The increased use of antibiotics for medical purposes is the primary factor in the growing problem of ever-hardier bacteria. But researchers have suspected for some time that the administering of drugs to livestock might also be contributing to the emergence of antibiotic-resistant infections in humans. Determining a cause-and-effect relationship for this subtler scenario is difficult, however. To that end, findings published today in the Proceedings of the National Academy of Sciences could prove insightful. According to the report, antibiotic use in farm animals may cause antibiotic resistance in humans to appear earlier than it would were the drugs not administered.

Nearly 80 percent of the antibiotics produced in the U.S. are used for agricultural purposes, sometimes to treat disease but often to promote growth in otherwise healthy animals. David L. Smith of the University of Maryland School of Medicine and colleagues attempted to quantify the consequences of using the same antibiotic in food animals and to treat people. Their mathematical model predicted that although agricultural antibiotic use has a small effect on the ultimate prevalence of antibiotic-resistant bacteria, it could hasten the emergence of resistance. The authors note that their model "indicates that the greatest impact occurs very early in the emergence of resistance, when [antibiotic-resistant] bacteria are rare" and possibly below the detection limits of current surveillance methods.

The results suggest that restricting the administration of antibiotics to livestock before resistance is established in humans could hinder the development of new antibiotic-resistant strains. Once such a strain is entrenched, however, agricultural use most likely has little effect, according to the model. As Marc Lipsitch of the Harvard School of Public Health and colleagues observe in an accompanying commentary, the theoretical results "support the adage that once the horse has fled the barn, it is too late to close the door. On the other side, their results also point to the role antibiotic use in food animals may have had in unlocking if not fully opening that door."

The researchers highlight some of their simplifying assumptions and caution that their model should not be taken as a precise risk assessment. Nevertheless, they conclude that regulating early agricultural antibiotic use "would likely extend the period that a drug can be used effectively in humans and reduce the demands for new antibiotics that must undergo an expensive discovery and approval process."