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Animal Antibiotics Under Scrutiny

Studies confirming animal-to-human transmission of drug-resistant bacteria boost concerns

by Britt E. Erickson
July 22, 2013 | A version of this story appeared in Volume 91, Issue 29

Credit: Shutterstock
Recently weaned piglets are routinely given antibiotics in their water to prevent bacterial infections.
Photo shows a group of young piglets drinking water at pig breeding farm.
Credit: Shutterstock
Recently weaned piglets are routinely given antibiotics in their water to prevent bacterial infections.

A growing body of scientific evidence suggests there is a link between the routine use of antibiotics in healthy farm animals and the rise in drug-resistant bacteria in humans. Until recently, however, it has been difficult to prove that such bacteria can be transmitted from livestock to humans.

A pair of groundbreaking studies now confirms that animal-to-human transmission of drug-resistant bacteria can occur. This evidence is putting new pressure on the Food & Drug Administration to go beyond its voluntary calls for limits on antibiotic use in animal production. It’s also giving some members of Congress more reason to push for legislation that addresses antibiotic use in animals.

In one study published earlier this month, researchers found a strain of drug-resistant Staphylococcus aureus bacteria associated with livestock in the noses of workers in North Carolina who handle antibiotic-fed animals but not in the noses of those who handle antibiotic-free animals (PLoS One 2013, DOI: 10.1371/journal.pone.0067641).

The researchers tested S. aureus isolates from the workers’ noses for resistance to a range of antibiotics and for genetic markers that suggest the bacteria came from livestock. These strains are resistant to multiple antibiotics, “including antibiotics that are used to treat human infections,” says Christopher D. Heaney, a professor of environmental health at Johns Hopkins Bloomberg School of Public Health and corresponding author of the paper.

In another study published earlier this year, a different group of researchers reported finding similar strains of drug-resistant bacteria in industrial livestock workers in Iowa (EMBO Mol. Med. 2013, DOI: 10.1002/emmm.201202413). That team used whole-genome sequencing to identify isolates of the superbug methicillin-resistant S. aureus (MRSA) in farmers and their livestock.

Scientists are concerned that drug-resistant S. aureus associated with livestock will find its way beyond livestock workers and into the community at large. S. aureus, commonly called staph, causes numerous illnesses in humans, including minor to life-threatening skin, blood, and respiratory infections.

Industry maintains that routine feeding of antibiotics to farm animals is needed to prevent disease and continue producing safe food. Antibiotics are commonly fed to chickens, pigs, and cows to boost production and promote animal growth in what many public health advocates say are overcrowded and unsanitary conditions.

Late last month, Mike Apley, a clinical pharmacologist at Kansas State University and a member of the National Cattlemen’s Beef Association, briefed congressional staffers about the use of antibiotics in the beef industry and clarified what he called “misconceptions and outright misrepresentations” about the way antibiotics are used in the cattle industry.

“Producers use antibiotics under the guidance of a veterinarian, and extensive regulations govern the use of animal health drugs,” Apley says. “Cattle producers and veterinarians utilize many tools including vaccines, herd health management, genetics, and animal nutrition.”

For its part, FDA believes that a voluntary approach will phase out medically unnecessary uses of antibiotics in farm animals faster, with less disruption to industry, than a ban. But a handful of lawmakers are not convinced that industry will go along with FDA’s wishes and have been calling for regulations.

The latest studies showing that drug-resistant bacteria can be transmitted from livestock to workers have strengthened congressional efforts to push for legislation that would ban the use of antibiotics in healthy animals. The studies have also prompted some lawmakers to call for measures to force FDA to make data on animal antibiotic use publicly available.

“Every new study that confirms the public health threat from antibiotic overuse in American livestock makes the FDA’s hands-off approach more indefensible,” says Rep. Louise M. Slaughter (D-N.Y.). Slaughter has repeatedly introduced legislation in the House of Representatives to restrict the use of antibiotics in animals.

Thus far, Slaughter’s bills have not seen much movement. Her latest attempt, the Preservation of Antibiotics for Medical Treatment Act (H.R. 1150), was introduced in March. The bill may get some extra attention because of the latest studies. “If FDA won’t protect Americans, then Congress has to step up to the plate by passing my bill to stop the overuse of antibiotics in food animals,” she says.

Late last month, Sen. Dianne Feinstein (D-Calif.) introduced similar legislation in the Senate. That bill, the Preventing Antibiotic Resistance (PAR) Act (S. 1256), would require animal producers to show that antibiotics are used to treat diseases, not fatten livestock. Such restrictions would apply only to antibiotics that are used in human medicine. Producers would be able to use any antibiotic to treat sick animals.

“When antibiotics are fed in low doses to animals, only the strongest, most resistant bacteria are left behind to reproduce,” Feinstein says. “By the time these resistant pathogens make their way from the animals into our communities, the infections [in humans] can be costly to treat or untreatable altogether.”

Hundreds of public health and environmental advocacy groups support Feinstein’s bill. “We have entered into an age where common bacterial diseases are once again becoming untreatable,” says Steven Roach, a senior analyst with Keep Antibiotics Working, a coalition of advocacy groups calling for restrictions on antibiotics in farm animals. “The PAR Act is desperately needed to reduce the dangerous overuse of antibiotics on farms, which leads to the emergence and spread of superbugs.”

Drug-resistant infections in humans are on the rise. They result in more severe illness and greater risk of death. But it is unclear to what degree antibiotic use on industrial animal farms is contributing to the problem.

The situation is particularly difficult to assess because FDA has been reluctant to share data with the public on the use of antibiotics in food-producing animals. Bills introduced in both the House and Senate would make that information publicly available, but they haven’t gained much traction.

In the House, Rep. Henry A. Waxman (D-Calif.) introduced the Delivering Antimicrobial Transparency in Animals Act (H.R. 820) in late February. The bill is similar to the Antimicrobial Data Collection Act (S. 895), introduced in the Senate in May by Sen. Kirsten E. Gillibrand (D-N.Y.). Both bills would require FDA to publicly report annual sales of antibiotics used in industrial animal farms.

Proponents of the bills tried to persuade lawmakers to include them as amendments in recently passed legislation that reauthorizes FDA to collect fees from the animal drug industry. Those attempts failed, however.

It remains unclear whether Congress will pass any of the current legislation related to antibiotics in farm animals this year. But because of the increasing awareness of the issue, consumer demand for antibiotic-free meat is growing. Such consumer pressure could turn out to be as effective as a federal ban in getting industry to stop using antibiotics on healthy animals.


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