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Background
E. coli O157:H7 is a member of the enterohemorrhagic E. coli (EHEC) group. These Shiga toxin-producing E. coli are responsible for the majority of very serious, food-related E. coli infections that cause severe damage to the lining of the intestine.
E. coli O157:H7 is very different from generic E.coli, which can be isolated from more than 95 percent of fecal material from all animals, including humans, and is not typically a foodborne pathogen.1 While generic E. coli is present in almost all fecal material, it also survives and grows very well outside the host. Therefore, finding generic E. coli does not necessarily indicate that fecal contamination is present. The focus of this page is on E.coli O157:H7 and other EHEC strains; not generic E. coli.
E. coli O157:H7 was first recognized as a human pathogen in 1982 when it was identified as the cause of two different outbreaks of hemorrhagic colitis. Of the 47 cases in two incidents, 33 individuals (70 percent) required hospitalization. The cause was traced back to undercooked ground beef and identified by serotype. Fresh produce can be a source of E. coli O157:H7, as demonstrated by a 2006 outbreak involving raw spinach, with 199 illnesses, 102 hospitalizations, 31 HUS (a severe kidney syndrome), and three deaths in the US.2
Symptoms and Disease Process
Typical E. coli O157:H7 symptoms include severe bloody diarrhea and abdominal cramps, with little or no fever. Symptoms usually occur three to four days after exposure, but onset ranges from two to nine days and generally last five to 10 days. Individuals can shed the organism for up to two weeks after they recover and become asymptomatic.
Milder illness may occur in some people; however, young children and the elderly are more likely to have severe symptoms. Severe infection can lead to hemolytic uremic syndrome (HUS), which is the leading cause of kidney failure among children in the United States, and may be fatal. HUS may also result in life-long complications.3
Primary Route of Transmission
Cattle are the main source of E. coli O157:H7, while other ruminants also carry the organism (ruminants are hoofed, even-toed, usually horned mammals of the suborder Ruminantia, such as cattle, sheep, goats and deer). It is a natural part of the bovine intestinal flora and is not considered a cattle pathogen. It can be found on a small number of cattle farms and can live in the intestines of healthy cattle. The incidence of carriage is more prevalent for animals raised in feedlot environments compared to pasture-raised animals, where incidence in adult cattle is extremely low.
Contamination of the water through uncontrolled runoff can occur. Therefore, understanding the source of water for irrigation is important when growing produce for fresh consumption. Meat can become contaminated during slaughter and organisms can be thoroughly mixed into beef when it is ground. Similarly, milk can be contaminated if the animal is a carrier.
The infective dose is extremely low. A dose between 0.3 and 15 organisms per gram of food was reported in a 1993 outbreak. Though most often associated with undercooked ground beef, additional sources of transmission include contaminated water, raw milk, unpasteurized apple cider and raw vegetables, specifically sprouts and salad vegetables. Waterborne outbreaks have also occurred, including swimming areas. From 1982 to 1998, the leading modes of transmission in order of frequency were ground beef, person-to-person, vegetable and salad bars, and water/swimming pools.
Control
Several interventions to reduce the prevalence of E. coli O157:H7 are practiced. The beef processing industry has implemented a number of food safety procedures to reduce or eliminate the presence of the organism on meat. Prevention of fecal contamination during slaughter and milking is an important step to minimize contamination, especially for products that will enter the food chain in the raw state. Washing hides prior to their removal has been shown to significantly reduce levels of contamination on the sides of beef. Carcass decontamination strategies include the use of FDA and USDA cleared antimicrobial treatments such as SANOVA® and Inspexx®, to reduce microbial count on meat and poultry surfaces, thermal pasteurization with hot water or steam, and steam vacuuming. These procedures can eliminate or significantly reduce the numbers of the organism on meat and poultry tissue surfaces. The use of FDA and USDA cleared antimicrobial treatments for beef trimmings prior to grinding is also a risk-reduction strategy. Since 1994, any raw ground beef and non-intact cuts of beef discovered to be contaminated with E. coli O157:H7 are considered to be adulterated per USDA regulations.
Thorough cooking will inactivate E. coli O157:H7, and is an effective strategy for cooked meat products. The prevention of cross-contamination from raw beef to cooked or ready-to-eat products is also critical. Proper personal hygiene and thorough hand washing is essential when handling beef since low doses can cause this severe illness.
For produce items that are consumed in the raw state, Good Agricultural Practices that avoid the potential for fecal contamination of crop-growing fields is critical. Produce-specific guidelines that can minimize the potential for product contamination with E. coli O157:H7 and other pathogens have been published for lettuce and leafy greens5 for example. Pasteurization of fruit juices is an effective control measure for these products.
The range of temperatures that support growth of E. coli O157:H7 is somewhat more limited than that for generic E. coli, with a minimum of 46°F (8°C) and a maximum of 113°F (45°C). It is more sensitive to heat than is Salmonella. E. coli O157:H7 is somewhat acid tolerant, and will survive well in fermented and acid foods.7
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References and Further Information
1U.S. Food and Drug Administration ?Bad Bug fact sheets?
2Centers for Disease Control. MMWR 55(Dispatch);1-2
3CDC Escherichia coli O157:H7.
4Commodity Specific Food Safety Guidelines for the Lettuce and Leafy Greens Supply Chain. 2006. (465 KB) 
5International Commission on Microbiological Specifications for Foods. Microorganisms in Foods 5, Characteristics of Microbial Pathogens. Blackie Academic and Professional, New York. 1996.
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