Hazard Identification

What are VTEC?

The verocytotoxin producing Escherichia coli (VTEC) are a group of strains within the species E. coli, some of which are highly pathogenic and capable of causing potentially serious foodborne infections in humans. E. coli are gram negative, non- sporeforming bacteria belonging to the family Enterobacteriaceae. Microbiologists recognise a small number of genera within the Enterobacteriaceae, including Escherichia species, as the coliform group.

E. coli are found as part of the normal human gut flora, as well as in the environment, and the presence of E. coli in processed product can indicate faecal contamination (the reason why E. coli is used as 'indicator' organism). Most strains of E. coli do not usually cause illness, but a minority have been associated with infections resulting in diarrhoea, or sometimes more severe illness.

There are four different groups of diarrhoea-causing E. coli grouped by virulence characteristics as follows:

Pathogenic E. coli
Enteropathogenic (EPEC)	Causing infantile gastroenteritis or summer diarrhoea mostly in the developing world.
Enterotoxigenic (ETEC)	Causing traveller's diarrhoea
Enteroinvasive (EIEC)	Causing a form of bacillary dysentery
Verocytotoxin-producing (VTEC) -- sometimes referred to as Shiga-like toxin-producing (STEC). This group includes a subset of serotypes often referred to as enterohaemorrhagic E. coli (EHEC)	Not all VTEC are associated with human disease, but those that are EHEC can cause haemorrhagic colitis (bloody diarrhoea).

The group of most concern in developed countries is the VTEC, so named because they produce one or more toxins that are toxic to vero cells (a tissue cell culture line derived from the kidneys of an African Green monkey). In excess of 200 VTEC have been described and some of these organisms have been associated with outbreaks of severe foodborne disease in many countries. The VTEC most frequently associated with causing foodborne illness is the serotype Escherichia coli O157:H7. Other important VTEC that have caused foodborne infections are O26, O103, O111 and O145.

Occurrence in foods

VTEC are usually associated with foods derived from cattle such as beef products, particularly minced/ground beef, and dairy products derived from raw milk. Although VTEC could be present on any raw beef product, minced meat products are considered more of a risk because the pathogen is transferred from the surface to the center of the product during the mincing process.

Studies in the USA and the UK have found that VTEC can be present, at least occasionally, on most farms. However, surveys of food commodities have found that the prevalence of the organism in beef and raw milk products is generally low. VTEC have also been found on fruits, vegetables and seeds and associated products. Fresh produce can be contaminated at any stage during cultivation or handling, possibly via contaminated water supplies, or cattle manure used as a fertilizer.

Hazard characterization

Effects on health

The incubation period for illness caused by VTEC can be between 1 and 14 days, although on average it is 3 - 4 days. The infective dose is thought to be very low, possibly just 10 cells. This is probably because these bacteria are unusually acid tolerant. Symptoms may be restricted to mild diarrhoea only, and some individuals may be asymptomatic.

However, VTEC infection can cause more serious symptoms in some 50% of those infected, especially in vulnerable groups. These symptoms include bloody diarrhoea, abdominal cramps, vomiting and very occasionally, fever. The illness typically resolves itself after 5 - 10 days, but in a small number of cases, particularly in young children under 5 years of age and the elderly, VTEC infection can lead to haemolytic uraemic syndrome (HUS), potentially resulting in kidney failure. HUS in children can also result in seizures, coma and sometimes death. Thrombotic thrombocytopaenic purpura (TTP) is a form of HUS typically developed by the elderly and includes fever, platelet loss and neurological symptoms. Around one third of individuals showing signs of VTEC infection are hospitalised and the average mortality rate from HUS caused by VTEC infections in the UK and in North America is 3 - 5 %.

Incidence and outbreaks

Fortunately, in view of its potentially serious symptoms, VTEC infections are comparatively rare. Nevertheless, in Europe between 1995 and 2002, incidence of infection more than doubled to 3.2 cases per 100,000 of the population, before leveling off. In 2005, just over 5,200 cases were reported in 25 countries. The UK, and Scotland in particular, have a higher incidence than many other European countries, but the reasons for this are not known. In the USA, O157 VTEC is reported separately from other VTEC, but approximately 3,500 VTEC cases were reported in 2005, giving an incidence of roughly 2.0 cases per 100,000.

