The concentration and prevalence of Escherichia coli O157 in cattle feces at the time of slaughter was studied over a 9-week period from May to July 2002. Fecal samples (n ‫؍‬ 589) were collected from the rectums of slaughtered cattle, and the animal-level prevalence rate was estimated to be 7.5% (95% confidence interval [CI], 5.4 to 9.6%) while the group prevalence was 40.4% (95% CI, 27.7 to 53.2%). Of the 44 infected animals detected, 9% were high shedders that contained E. coli O157 at concentrations of >10 4 CFU g ؊1 . These 9% represented >96% of the total E. coli O157 produced by all animals tested. All isolates possessed the vt 2 gene, 39 had the eaeA gene, and a further five had the vt 1 gene also. The presence of high-shedding animals at the abattoir increases the potential risk of meat contamination during the slaughtering process and stresses the need for correctly implemented hazard analysis and critical control point procedures.Escherichia coli O157 was first identified as a food-borne pathogen in 1982 during an outbreak that was traced to contaminated hamburgers (20). The pathogen is associated with a range of symptoms, including watery or bloody diarrhea, vomiting, hemorrhagic colitis, and hemolytic uremic syndrome, which is characterized by acute renal failure affecting mainly children and the immunocompromised (7). While the majority of foods linked to human outbreaks of E. coli O157 are not assessed quantitatively, some studies have indicated a low infective dose (1, 26), highlighting the need for stringent control of contamination during food production.Cattle and other ruminants have been established as major natural reservoirs for E. coli O157 (18) and play a significant role in the epidemiology of human infections (7). It has been estimated that 1 to 4% of United Kingdom cattle are infected at slaughter (3,19), although more recently a prevalence rate of 8.6% has been reported from a farm study in Scotland (25). In the United States, breeding herd prevalences of 1% (21) and 9.3% (6) have been recorded, whereas in feedlot animals, rates have varied between 2.8% (4) and 35.8% (5). Prevalences in the summer months were usually greater than in the winter months. A number of environmental and food-borne sources have caused E. coli O157 incidents, with many attributed to the consumption of food of bovine origin (22) or with either direct or indirect contact with cattle and other farm animals (13).The concentration at which E. coli O157 is shed in feces varies from animal to animal as demonstrated in a North American study with calves (29), where a range from 10 2 to 10 5 CFU g Ϫ1 was observed. High-shedding sheep (excreting Ͼ10 4 CFU g Ϫ1 ) were responsible for the New Deer E. coli O157 outbreak in Scotland (16,23). High-shedding animals pose an elevated risk of contaminating the food chain if presented to slaughter. However, little published data are available on the concentration of E. coli O157 in cattle feces at the time of slaughter.The health risk from E. coli O157 and other pathogens is mi...