Antimicrobials in farming

I wish to applaud Barbara Sibbald for her editorial on antimicrobial use in agriculture and its implications for human health.1 I was interested to learn about the degree to which Canada lags behind other countries in regulating the agricultural use of antimicrobials.

I have 2 comments on the editorial: With respect to the article’s statement that “…quinupristin–dalfopristin, [is] our last line of defence if Staphylococcus aureus or Enterococcus infections develop resistance to vancomycin,”1 I note that vancomycin-resistant Enterococcus has been recognized for over 2 decades;2 strains of S. aureus with intermediate-level resistance to vancomycin were described in 1997, and high-level vancomycin resistant S. aureus was described (outside the laboratory) in 2002.3 The role that agricultural use of avoparcin played in the genesis of vancomycin-resistant Enterococcus in humans is controversial, but recognition of the potential links between avoparcin and vancomycin-resistant Enterococcus likely contributed to European decisions to remove this antibiotic from feeds, as mentioned in the article.4 Fortunately, however, quinpristin-dalfopristin is not the only agent with activity against multiresistant gram positive microbes. Daptomycin and linezolid are available in Canada, and other agents, such as ceftaroline, have been approved in the United States and await approval in Canada.

The editorial did not mention multi-drug resistant gram negative pathogens, including Acinetobacter species, Pseudomonas aeruginosa, and the Enterobacteraceae, including recently described strains with a novel plasmid-borne carbapenemase, some of which have actually reached a point of being effectively pan resistant to available antimicrobials.5 Such microbes are environmentally abundant in agricultural runoff, surface waters and sewage. Particularly given the tendency of resistance determinants to cluster at the microbial level, the environmental contamination associated with agricultural antibiotic use provides a source of selective pressure that gives a competitive advantage to resistant microbes.