Transmission of and Infection with E. coli
How does food become contaminated with E. coli?
Beef and dairy cattle are known reservoirs of E. coli O157:H7 and non-O157 Shiga toxin-producing strains of E. coli. [58, 59]
In reviews of STEC occurrence in cattle worldwide, the prevalence of non-O157 STEC ranged from 4.6 to 55.9% in feedlot cattle, 4.7 to 44.8% in grazing cattle, and 0.4 to 74% in dairy cattle feces. The prevalence in beef cattle going to slaughter ranged from 2.1 to 70.1%. While most dairy cattle-associated foodborne disease outbreaks are linked to milk products, dairy cattle still represent a potential source of contamination of beef products when they are sent to slaughter at the end of their useful production life (termed “cull” or “spent” dairy cows); this “dairy beef” is often ground and sold as hamburger.
The high prevalence of E. coli O157 and non-O157 STEC in some cattle populations, combined with the lack of effective on-farm control strategies to reduce carriage, represents a significant risk of contamination of the food supply and the environment. Non-O157 STEC are also harbored in other ruminants, including swine. 
How do beef products become contaminated with E. coli?
Numerous Shiga toxin-producing E. coli serotypes known to cause human illness are of bovine origin, thus putting the beef supply at-risk. Both E. coli O157:H7 and non-O157 STEC may colonize the gastrointestinal tract of cattle, and potentially contaminate beef carcasses during processing. Although not as well studied, the risk factors for contamination of beef products from cattle colonized with non-O157 STEC are probably the same or very similar to E. coli O157:H7. For example, cattle hides contaminated with E. coli O157:H7 during slaughter and processing are a known risk factor for subsequent E. coli O157:H7 contamination of beef products. One study showed that the prevalence of non-O157 STEC (56.6%) on hides is nearly as high as that found for E. coli O157:H7 (60.6%). 
A review of published reports from over three decades found that non-O157 STEC were more prevalent in beef products compared with E. coli O157.  In this study, the prevalence of non-O157 STEC ranged from 1.7 to 58% in packing plants, from 3 to 62.5% in supermarkets, and an average of 3% in fast food restaurants. In a recent survey of retail ground beef products in the United States, 23 (1.9%) of 1,216 samples were contaminated with non-O157 STEC.  In another study, researchers found a 10 to 30% prevalence of non-O157 STEC in imported and domestic boneless beef trim used for ground beef. 
Environmental Sources of E. coli
E. coli O157:H7 bacteria and other pathogenic E. coli are believed to mostly live in the intestines of cattle, but these bacteria have also been found in the intestines of chickens, deer, sheep, and pigs. [1, 35] A 2003 study on the prevalence of E. coli O157:H7 in livestock at 29 county and three large state agricultural fairs in the United States found that E. coli O157:H7 could be isolated from 13.8% of beef cattle, 5.9% of dairy cattle, 3.6% of pigs, 5.2% of sheep, and 2.8% of goats.  Over seven percent of pest fly pools also tested positive for E. coli O157:H7.  Shiga toxin-producing E. coli does not make the animals that carry it ill, the animals are merely the reservoir for the bacteria.  
Food Products Implicated in Previous Non-O157 STEC Outbreaks
There is a paucity of information on the vehicles of transmission for human non-O157 STEC infections, but contaminated raw dairy products, produce, and water have been implicated in the United States.  A review of non-O157 STEC in Connecticut showed that exposures, including ground beef, were similar in both non-O157 STEC and E. coli O157:H7 cases, suggesting that the routes of transmission are similar (CDC 2007). Considering the relatively high prevalence of both E. coli O157:H7 and non-O157 STEC in cattle populations and their products, it is not surprising that ground beef and other beef products could be a common food vehicle.
Non-O157 STEC outbreaks attributed to ground beef and its sausage products have been documented outside the United States including Argentina, Australia, Germany, and Italy. These beef-related outbreaks involved 8 STEC serogroups (O1, O2, O15, O25, O75, O86, O111, and O160). HUS cases were reported in five of the six outbreaks, mostly striking children and the elderly.