The Need for International Standards.

Unfortunately, Denmark’s comprehensive reform of antibiotic use in agriculture doesn’t necessarily mean Danes are safe from antibiotic-resistant pathogens carried in animals or meat. That’s illustrated by recent work⁴⁰ by Yvonne Agersø tracking the emergence of bacteria that carry a gene for the production of extended spectrum β-lactamase (ESBL) enzymes, which confer resistance to both penicillins and cephalosporins.

Cephalosporins have been widely used as growth promoters in chickens in some parts of the world, but were never used on Danish poultry. Yet recent data show a dramatic rise in the incidence of ESBL-producing Escherichia coli bacteria in chicken meat sold in Denmark. In 2012 testing showed that 61% of samples of imported chicken were contaminated with ESBL-producing E. coli, but the same kinds of microbes also were identified in 36% of samples from poultry raised in Denmark, even though the chickens had never received cephalosporins.⁴¹

Agersø and her colleagues tracked the resistant microbes back through generations of birds.⁴⁰ The grandparents of the contaminated Danish chickens had been imported from Scotland, where they were treated with cephalosporins very early in life, and resistant bacteria passed from one generation to the next. A Swedish team recently reported similar findings for chickens in that country.⁴² The findings point up the need for international standards restricting the agricultural use of antibiotics.