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. Last Updated: 07/27/2016

Report: Animal Drugs Pose Threat

BALTIMORE -- The widespread use of antibiotics on farm animals may be shortening the length of time the drugs are useful in treating human disease, according to researchers from the University of Maryland School of Medicine.

In a report published Tuesday in a national scientific journal, the researchers said it is difficult to say how large a role farms are playing in the growing crisis of antibiotic resistance in the treatment of infections.

But David Smith, an assistant professor of epidemiology, said there is growing evidence that the heavy use of antibiotics on chickens, pigs and cattle is shortening the "honeymoon period'' before drugs lose their effectiveness in people.

For this reason, new antibiotics should be reserved exclusively for human use until they are no longer effective in people, he said.

"We think the most important policy implication is that you shouldn't use antibiotics as growth promoters in animals before you use those drugs for humans,'' Smith said.

Over time, antibiotics inevitably lose their effectiveness as bacterial strains evolve that are genetically resistant. The overuse of antibiotics by doctors is generally seen as a chief culprit in the development of bacteria strains that are difficult or, in some cases, impossible to treat.

The role played by farms has been hotly debated. According to the Union of Concerned Scientists, about 70 percent of all antibiotics produced in the United States are fed to healthy livestock to promote growth.

The researchers used a computer model to assess the role played by farms in the problem of antibiotic resistance in humans. Smith said the paper, published in this week's Proceedings of the National Academy of Sciences, was not intended as a policy statement.

"The point is that the farm or the hospital is an evolutionary incubator, a place where the ideal circumstances exist for antibiotic resistance to evolve.''

The U.S. Food and Drug Administration is planning hearings on its proposal for a regulatory ban on the use of flouroquinolones, a family of antibiotics that includes Cipro, by poultry farmers. The proposal is being challenged by Bayer, which manufactures the drug.

Ellen Silbergeld, a co-author who is now a professor of environmental science at the Johns Hopkins University's Bloomberg School of Public Health, said it is unclear what effect the paper might have on that decision. While agricultural use clearly plays a role, measuring that role remains difficult.

There are several ways that drug-resistant bacteria in animals can cause problems in humans, she said: People can acquire the bacteria by eating meat, but they might also be exposed through animal waste that gets into the water and air.