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Sources and transmission
Most available information on EHEC relates to serotype O157:H7, since it is easily differentiated biochemically from other E. coli strains. The reservoir of this pathogen appears to be mainly cattle. In addition, other ruminants such as sheep, goats, deer are considered significant reservoirs, while other mammals (pigs, horses, rabbits, dogs, cats) and birds (chickens, turkeys) have been occasionally found infected.
E. coli O157:H7 is transmitted to humans primarily 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 (with beef and other meat products, contaminated surfaces and kitchen utensils), will also lead to infection. Examples of foods implicated in outbreaks of E. coli O157:H7 include undercooked hamburgers, dried cured salami, unpasteurized fresh-pressed apple cider, yogurt, cheese made from raw milk.
An increasing number of outbreaks are associated with the consumption of fruits and vegetables (sprouts, spinach, lettuce, coleslaw, salad) whereby contamination may be due to contact with faeces from domestic or wild animals at some stage during cultivation or handling. EHEC has also been isolated from bodies of water (ponds, streams), wells and water troughs, and has been found to survive for months in manure and water-trough sediments. 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. Visiting farms and other venues where the general public might come into direct contact with farm animals has also been identified as an important risk factor for EHEC infection.
Prevention
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 household kitchens.
Industry
The number of cases of disease might be reduced by various mitigation strategies for ground beef (for example, screening the animals pre-slaughter to reduce the introduction of large numbers of pathogens in the slaughtering environment). Good hygienic slaughtering practices reduce contamination of carcasses by faeces, but do not guarantee the absence of EHEC from products. Education in hygienic handling of foods for workers at farms (1), abattoirs and those involved in the food production is essential to keep microbiological contamination to a minimum (2). The only effective method of eliminating EHEC from foods is to introduce a bactericidal treatment, such as heating (e.g. cooking, pasteurization) or irradiation.
Household
Preventive measures for E. coli O157:H7 infection are similar to those recommended for other foodborne diseases. Basic good food hygiene practice, as described in the WHO Five keys to safer food, can prevent the transmission of pathogens responsible for many foodborne diseases, and also protect against foodborne diseases caused by EHEC. Such recommendations should in all cases be implemented, especially "Cook thoroughly" so that the centre of the food reaches at least 70°C. Make sure to wash fruits and vegetables carefully, especially if they are eaten raw. If possible, vegetables and fruits should be peeled. Vulnerable populations (e.g. small children, the elderly) should avoid the consumption of raw or undercooked meat products, raw milk and products made from raw milk.
Regular hand washing, particularly before food preparation or consumption and after toilet contact, is highly recommended, especially for people who take care of small children, the elderly or immunocompromised individuals, as the bacterium can be passed from person-to-person, as well as through food, water and direct contact with animals.
A number of EHEC infections have been caused by contact with recreational water. Therefore, it is also important to protect such water areas, as well as drinking-water sources, from animal waste (3).
WHO response
During E. coli outbreaks, such as the ones in Europe in 2011, WHO has responded by:
- supporting the coordination of information sharing and collaboration through International Health Regulations and the International Food Safety Authorities Network (INFOSAN) worldwide;
- working closely with national health authorities and international partners, providing technical assistance and the latest information on the outbreak.
In terms of prevention, WHO has responded with a global strategy to decrease the burden of foodborne diseases. WHO developed the Five keys to safer food message. The Five keys and associated training materials provide countries with materials that are easy to use, reproduce and adapt to different target audiences.
1. FAO/WHO [Food and Agriculture Organization of the United Nations/World Health Organization]. Microbiological hazards in fresh leafy vegetables and herbs: meeting report. Microbiological Risk Assessment Series No. 14, Rome, 2008.
2. Codex Recommended International Code of Practice-General Principles of Food Hygiene (CAC/RCP 1-1969). In: FAO and WHO. Food Hygiene Basic Texts. Fourth edition, 2009.
3. WHO. Five Keys for growing safer fruits and vegetables: promoting health by decreasing microbial contamination, 2011 (in press).