Antibiotic Use in Agriculture is Helping Drive Antibiotic Resistance in Humans, Experts Say

With new cases of antibiotic-resistant infections turning up in the nation's hospitals, schools, oceans—and even professional sports locker rooms—scientists are facing an uphill battle against harmful bacteria.

While experts agree that antibiotic overuse is one of the key contributors to the recent rise in drug-resistant infections, research is raising the prospect that agricultural use is as important as hospital use and has sparked attention from America's heartland all the way to Washington, D.C.

Speaking 24 February at a Capitol Hill briefing co-organized by AAAS, three top experts warned that the increased use of antibiotics in agricultural feed contributes to drug-resistant infections in humans. But just how extensive that relationship is has yet to be determined.

David Wallinga

David Wallinga, director of the Food and Health Program at the Institute for Agriculture and Trade Policy, said that antibiotics are regularly added to livestock feed to prevent infections and increase animal growth rates throughout Europe and the United States. These drugs are used in high doses in crowded stockyard conditions and are often identical to antibiotics used by humans to combat E. coli and S. aureus (staph).

Wallinga said that these conditions are breeding grounds for antibiotic-resistant infections that can be transmitted to the general population through the food we eat, interactions with agricultural workers, or groundwater and soil from the farms.

"There is a scientific consensus that antibiotics used in agriculture do contribute to rising resistance transmitted to humans," said Wallinga. And, he predicted, that may mean a proliferation of untreatable infections driving up healthcare costs.

Tara Smith

At the afternoon briefing for congressional staffers in the Dirksen Senate Office Building, Wallinga spoke alongside Glenn Morris, director of the Emerging Pathogen Institute at the University of Florida, and Tara Smith, assistant professor of epidemiology at the University of Iowa. Co-organized by the nonprofit Union of Concerned Scientists, the briefing explored the connection between the use of antibiotics in agriculture and the rise in drug-resistant infections in humans.

Joanne Carney, director of the AAAS Center for Science, Technology, and Congress, said that the briefing was timely due to legislation expected to be reintroduced in the Senate later this year. Originally sponsored by Senators Edward Kennedy (D-Massachusetts), Sherrod Brown (D-Ohio), Jack Reed (D-Rhode Island), and Olympia Snowe (R-Maine), the bill would phase out some agricultural antibiotic use over two years and require pharmaceutical companies to document the type and quantity of drugs they sell to livestock feed producers.

As with this bill, Carney said that policymakers frequently rely on expert advice from scientists when drafting public health initiatives.

"With healthcare reform a high priority for the White House and Congress, it is important for policymakers to better understand the science behind antimicrobial resistance in agriculture and its impact to public health," said Carney.

Smith said that while everyone should be concerned about the potential spread of antibiotic-resistant bacteria, Iowa is especially vulnerable due to her state's livestock population and economic dependence on agriculture. The state's 3 million residents co-exist with 92 million farm animals—19 million swine, 65 million chickens, 4 million turkeys, and 4 million cows. In addition, Iowa is the county's leading producer of pork and eggs.

Smith outlined a study published last month in the Public Library of Science in which she led a team surveying swine and agricultural workers to see if they carried antibiotic-resistant bacteria.

The team traveled to seven farms throughout Iowa that comprised a closed farming system—the complete agricultural process from the hog's birth to slaughter—and swabbed the nasal passages of the animals and human workers. Of the 209 nasal swabs obtained from pigs, 70% showed Methicillin-resistant S. aureus (MRSA) bacteria.

In addition, 64% of workers from the farm system tested positive for MRSA. After analyzing the bacteria, the team found that the bacteria were genetically similar, suggesting that there was transmission between humans and animals.

"Due to thousands of animals being confined to a small space, agricultural settings are a reservoir of resistance with a potential to spread dangerous bacteria to the larger community," said Smith. She said that 18,000 deaths and 94,000 infections were attributed to MRSA in 2005.

MRSA was first documented in swine in 2005 when a six-month-old hospital patient in the Netherlands tested positive for MRSA. After learning that her family lived on a pig farm, researchers swabbed the farm's workers, with 23% testing positive—a rate 760 times higher than in the general population. In addition, the swine tested positive for the same MRSA strain. Smith said that other swine-related MRSA has been found in Denmark, Germany, Austria, France, and Canada.

Glenn Morris

While transmission of MRSA between swine and farm workers is cause for alarm, Morris said that researchers disagree about the relative danger it poses to the general population, with many experts attributing the rise in antibiotic-resistance to exposure to people in hospitals.

People interact with agriculture at many points, he said—when they eat meat, drink groundwater tainted by farm runoff, or interact with farm workers. But he said that the bacteria are relatively low-risk as people frequently host MRSA colonies and don't get sick.

Conversely, most people will not be exposed to people colonized with the stronger MRSA found more often at hospitals.

In a paper he said generated more controversy than any other he has written, Morris developed a mathematical model in which he argues agricultural use of antibiotics results in more rapid emergence of resistance in human populations.

This suggests that a large number of people exposed to a small risk through food may generate many more cases of antibiotic resistance than a small number of people exposed to a high risk at hospitals.

"We may not all go to the hospital, but we all eat," said Morris. The implication, he suggested, is that farms may provide a much larger setting for bacteria to evolve and become resistant to antibiotics.

Benjamin Somers

13 March 2009