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Prevalence of Escherichia coli O157 on cattle entering the slaughter floor may range from 10 to > 70%. This study was conducted to determine the effect of E. coli O157 prevalence in fecal pats collected from feedlot pen floors on subsequent E. coli O157 prevalence on carcasses at various points in the slaughter process. Fecal pats from the feedlot pen floor were collected within 3 days before slaughter. During cattle processing at the slaughter facility, additional samples were collected from the hide, from the colon, and from the carcasses before and after evisceration and after final decontamination. Of 15 lots (a group of cattle from the same pen from a feedlot) sampled, 87% had at least one positive fecal pat from the feedlot floor, 47% had a positive hide sample, 73% had a positive colon/fecal sample, and 47% had a positive carcass sample preevisceration; however, only 8% of lots had a positive carcass sample postevisceration or after final intervention. Of the total samples tested (n = 1,328), 24.7, 14.7, 27.6, 10.1, 1.4, and 0.3% of fecal pats from the feedlot floor, hide, colon, preevisceration, postevisceration, and final intervention samples, respectively, were positive for E. coli O157. Pens with greater than 20% positive fecal pats from the feedlot floor had 25.5% hide, 51.4% colon, and 14.3, 2.9, and 0.7% carcass samples positive at preevisceration, at postevisceration, and after final intervention, respectively. However, fecal pats from feedlot floor samples that contained less than 20% positive fecal samples showed lower pathogen prevalence, with 5.0% hide, 7.5% colon, and 6.3, 0, and 0% carcass positive samples at preevisceration, postevisceration, and post-final intervention, respectively. Data from this study can be used as part of risk assessment processes in order to identify mitigation strategies to minimize prevalence of E. coli O157 on fresh beef carcasses.

Determination of Escherichia coli O157 prevalence immediately prior to shipment and harvest is an important facet of the ecology of this organism, which requires further elucidation. As part of a larger study to measure the effects of within-pen prevalence of E. coli O157 on subsequent carcass contamination, fecal samples from 15 pens of cattle in each of 12 different feedlots in three states (Colorado, Nebraska, and Montana) were collected from June through September 2002. Thirty fresh fecal samples were collected from each pen floor within 36 h of shipment to slaughter. Fecal samples underwent standard enrichment, immunomagnetic separation, and isolation procedures for E. coli O157. Multivariable logistic regression was used to determine which factors best predicted pen-level positive culture results, and to estimate the magnitude of association between each factor and the outcome, while adjusting for other factors in the model. Thirteen (86.7%) of the 15 pens had at least one positive sample, and the within-pen prevalence of E. coli O157 in positive pens ranged from 3.3% to 77.8%. The odds of E. coli O157 positive fecal samples from cattle fed brewers grains were six times that for cattle not fed brewers grains. The odds of E. coli O157 positive fecal samples from pens of cattle from Central Nebraska was nine times that for pens of cattle from Eastern Colorado. These data demonstrate that the presence of E. coli O157 in fecal samples from finished feedlot cattle is associated with feeding of brewers grain and geographic location. Additional studies to further characterize geographic distribution of E. coli O157 and to investigate pen-level intervention strategies should be conducted.