SR9095 - Antimicrobial Resistance in Selected Bacteria of Beef Cattle and Associations with On-Farm Antimicrobial Use

Researcher:

Dr. Scott McEwen, Dept. of Population Medicine, University of Guelph

Objectives:

1. To identify the prevalence of antimicrobial resistance in fecal bacteria of beef cattle on selected cow-calf and feedlot farms.
   

2. To identify the relationship between observed resistance and antimicrobial use on those farms.
   

3. To conduct an intensive study on a small number of farms to identify the incidence, persistence and spread of resistance among individual animals.

Expected Benefits:

1. The proposed research will determine the associations between antimicrobial use on beef farms and antimicrobial resistance in enteric bacteria.
   

2. To provide information for qualitative and quantitative risk assessments of the attendant human health risks.

Summary of Research Results:

Accurate quantities of antimicrobials were necessary for understanding antimicrobial resistance patterns in generic fecal Escherichia coli collected from the same farms. For this project, antimicrobial use was documented descriptively at the farm level (to indicate the number of farms using the various antimicrobial products), and more quantitatively the mass (kg) of active ingredient for the various antimicrobial products was determined, and evaluated for the various farm operation types as measures of different consumption patterns for the main stages of beef cattle production. Further evaluation into rates of antimicrobial usage taking into consideration farm herd sizes and length of time in the study are underway. Key findings were that on all farm types, the predominant injectable antimicrobial used was oxytetracycline, whereas fluoroquinolone and cephalosporin use were minimal. Penicillin was commonly used at greater than the labeled dose on all farm types.

Interesting points to note from the available antimicrobial susceptibility testing results (of the generic E.coli isolates) were that the percentage of resistant isolates was the highest for sulphamethoxazole and tetracycline (the highest being tetracycline) and that this was consistent for both the pooled farm samples as well as the individual animal samples. Also to note was that resistance was relatively low to the cephalosporins, ciprofloxacin and nalidixic acid. On a very general level, this pattern corresponds to the antimicrobial usage information for tetracycline, cephalosporins and fluoroquinolones. Further exploration of the resistance patterns in relation to antimicrobial usage is underway.

Finally, a collection of enteric bacteria from individual animals and pooled samples has been established, for further analysis of genetic markers of antibiotic resistance, pending funding from other sources.

It is important to recognize that the study farms were a volunteer population and care must be taken not to generalize these results to all beef farms in Ontario.