Use of rope devices to describe and explain the feedlot ecology of Escherichia coli O157:H7 by time and place.

Escherichia coli O157:H7 is an important pathogen of humans, and cattle populations serve as an important reservoir for human exposure. The organism is ubiquitous to feedlot cattle populations, although the nature of its occurrence is quite dynamic. Why E. coli O157:H7 varies by time and place in fed cattle is poorly understood. This study was designed to describe and explain the occurrence of E. coli O157:H7 by pen-level factors of time and place. From each pen, we cultured seven ropes placed within pens for cattle to rub and chew (ROPES), in order to classify the pens as high or low prevalence in longitudinal studies conducted during the summer and winter feeding periods of 2 full years. We observed differences in occurrence of ROPES-positive pens by season, weeks within season, and feedyard. ROPES-positive pens clustered temporally. Factors associated with ROPES-positive pen-weeks during both the summer and winter feeding periods were feedyard, prior 7-day mean air temperature, recovery of E. coli O157:H7 from the composite fecal sample, and recovery of E. coli O157:H7 from the water tank. Pens of summer-fed cattle were less likely to be ROPES-positive for E. coli O157:H7 if the ROPES were positive for Salmonella spp. The condition of the pen surface was associated with the likelihood for winter-fed pens of cattle to be ROPES-positive. We were able to monitor these pens of cattle using ROPES at minimal cost and without disturbing individual cattle. These observations improve our understanding of the ecology of E. coli O157:H7 in fed cattle, and also illustrate the importance of designing and analyzing observational studies and clinical trials to account for time- and place-dependent variables that affect the probability of detecting E. coli O157:H7.

Use of rope devices to describe and explain the feedlot ecology of Salmonella by time and place.

Salmonella spp. are important zoonotic pathogens, and cattle can serve as an important source of this organism for human exposure through food. This study was designed to describe and explain the occurrence of Salmonella spp. by pen-level factors of time and place using a pen-level test-method previously validated for E. coli O157:H7. From each pen, we cultured seven ropes placed within the pen for cattle to rub and chew (ROPES), in order to classify the pens as ROPES-positive or ROPES-negative each week in longitudinal studies conducted during the summer and winter feeding periods of 2 full years. We observed differences in occurrence of ROPES-positive pens by week within each season such that, at times, a high proportion of pens were of the same ROPES-status even though feedyards were separated by 50-200 km. Factors associated with ROPES-positive pen-weeks during both the summer and winter feeding periods were the condition of the pen surface and recovery of Salmonella spp. from the water tank. The probability for pens of summer-fed cattle to be ROPES-positive for Salmonella spp. increased as the number of cattle in the pen increased and decreased when the pen was ROPES-positive for E. coli O157:H7. Pens of winter-fed cattle differed by feedyard in the probability of their being ROPES-positive for Salmonella spp. We were able to monitor these pens using ROPES at minimal expense, without disturbing individual cattle, and observe important time and place relationships of Salmonella spp. in fed cattle. These observations illustrate the importance of accounting for time- and place-dependent variables that affect the probability of detecting Salmonella spp. when designing and analyzing observational studies and clinical trials.