VTEC outbreaks, particularly those caused by E. coli O157:H7, have frequently been associated with undercooked minced (ground) beef products such as hamburgers - it has been dubbed 'hamburger disease'. However, VTEC outbreaks have also been caused by a wide variety of other foods such as cooked meats, raw and recontaminated pasteurised milk, cheese, yoghurt, mayonnaise, unfermented apple cider, unpasteurised apple juice, melon, salad leaves such as lettuce and spinach, parsley, coleslaw, venison jerky, salami and alfalfa sprouts. Contaminated water sources are also a common source of VTEC outbreaks.

Sources

The main infection reservoir for O157 VTEC is recognised as cattle, which, together with other ruminants such as sheep and camels, are apparently healthy carriers of VTEC. Studies have found that the organism is more likely to be found in cattle faeces during the spring than in the winter. Other animals have also been found to excrete VTEC, including goats, deer, horses, dogs, cats, rats, seagulls, pigeons, and geese. E. coli O157 has also been isolated from houseflies. A number of outbreaks have been associated with direct contact with infected animals in petting zoos.

Contamination of water supplies with animal faeces has led to outbreaks linked to drinking water and wells, as well as from recreational waters such as lakes, paddling pools and water parks. Soil manured with animal faeces, or in fields where animals have been grazing, can be contaminated with VTEC and contamination may be transferred to crops.

Person to person spread via the faecal-oral route has also occurred causing outbreaks in institutions and child-care settings such as nurseries. Asymptomatic carriers, a state where individuals show no clinical symptom of the disease but are capable of infecting others, have also been reported.

Growth and survival in foods

VTEC can grow over the temperature range 7 - 46 ^ºC (although some sources suggest possibly up to 50 ^ºC) with an optimum of 37 ^ºC. Some isolates of E. coli O157:H7 have been reported to grow in raw milk at 8 ^ºC. E. coli O157:H7 also grows poorly at 44 - 45 ^ºC, so that traditional methods to detect E. coli in food may not pick up this important pathogen.

VTEC survive well at chilled and frozen temperatures. Low temperature is reported to be the primary trigger for VTEC to enter a 'viable non culturable' state (VNC) in water. A VNC state means that normal methods of detection are unable to recover the organism, but it is still able to cause illness.

VTEC are unusual amongst E. coli because they are relatively acid tolerant. The minimum pH for the growth of E. coli O157 under otherwise optimum conditions is reported as 4.0 - 4.4, although the minimum value is affected by the acidulant and acetic and lactic acids are more inhibitory than hydrochloric acid. The organism is able to survive acid conditions (down to 3.6) and has been reported to survive for two months at 4 ^ºC at a pH of 4.5.

The minimum reported water activity for the growth of VTEC is 0.95. Salt (NaCl) at 8.5 % inhibits the growth of E. coli O157 and growth is retarded at 2.5 %. VTEC are very resistant to desiccation and are able to survive many drying and fermentation processes. Outbreaks have been associated with salami and jerky type meat products.

VTEC are facultative anaerobes (able to grow with or without the presence of oxygen). Modified atmosphere packaging has little effect on the pathogen although it is reported that it is inhibited on meat packaged under 100 % CO2.

VTEC are not notably resistant to preservatives and sanitizers typically used in the food industry. Organic acids (acetic and lactic acid) are used in the US to decontaminated beef carcasses.

Thermal resistance

VTECs are not heat resistant organisms. For E. coli O157, D_{57 ^ºC} values of 5 mins, and D_{63 ^ºC} values of 0.5 mins have been reported in meat.

VTEC present on the surface of the product are likely to be inactivated rapidly during cooking, but cells at the center of ground meat products and rolled meat joints will only be inactivated if the center of the product is sufficiently heated. Advice has been given in the US and the UK on the cooking of hamburgers (meat patties, beef burgers) to ensure the complete inactivation of the pathogen. In the US, this advice is that they should reach an internal temperature of 71 ^ºC throughout, and in the UK it is recommended that they be cooked to 70 ^ºC for 2 minutes, or the equivalent, in all parts of every burger.

Control options

The control of VTEC starts on the farm with the implementation of good agricultural practices. This can help reduce the shedding of E. coli O157 from cattle. Good agricultural practices are extremely important for the production of fresh fruits, salad stuffs and vegetables. It is very important to minimize faecal contamination of all food commodities.

Processing

It is safe to assume that raw products of bovine origin (such as fresh meat and raw milk) are potentially contaminated with VTEC and to treat them accordingly using a HACCP approach. Good hygienic practices should be implemented when handling beef carcasses and the controlled use of chilled temperatures will prevent the growth of VTEC in these products. The possible survival of VTEC should also be considered during the development of products such as bovine milk cheeses and fermented meat products. There are published guidelines for producers of such foods, but the use of unpasteurised milk is best avoided.

