Global demand for meat is expected to nearly double as the human population surpasses 9 billion in 2050. If these demands are to be satisfied, livestock producers must continue to expand production through sustainable intensification. For beef, this means an increase in the use of high density intensive feedlot operations. Such systems transport cattle from vast geographical distances and concentrate them in pens containing up to 200 head, a practice that increases the likelihood of disease transmission. Cattle in feedlots are also fed high-grain diets, a practice that improves production efficiency, but also increases the risk of bloat, acidosis and the development of liver abscesses. To reduce the occurrence of these diseases, subtherapeutic antibiotics are commonly included in the diet of North American feedlot cattle, a practice increasingly scrutinized by consumers. However, many of the antibiotics used for this purpose are not the same as those used to control bacterial infections in humans.Furthermore, antibiotic resistant bacteria can be isolated from the intestinal tract of cattle and humans that are not receiving antibiotics and administration at subtherapeutic levels does not always result in resistance. Perhaps of greater importance is the nature of the mobile genetic elements that transfer resistance to multiple antibiotics among bacterial species. Transfer of these elements to bacteria that cause infection could have serious implications for the efficacy of antibiotic therapy for both humans and livestock. To date, there is no evidence that the emergence or transmission of these mobile elements is associated with the subtherapeutic use of antibiotics in beef cattle.
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