HEALTH TOPICS A TO Z

Escherichia coli (E. coli) is a bacterium that is commonly found in the gut of humans and warm-blooded animals. Most strains of E. coli are harmless. Some strains however, such as enterohaemorrhagic E. coli (EHEC), can cause severe foodborne disease. It is transmitted to humans primarily through consumption of contaminated foods, such as raw or undercooked ground meat products and raw milk. Its significance as a public health problem was recognized in 1982, following an outbreak in the United States of America. EHEC produces toxins, known as verotoxins or Shiga-like toxins because of their similarity to the toxins produced by Shigella dysenteriae. EHEC can grow in temperatures ranging from from 7°C to 50°C, with an optimum temperature of 37°C. 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 70°C or higher. E. coli O157:H7 is the most important EHEC serotype in relation to public health; however, other serotypes have frequently been involved in sporadic cases and outbreaks.

The diseases caused by EHEC

Symptoms of the diseases caused by EHEC include abdominal cramps and diarrhoea that may in some cases progress to bloody diarrhoea (haemorrhagic colitis). Fever and vomiting may also occur. The incubation period can range from three to eight days, with a median of three to four days. Most patients recover within 10 days, but in a small proportion of patients (particularly young children and the elderly), the infection may lead to a life-threatening disease, such as haemolytic uraemic syndrome (HUS). 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%. Overall, HUS is the most common cause of acute renal failure in young children. It can cause neurological complications (such as seizure, stroke and coma) in 25% of HUS patients and chronic renal sequelae, usually mild, in around 50% of survivors.

The incidence of EHEC infections varies by age group, with the highest incidence of reported cases occurring in children aged under 15 years (0.7 cases per 100 000 in the United States). Sixty-three to 85% of cases are a result of exposure to the pathogen through food. The percentage of EHEC infections which progress to HUS varies between sporadic cases (3%-7%) and those associated with outbreaks (20% or more). In epidemiological terms, there is generally a background of sporadic cases, with occasional outbreaks. Some of these outbreaks have involved a high number of cases, such as in Japan in 1996, where an outbreak linked to contaminated radish sprouts in school lunches caused 9 451 cases. Data on the situation in developing countries are limited, as surveillance for this pathogen is not done routinely.

Sources of infection

Most available information 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 and other ruminants such as camels. It 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 and milk. An increasing number of outbreaks are associated with the consumption of fruits and vegetables (sprouts, 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.

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. Available data are not sufficient to enable the recommendation of specific intervention methods on the farm in order to reduce the incidence of EHEC in cattle. However, risk assessments conducted at national level have predicted that the number of cases of disease might be reduced by various mitigation strategies for ground beef (for example, screening the animals preslaughter 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 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 impossible, but the education of farm workers in principles of good hygienic practice should be carried out 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. Some countries implement the policy that raw ground beef is considered contaminated if it is found to contain E. coli O157:H7.

Preventive measures for E. coli O157:H7 infection are similar to those recommended for other foodborne diseases (see basic food hygiene practice described below). However, some of the measures may need to be reinforced for EHEC, particularly in view of its importance in vulnerable groups such as children and the elderly. Since a number of EHEC infections have been caused by contact with recreational water, it is also important to protect such water areas, as well as drinking-water sources, from animal wastes.

Recommendations to reduce the public health risk

To ensure that those who come directly or indirectly into contact with food are not likely to contaminate it with EHEC, food handlers should follow the Recommended International Code of Practice, General Principles of Food Hygiene (CAC/RCP 1-1969, Rev. 3-1997, Amd. (1999); Section VII - Establishment: personal hygiene), contained in: Joint FAO/WHO Food Standards Programme, Codex Alimentarius Commission. General requirements (food hygiene). FAO/WHO, Rome, 2001 (Second edition) .

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 at least the centre of the food reaches 70°C.

Specific recommendations to sprout producers

In recent years, the popularity of sprouted seeds has increased significantly owing to their nutritional value. However, reports of foodborne outbreaks associated with such raw vegetable sprouts have raised concerns among public health agencies and consumers. Outbreak investigations have indicated that pathogens found on sprouts most likely originate from the seeds. The seed may be contaminated in the field or during harvesting, storage or transportation. During the germination process in sprout production, low levels of pathogens present on seeds may quickly reach levels high enough to cause disease. Therefore specific care is needed. Guidance is available in the Codex Code of Hygienic Practice for Fresh Fruits and Vegetables, Annex for sprout production (document CAC/RCP 53-2003 which can be obtained on request from the Secretariat of the Codex Alimentarius Commission, codex@fao.org).