Antimicrobial Use and Resistance in Enteric Bacteria

Antimicrobial drugs are commonly used in food animal production for the treatment of clinical disease and for the enhancement of performance. Recently, the use of antimicrobial drugs in food animals has been implicated as an important cause for the emergence of antimicrobial-resistant bacteria of public health concern, and there has been increasing scrutiny of antimicrobial drug use in animals because of perceived abuse. We believe that livestock producers and veterinarians have a strong desire to continue to be good stewards of public health and produce wholesome food products. However, objective information is desperately needed regarding the impact of antimicrobial drug use practices in agriculture which can be used to make science-based recommendations for management changes. The overall objective of this research is to quantify the effect of common antimicrobial use practices on the development and dissemination of anti microbial-resistant bacteria in feedlot cattle populations.
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The specific aims are to quantify changes in the prevalence of antimicrobial resistance in Salmonella and non-type-specific E. coli (NTSEC) recovered from the feces of feedlot cattle after they are treated with antimicrobial drugs (orally and/or parenterally), to compare the prevalence of antimicrobial resistance in enteric Salmonella and NTSEC between cattle that are treated with antimicrobial drugs and cattle that are managed without antimicrobial drug exposure, and to determine whether returning feedlot cattle to their pen of origin after therapeutic treatment with antimicrobial drugs is associated with increasing prevalence of resistant Salmonella and NTSEC recovered from feces in those pens. Prospective longitudinal studies of cattle in a commercial feedlot will be used to accomplish the objectives and specific aims of this research using both individual and pen level observations. Samples will be collected at multiple time points throughout the feeding period to allow comparisons over time in addition to the comparisons among exposure groups. Thirty pens of cattle will be enrolled in the study during each year of this 3 year project. Salmonella and NTSEC will be recovered from fresh fecal samples that are collected from the pen floor, as well as by rectal retrieval from individually identified cattle. All fecal samples will be cultured for the presence of Salmonella and NTSEC. The antimicrobial susceptibility profile of all isolates will be determined using the panel of antimicrobial drugs used by the NARMS-EB. The association between antimicrobial drug treatment and the occurrence of antimicrobial resistance will be evaluated using multivariable analyses.