Assessing the Incidence of Antimicrobial Resistant E. coli and Salmonella Bacterial Isolates in Cull Cows from Ontario Free Stall Dairy Herds

The objective of this project is to determine the incidence of antimicrobial resistant E. coli and Salmonella spp. in cull dairy cows entering the food system as beef using an incidence density case-control observational study design. A second objective is to identify key aspects of facility design and cow management that are associated with herd level antimicrobial drug use and the incidence of resistance development.
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Surveys assessing various aspects of antimicrobial use will be mailed to all producers with free-stall dairy barns and all livestock veterinarians in the province of Ontario. Additionally, 25 free-stall herds are currently enrolled in a longitudinal study in which producers track individual and herd level drug use for a one year period using a paper-based treatment record and a garbage can auditing (GCA) system. Cull cows from the study herds will be assessed with regard to previous exposure to antimicrobial drugs and fecal samples will be collected and cultured for E. coli and Salmonella isolates will be typed. The incidence of resistant isolates in cull cows (cases) will be determined and compared to the incidence in cows remaining in the herd (controls). Cow and herd level data will be collected to be included as predictor variables in a model that will assess the association of the incidence of antimicrobial drug resistant E. coli and Salmonella in cull cows with specific risk factors.