157:H7" in the name of this bacterium refers to specific chemical compounds that are found on its surface that distinguishes it from other strains of E. coli.
2. What illnesses does E. coli O157:H7 cause ?
Symptoms of the illness caused by E.coli O157:H7 include abdominal cramps and watery diarrhoea that can develop into bloody diarrhoea (haemorrhagic colitis ). Fever and vomiting may also occur and most patients recover within 10 days. However, in a small proportion of patients, particularly young children and the elderly, the infection can result in life-threatening complications, such as haemolytic uraemic syndrome (HUS), for example.
HUS is characterized by acute renal failure, haemolytic anaemia and thrombocytopenia. It is estimated that up to 10% of patients with EHEC infection may develop HUS, with a case-fatality rate ranging from 3% to 5%. The incubation period for the illness can range from 3 to 8 days, with a median of 3-4 days.
3. Sources of infection
The reservoir of this pathogen appears to be mainly cattle. It is transmitted to man principally through consumption of contaminated foods, such as raw or undercooked ground meat products and raw milk. Faecal contamination of water and other foods, as well as cross-contamination during food preparation will also lead to infection. Examples of foods implicated in outbreaks of E. coli O157:H7 include hamburgers, fresh-pressed apple cider, yogurt, cheese, dried cured salami, and cooked maize. EHEC can survive and grow on salad vegetables.
Waterborne transmission has been reported, both from contaminated drinking water and from recreational waters. Person-to-person contact is an important mode of transmission through the oral-faecal route. An asymptomatic carrier state has been reported, where individuals show no clinical signs of disease but are capable of infecting others. The duration of excretion of EHEC is about one week or less in adults, but can be longer in children
4. Control and Prevention Methods
The prevention of infection requires control measures at all stages of the food chain, from agricultural production on the farm, to processing, manufacturing and preparation of foods in both commercial establishments and the domestic environment.
There are insufficient data to recommend specific intervention methods on the farm in order to reduce the incidence of EHEC in cattle. Good hygienic slaughtering practices will reduce contamination of carcasses by faeces but will not guarantee the absence of EHEC from products. Education in hygienic handling of foods for abattoir workers and those involved in the production of raw meat is essential to keep microbiological contamination to a minimum. Similarly, prevention of contamination of raw milk on the farm is virtually not feasible but education of farm workers in principles of good hygienic practice is necessary in order to keep contamination to a minimum. The only effective method of eliminating EHEC from foods is to introduce a bactericidal treatment, such as heating (e.g. cooking or pasteurization) or irradiation.
Preventive measures for of E.coli O157:H7 infection are similar to other foodborne diseases. However, some of the measures may need to be reinforced for this infection, particularly because of the importance of EHEC infection in vulnerable groups such as children and elderly.
Recommendations to the public and travellers
A WHO Guide on Safe Food for Travellers is available from the Sales and Distribution Office, Geneva, Switzerland, that gives practical advice for safeguarding health when travelling.
Recommendations to professional and domestic food handlers:
More information for food handlers is given in the WHO Guide on Hygiene in Food Service and Mass Cartering Establishments.
Further information of Food Safety can be obtained at the WHO WWW Site (hppt:/www.who.ch/programmes/fnu/fos/gldnrls.htm) or E-mail at firstname.lastname@example.org.
For further information please contact Valery Abramov, Health Communications and Public Relations, WHO, Geneva. Tel. (41 22) 791 2543, or Fax (41 22) 791 4858.
All WHO press releases, fact sheets and features are available on the Internet on the WHO Home Page http:www.who.ch/
THE WHO GOLDEN RULES FOR SAFE FOOD PREPARATION
1. Choose foods processed for safety
While many foods, such as fruits and vegetables, are best in their natural state, others simply are not safe unless they have been processed. For example, always buy pasteurized as opposed to raw milk and, if you have the choice, select fresh or frozen poultry treated with ionizing radiation. When shopping, keep in mind that food processing was invented to improve safety as well as to prolong shelf-life. Certain foods eaten raw, such as lettuce, need thorough washing.
2. Cook food thoroughly
Many raw foods, most notably poultry, meats, and unpasteurized milk, are very often contaminated with disease-causing pathogens. Thorough cooking will kill the pathogens, but remember that the temperature of all parts of the food must reach at least 70C. If cooked chicken is still raw near the bone, put it back in the oven until il's done -- all the way through. Frozen meat, fish, and poultry must be thoroughly thawed before cooking.
3. Eat cooked foods immediately
When cooked foods cool to room temperature, microbes begin to proliferate. The longer the wait, the greater the risk. To be on the safe side, eat cooked foods just as soon as they come off the heat.
4. Store cooked foods carefully
If you must prepare foods in advance or want to keep leftovers, be sure to store them under either hot (near or above 60C) conditions. This rule is of vital importance if you plan to store foods for more than four or five hours. Foods for infants should preferably not be stored at all. A common error, responsible for countless cases of foodborne disease, is putting too large a quantity of warm food in the refrigerator. In an overburdened refrigerator, cooked foods cannot cool to the core as quickly as they must. When the centre of food remains warm (above 10C) too long, microbes thrive, quickly proliferating to disease-producing levels.
5. Reheat cooked foods thoroughly
This is your best protection against microbes that may have developed during storage (proper storage slows down microbial growth but does not kill the organisms). Once again, thorough reheating means that all parts of the food must reach at least 70C.
6. Avoid contact between raw foods and cooked foods
Safely cooked food can become contaminated through even the slightest contact with raw food. This cross-examination can be direct, as when raw poultry meat comes into contact with cooked foods. It can also be more subtle. For example, don't prepare a raw chicken and then use the same unwashed cutting board and knife to carve the cooked bird. Doing so can reintroduce all the potential risks for microbial growth and subsequent illness present prior to cooking.
7. Wash hands repeatedly
Wash hands thoroughly before you start preparing food and after every interruption -- especially if you have to change the baby or have been to the toilet. After preparing raw foods such as fish, meat or poultry, wash again before you start handling other foods. And if you have an infection on your hand, be sure to bandage or cover it before preparing food. Remember too, that household pets -- dogs, birds, and especially turtles -- often harbour dangerous pathogens that can pass from your hands into food.
8. Keep all kitchen surfaces meticulously clean
Since foods are so easily contaminated, any surface used for food preparation must be kept absolutely clean. Think of every food scrap, crumb or spot as a potential reservoir of germs. Cloths that come into contact with dishes and utensils should be changed every day and boiled before reuse. Separate cloths for cleaning the floor also require, frequent washing.
9. Protect foods from insects, rodents, and other animals
Animals frequently carry pathogenic microorganisms which cause foodborne disease. Storing foods in tightly sealed containers is your best protection.
10. Use pure water
Pure water is just as important for food preparation as for drinking. If you have any doubts about the water supply, boil water before adding it to food or making ice for drinks. Be especially careful with any water used to prepare an infant's meal.
The World health Organization regards illness due to contaminated food as one of the most widespread health problems in the contemporary world. In infants and the elderly, the consequences can be fatal. Protect your family by following these simple rules. They will reduce the risk of foodborne disease significantly.
For further information please contact the Food Safety Unit, World Health Organization, 1211 Geneva 27, Switzerland.