| The research described in this thesis investigated antimicrobial (AM) use practices in beef production in Canada, as well as risk factors for antimicrobial resistance (AMR) in generic Escherichia coli isolated from beef cattle. A study of volunteer cow-calf and feedlot producers was conducted to collect farm-level AM use information, to collect samples for E. coli isolation and susceptibility testing, and to evaluate risk factors for AMR. Oxytetracycline, penicillin, macrolides, florfenicol, and spectinomycin were AMs most commonly used by injection; most commonly used in-feed were monensin, tylosin, lasalocid, and tetracyclines. No isolates were resistant to ceftriaxone, ciprofloxacin, gentamicin, or nalidixic acid, and < 1% of the pooled isolates were resistant to ceftiofur. On feedlots, routine use of tetracycline in-feed was a risk factor for streptomycin, sulfamethoxazole, and tetracycline resistance. Stochastic models, based on data from research studies supplemented with national cattle statistics and industry information, were used to estimate national group non-therapeutic use (including metaphylactic, prophylactic and growth promotion uses) for the calendar year 2007. AMs designated by Health Canada as of very high importance in human medicine comprised 0% of estimated total use, whereas AMs designated as of low importance in human medicine comprised > 66% of total use. The majority of estimated use occurred during the feedlot stage of production (69%). A mathematical model was built to estimate the average probability that cattle complete the feedlot stage of beef production colonized with ceftiofur-resistant E. coli under conditions involving various ceftiofur use practices, different beef production stages, and Canadian beef production conditions. The model estimated an average of 900 kg of ceftiofur was used in 2007 in beef production in Canada (5th percentile 300 kg; 95th percentile 1,921 kg). The probability that a feedlot animal would be colonized with ceftiofur-resistant E. coli at the end of the feedlot stage of production was 1.40 x 10-4 (5th percentile = 1.05 x 10-7; 95th percentile = 7.09 x 10-4). Nationally, this corresponded to 511 feedlot animals in 2007 carrying ceftiofur-resistant E. coli at the end of the feedlot stage (5th percentile = 0; 95th percentile = 2,601). |
