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Report Says Ag Antibiotic Use and Impact on Resistance Are High

A recent report suggests that use of antibiotics in animal feed is a large contributor to the increase in resistance to these drugs. (James A. Sugar/Corbis)

Nontherapeutic administration of antibiotics to livestock dominates how these drugs are used—exceeding their use in human medicine, according to estimates in a new report from the Union of Concerned Scientists (UCS), a public interest group headquartered in Cambridge, Mass. The findings "suggest that agricultural use of antibiotics is likely to be a larger part of the antibiotic resistance problem than is currently thought," says study director Margaret Mellon of the UCS office in Washington, D.C.

"When we began looking into antibiotics and meat production, we assumed that someone in the government had the data," Mellon says. "But, except for pesticidal uses of antibiotics, no one did." In any case, the UCS report, "Hogging It: Estimates of Antimicrobial Abuse in Livestock," is based on an analysis of information from publicly available sources, including the Food and Drug Administration (FDA), the Centers for Disease Control and Prevention (CDC), and the U.S. Department of Agriculture. It concludes that the generally accepted picture of antibiotic usage rests on a "major misconception," she says. "It is livestock use, not human medicine, that is hogging antibiotics."

FDA Planning To Halt Ag Use of Two Antibiotics

FDA officials were set to convene a public meeting to consider related issues late in January, part of a continuing discussion of a 1999 agency proposal to change how it regulates the use of antibiotics in agricultural settings. Late last year, for instance, agency officials announced plans for withdrawing two fluoroquinolone antibiotics from use by poultry farmers to treat respiratory problems in chickens and turkeys—the first time it acted to control veterinary use of antibiotics to which bacteria have grown resistant (ASM News, January 2001, p. 9).

Meanwhile, many experts have cited 50 million pounds as a consensus figure when estimating the total amount of antibiotic usage for medical and agricultural purposes in the United States. Although the UCS report revises that figure downward to 35 million pounds, it concludes that 70% of all antibiotics goes each year for nontherapeutic treatments of livestock—principally to promote the growth of cattle, hogs, and poultry. Moreover, the report's estimate of antibiotics going for clinical uses is reduced sharply from 32-33 million pounds to 3 million pounds, making that use less than 10% of the overall total, according to Charles Benbrook, a consultant from Sandpoint, Idaho, and a coauthor of the UCS report. That figure was extrapolated from CDC estimates of total prescriptions written, with an average 8 g of antibiotics per prescription for patients being treated in hospitals or in their communities.

The use of antibiotics as growth promotants and in other subtherapeutic dosing regimens to treat livestock and poultry is at the center of this controversy. Many public health experts argue that low doses of antibiotics enable initially rare mutant bacteria to develop resistance and become a public health threat. However, veterinarians, ranchers, feed suppliers, drug manufacturers, and others have argued that such low-dose usage of antibiotics contributes little in the overall development of resistance. They contend that improper uses of such drugs in hospital and community settings represent the chief source of resistance development among microbial pathogens.

Amid this debate is uncertainty about how much of those drugs is being produced and where they are being used. "We stand by our numbers, which are based on direct numbers from companies," says Carole Throssell, a spokeswoman for the Animal Health Institute (AHI) in Washington, D.C., a trade organization representing companies that produce animal health products. "We are trying to do everything we can to make sure that antibiotics are used prudently and safely." According to AHI, 17.8 million pounds of antibiotics are used each year on farm animals, with 14.8 million pounds, or 83%, used therapeutically, to treat sick animals or for prophylactic treatments, and the remainder used as growth promotants. "We agree with [UCS] that the 50-million-pound total needs to be looked at again," she says.

"I'm delighted that attention is being given to the problem because we don't have this data and need to get it," says Stuart Levy of Tufts University Medical School in Boston, Mass. "The numbers are important, so is how, why, and where antibiotics are used, and which microorganisms are involved with what drug combinations. But whatever the numbers, we also know there is plenty of misuses of antibiotics in people. So, as strong as I've been on this issue, I get upset when we try to put too much of the blame [for resistance] on the animal [uses of antibiotics]."

"It is nice to have some numbers," agrees Julian Davies, another expert in antibiotic resistance issues, who is at the University of British Columbia in Vancouver, British Columbia, Canada. "It is not clear that they are completely correct, but close enough!" No matter how many tons of antibiotics are being used in agriculture, he adds, their impact on antibiotic resistance is "still too much We are past the point of no return whichever way you look at it. The critical thing for the future is to keep new antibiotics out of nonhuman uses."

"There is no question that overuse of antibiotics, whether in human medicine or in agriculture, is a serious problem," says Abigail Salyers of the University of Illinois. "Unfortunately, many of the physician/medical microbiology persuasion think that antibiotic resistance is a problem confined to and originating in hospitals—or maybe a little bit in the community. It is important to ask how much the nonmedical uses of antibiotics affect resistance levels. The UCS report is a start, but it is going to take a lot more to convince people . . . that the ecology of resistance is the critical issue."

Levy, Salyers, and other experts in this field who formed the Alliance for the Prudent Use of Antibiotics several years ago say that these issues are further complicated because antibiotic resistance genes are being redistributed in nature. Moreover, some antibiotics reside stably in the environment after they have been used—no matter whether that use was in clinical or agricultural settings, according to Levy. "So, again, it's not just a matter of numbers, but what is the `life' of antibiotics after treatment."

Jeffrey L. Fox