The meat industry defends its reliance on routine antibiotic use by flatly denying the practice poses any public health problem. The view is summed up by this 2010 National Pork Producers Council newsletter: "[T]here are no definitive studies linking the use of antibiotics in animal feed to changes in resistance in humans." The claim, I guess, is that the drug-resistant bacteria that evolve on antibiotic-laden feedlots stay on those feedlots and don't migrate out.
That contention is looking increasingly flimsy. My colleague Julia Whitty recently pointed to a new study showing that a particular antibiotic-resistant pathogen "likely originated as a harmless bacterium living in humans, which acquired antibiotic resistance only after it migrated into livestock." In its new, harmful form, Julia reported, the bacterial strain "now causes skin infections and sepsis, mostly in farm workers."
And humans aren't the only creatures paying the price of routine antibiotic use. A research team from the Pacific Northwest has found that terrestrial pathogens, including strains of E. coli resistant to multiple antibiotics, are now infecting sea mammals. The researchers collected and performed autopsies on more than 1,600 stranded seals and otters over 10 years. They found that infectious diseases accounted for 30 to 40 percent of the deaths. "Comparing the diseases found in marine mammals with terrestrial mammals has identified similar, and in many cases genetically identical disease agents," the researchers report.