Numerous scientific studies provide strong evidence that antibiotic use on farms—as in any other setting—leads to the emergence of resistant bacteria.
A recent article I authored, together with several colleagues from The Pew Charitable Trusts’ antibiotic resistance project, systematically analyzes scientific evidence regarding the link between antibiotic use in animal agriculture and public health. Our review of the literature reaffirms what scientists have known for decades: Antibiotic use in animal agriculture, as in any setting, contributes to the emergence of resistance. While antibiotics are sometimes needed to protect animal health, these findings underscore the importance of discovering ways to minimize the need for antibiotics in food animals.
However, the connection between antibiotic use in animal agriculture and a public health risk caused by drug-resistant bacteria is a complex one, involving multiple steps and transmission routes, with various external factors able to influence how quickly resistance emerges.
Given this complexity, Pew’s analysis explores the essence of this issue in three key questions, synthesizing what is known and the strength of supporting evidence, and highlighting key areas where more information is needed.
- Does antibiotic use on farms and feedlots lead to the emergence of resistant bacteria? Yes.
Are resistant bacteria on farms or feedlots infecting humans? Yes.
- What’s known: Numerous scientific studies provide strong evidence that antibiotic use on farms—as in any other setting—leads to the emergence of resistant bacteria. This research includes randomized controlled trials in which some animals are given antibiotics under tightly controlled experimental conditions while others are not, and bacteria from the exposed and unexposed animals are monitored for the emergence of resistance. It also includes observational studies where researchers draw conclusions by comparing bacteria from animals that have received antibiotics on commercial operations to animals on comparable operations that did not.
- What’s missing: Although the link between antibiotic use and the emergence of resistance is clear, a number of data gaps remain. This makes predicting how quickly exposure to a given antibiotic will lead to resistant bacteria extremely difficult. For instance, some types of bacteria and antibiotics are more prone to the emergence of resistance than others, and sometimes discontinuing the use of an antibiotic leads to a rapid drop in resistance while in others it does not. Some evidence further suggests that how an antibiotic is administered, prior exposure to antibiotics, and other factors such as the type of feed may influence how quickly resistance emerges or disappears.
Are infections with these resistant bacteria worse than if the bacteria were not resistant? Yes.
- What’s known: A variety of studies provide strong evidence that resistant bacteria can be transferred from food-producing animals to humans through direct contact, food, or the environment. For instance, outbreaks of foodborne illness have been traced back to the original source on the farm, where the pathogen was found on the animals and/or in the environment.
Additionally, bacteria can readily share resistance traits with other types of bacteria, which has been demonstrated in test tubes, laboratory animals, and the gut of human volunteers. Consequently, resistance originating even in bacteria that are themselves harmless to humans can pose a public health risk.
- What’s missing: How important each of these transmission pathways is for public health largely remains to be determined. The proportion of resistant pathogens that emerged from harmless bacteria from animal-associated sources and that eventually make humans sick is also not known.
- What’s known: Strong epidemiological evidence demonstrates that infections with resistant foodborne bacteria such as Salmonella or Campylobacter lead to worse health outcomes, including a higher risk of hospitalization and death.
- What’s missing: Why the health outcomes are worse is not clear in all cases and may include delayed treatment onset or the need to choose a less-desirable antibiotic when the bacteria have become resistant to the preferred options. In some cases, resistant bacteria may also have other traits that make them more dangerous to humans.
Scientific findings show a relationship between antibiotic use on farms and a public health risk of drug-resistant infections in people, even if data to quantify the contribution of this compared to other sources, such as human health care, are not sufficient. In spite of the gaps in information, no doubt remains that antibiotic use in animal agriculture contributes to the problem of drug resistance and that these drugs must therefore be used judiciously.
Animal producers and other stakeholders have made important progress in recent years by taking steps such as eliminating the use of medically important antibiotics for growth promotion, expanding oversight by veterinary professionals, and advancing judicious use practices like antibiotic alternatives. But more work is needed to ensure that antibiotic use in livestock settings fully protects animal health while guaranteeing that these drugs remain effective for treating infections in both humans and animals for generations to come.
Karin Hoelzer is a senior officer in health programs supporting Pew’s safe food and antibiotic resistance projects.