US regulations require abattoirs and meat processing establishments to implement a step to eliminate E. coli O157:H7 and this can include decontamination. Non-intact raw beef products (as well as intact raw beef products intended to be processed into non-intact raw beef products) found positive for E. coli O157:H7 are considered 'adulterated' and are recalled.

It is important to ensure that heat processes (where appropriate) are designed to inactivate any VTEC. Cross contamination between raw and processed product must be avoided.

Product use

Consumers should be advised of the risks associated with raw meat products, in particular those made from minced/ground meat, and that all beef products need to be thoroughly cooked. Advice has been given on the required internal cooking temperature for burgers (see thermal inactivation). In the US consumers are advised that checking the colour of meat patties or burgers (brown as opposed to pink or red) is not a reliable indication that the product has reached a safe temperature and that they should use a thermometer to check that the required temperature has been reached.

Consumers should be advised to avoid unpasteurised dairy products, juice or cider, and to wash fruit and vegetables well (although washing may not remove all contamination). Vulnerable groups (the young, elderly and the immunocompromised) should be advised not to eat raw or lightly cooked sprouts (such as alfalfa and mung beans).

Legislation

EU regulations have some general requirements for E. coli as an indicator of faecal contamination in some products. These requirements giving maximum levels for E. coli in some products do not pertain specifically to VTEC, but the presence of these organisms in any product that will not receive a heat treatment prior to consumption is unacceptable. The UK Health Protection Agency has issued guidelines for the microbiological quality of ready-to-eat foods and these state that in these products E. coli O157 and other VTEC should be absent in 25 g.

The US Food Code (2005) requires food to be safe and unadulterated and product that will not be heated prior to being consumed would need to be absent from VTEC to conform to this requirement. In addition, E. coli O157:H7 is considered an adulterant in non-intact raw beef products (ground, minced or chopped), as well as intact raw beef products intended to be processed into non-intact raw beef products.

Sources of Further Information

Published

Desmarchelier P.M., Fegan N.
Enteropathogenic Escherichia coli
In Foodborne Microorganisms of Public Health Significance. 6^th Edn. Ed. Australian Institute of Food Science and Technology, Waterloo DC. AIFST, 2003, 267 -- 310.

Bell C., Kyriakides A.
E. coli: a practical approach to the organism and its control in foods.
London. Blackie, 1998.

On the web

Risk profile: Shiga-like toxin producing Escherichia coli in leafy vegetables. Institute of Environmental Science and Research Limited. (February 2006)
http://www.nzfsa.govt.nz/science/risk-profiles/FW0456_STEC_in_leafy_veges_February_2006_Final_version_to_NZFSA.pdf

Risk profile: Shiga-like toxin producing Escherichia coli in uncooked comminuted fermented meat products. Institute of Environmental Science and Research Limited. (February 2003)
http://www.nzfsa.govt.nz/science/risk-profiles/escherichia-coli-meat.pdf

Risk profile for enterohemorragic E. coli including the identification of the commodities of concern, including sprouts, ground beef and pork. Food and Agriculture Organization of the United Nations/World Health Organization. (February 2003)
ftp://ftp.fao.org/codex/ccfh35/fh0305de.pdf

Risk profile: Shiga toxin-producing Escherichia coli in red meat and meat products. Institute of Environmental Science and Research Limited. (August 2002)
http://www.nzfsa.govt.nz/science/risk-profiles/stec-in-red-meat.pdf

Comparative risk assessment for intact (non tenderized) and non intact (tenderized) beef: technical report. US Department of Agriculture's Food Safety and Inspection Service (FSIS) (March 2002)
http://www.fsis.usda.gov/PDF/Beef_Risk_Assess_Report_Mar2002.pdf

Draft risk assessment of the public health impact of Escherichia coli O157:H7 in ground beef. US Department of Agriculture's Food Safety and Inspection Service (FSIS). (October 2001).
http://www.fsis.usda.gov/OPPDE/rdad/FRPubs/00-023N/00-023NReport.pdf

Guidelines for the microbiological quality of some ready-to-eat foods sampled at the point of sale. Health Protection Agency. (September 2000)
http://www.hpa.org.uk/cdph/issues/CDPHvol3/No3/guides_micro.pdf