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Antimicrobial use and resistance in Canadian cow-calf herds



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Antimicrobial resistance (AMR) is a concern to human health and has been a growing concern for the public and livestock producers in recent years. The existing literature regarding antimicrobial use (AMU) and AMR in cow-calf herds, a critical component of the beef supply chain, is limited and much of it is more than a decade old. The goal of this thesis is to address the existing gaps by examining AMU practices on Canadian cow-calf operations and AMR in two enteric species important to human health: E. coli and Enterococcus. In Chapter 2, a survey was used to collect AMU data from herds across the country. AMU data for the period of July 1, 2019, to June 30, 2020, was collected from 146 herds. Ninety-nine percent (145/146) reported the use of an antimicrobial at least once during the study period; however, frequency of use within herds was low. The antimicrobial most likely to be reported as used by participating herds was oxytetracycline (81%, 118/146). Category I antimicrobials were used at least once by 33% (48/146) of herds, with no herds reporting treatment of more than 30% of animals with a Category I antimicrobial. Factors such as calving season and herd type were shown to influence AMU practices. Overall, AMU practices were similar to previous studies examining AMU in cow-calf herds. In Chapter 3, AMR patterns in fecal E. coli were examined in samples collected from cows and calves in the spring and fall of 2021 from western Canadian herds. In total, 1,551 E. coli isolates were obtained from 809 calves and 746 cows, resulting in an isolation rate of 99.7%. AMR susceptibility testing was completed using the NARMS panel for gram-negative bacteria. Overall, 15% (231/1551) of the recovered isolates were resistant to a single antimicrobial. Resistance was found at least once in nearly every herd (90%, 35/39). Resistance of E. coli to Category I antimicrobials was very infrequent, with tetracycline being the most common resistance target. Calves were more likely to display resistance than cows, with a higher proportion of calves also displaying multiclass resistance. Additionally, calves in the spring were more likely to display resistance compared to calves in the fall. In the fourth chapter, antimicrobial resistance patterns were described for Enterococcus. Enterococcus has not previously been studied in Canadian cow-calf herds. Recovery rates for Enterococcus were good (97%), with 1,505 isolates recovered from 1,555 animals consisting of 809 calves and 746 cows. Resistance of isolates to at least one antimicrobial was 98% in the spring and 96% in the fall. The antimicrobials of quinupristin/dalfopristin and tetracycline were common resistance targets in both cows and calves. When summarized at the herd level, multiclass resistance and resistance to Category I antimicrobials was greater in calves than in cows. AMR resistance in Enterococcus is complicated by questions regarding the role of intrinsic resistance in observed susceptibility data. There are also concerns that current minimum inhibitory concentration (MIC) breakpoints are not accurate for all Enterococcal species further complicating the interpretation of the study findings. Future studies will be required to better understand the prevalence of resistance amongst different bacteria of interest in cow-calf herds.



Antimicrobial use, Antimicrobial resistance, Beef cattle, Cow-calf



Master of Science (M.Sc.)


Large Animal Clinical Sciences


Large Animal Clinical Sciences


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