Ellen Silbergeld, Eng '72 (PhD), recalls that she did not want to go to the seminar. She was a professor of epidemiology at the University of Maryland School of Medicine in 1999 when her department's chairman needed an audience for the seminar's presenter, a candidate for a faculty position. Silbergeld recalls the chairman saying, "Please, just sit in the room. You can come to lunch." So she sat in the room, and something caught her attention. The seminar was on hospital-acquired infections, but the presenter mentioned in passing that some drug-resistant infections came from food. That seemed odd. Silbergeld knew you could pick up Salmonella from, say, tainted chicken salad. But how would that Salmonella have become resistant to antibiotics? She turned to a colleague and asked. Because, he said, factory chicken farms routinely feed antibiotics to their flocks, to accelerate growth, and the drugs generate resistance.
Ten years later, Silbergeld, now a professor of environmental health sciences at the Bloomberg School of Public Health, is one of several researchers at Johns Hopkins and around the world assembling evidence that the industrial farming of chickens, pigs, and cattle is cultivating more than poultry and livestock — it's cultivating bacteria that medicine is losing the ability to fight. Antimicrobial drugs, including antibiotics like penicillin, ciprofloxacin, and methicillin, kill pathogenic bacteria. But they simultaneously drive the resistance that is bacteria's defense, especially when administered in low, subtherapeutic doses. Scientists estimate that 50 percent to 80 percent of all antimicrobials in the United States are not used by doctors to treat sick people or animals but are added to farm animal feed, mostly in such subtherapeutic dosages. Public health researchers like Silbergeld are convinced that this nontherapeutic use of antimicrobials is building dangerous genetic reservoirs of resistance. If they are right, industrial agriculture is fostering and dispersing drug-resistant bacteria that impair medicine's ability to protect the public from them.
The United States Department of Agriculture (USDA) estimates that livestock and poultry produce 335 million tons of manure per year, which is one way resistant pathogens get out of animals and into the environment. That's 40 times as much fecal waste as humans produce annually. Farms use it for fertilizer and collect it in sheds and manure lagoons, but those containment measures do not prevent infectious microbes from getting into the air, soil, and water. They can be transported off the farms by the animals themselves, houseflies, farm trucks, and farm workers, and by spreading manure on other fields. Out in the environment, they form a sort of bank of genetic material that enables the spread of resistance.