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<ol><li>Determine the prevalence and diversity of Salmonella in multiple environmental locations within dairy farms throughout Texas.</li>
<li>Characterize the antimicrobial susceptibility status of Salmonella isolates across dairy farm environmental locations.</li>
<li>Establish the degree to which environmental contamination reflects carriage of resistant Salmonella among cull dairy cattle.</li></ol>
<p>Dairy cattle are a key reservoir of several Salmonella serovars that are leading causes of human salmonellosis, including multidrug‐resistant (MDR) S. enterica serovar Newport and S. enterica serovar Typhimurium (Gupta et al., 2003; Dechet et al., 2006; Varma et al., 2006; Karon et al., 2007). Administration of antimicrobial agents to dairy cattle and other food animals is considered to be one of the driving factors for antimicrobial resistance among Salmonella and other pathogens, based on exertion of local selection pressure in agricultural settings (Holmberg et al., 1984; Cohen and Tauxe, 1986; Angulo et al., 2000; Threlfall et al., 2000; White et al., 2001). Antimicrobial agents that are currently licensed for use in dairy cattle in the United States include enrofloxacin, florfenicol, and various penicillins, cephalosporins, macrolides, sulfonamides, and tetracyclines; extra‐label use of some additional drugs is also permitted under certain circumstances. However, a number of studies have failed to find conclusive evidence that the use of antimicrobial agents in animal production systems leads to a sustained increase in
antimicrobial resistance among pathogens within the gastrointestinal tract of animals (Ray et al., 2006; Singer et al., 2008; Daniels et al., 2009; Heider et al., 2009; Mann et al., 2011; Morley et al., 2011). In fact, recent experimental work suggests that the environment could play a central role in antimicrobial selection pressure. Excreted ceftiofur metabolites were found to exert substantial selection pressure that promoted resistance in the environment, and cattle subsequently became infected with resistant bacterial strains through environmental exposure (Subbiah et al., 2012; Call et al., 2013). This suggests that there is likely to be a discrepancy among environmental reservoirs in their ability to promote the emergence and persistence of antimicrobial resistance on dairy farms. Identification of high‐risk environmental reservoirs within dairies would provide an opportunity to mitigate the burden of antimicrobial resistance at the pre‐harvest level.</p>