Livestock's role in antibiotic resistance.

Increasingly, infectious disease specialists have been campaigning against excessive use of antibiotics. They argue, the greater the exposure of bacteria to these drugs, the greater will be the chance that the microbes become resistant to them. While public attention has focused on the prescription practices of doctors, about one-quarter of the antibiotics dispensed in the United States aren't targeted at diagnosed disease. Instead, they're administered in sub-therapeutic doses to promote weight gain in apparently healthy livestock.

In the past year, several strains of pathogenic bacteria with resistance to nearly ail known antibiotics have emerged. Because most human antibiotics are also administered to animals, the National Research Council (NRC)and the Institute of Medicine (IOM) in Washington, D.C., convened an expert panel to explore drug use in livestock--especially growth-promoting, subtherapeutic applications--as a factor behind antibiotic resistance in foodborne bacteria.

The panel reports that cases of antibiotic-resistant human disease have "clearly occurred" due to bacteria from livestock treated with the drugs. Data indicate that growth-promoting use of antibiotics has fostered at least some of that resistance, says panel member George W. Beran, a veterinarian at Iowa State University in Ames.

However, Beran observes, the extent to which agricultural use is diminishing antibiotics' utility in fighting human disease "has not yet been quantified with hard data." It remains unclear whether the documented cases reflect a widespread problem or just a few isolated outbreaks, he says. Indeed, the new report concludes that antibiotic use in livestock "does not appear to constitute an immediate public-health concern," but it cautions, "additional data might alter this conclusion."

Hoping to resolve the uncertainties, the panel calls for a standing task force. Its members, to be recruited from both human and veterinary medicine, would be charged with collecting and analyzing data not only on the emergence of resistant strains of bacteria but also on the ways and the places that specific antibiotics have been used.

In the meantime, the panel calls for "propitious use of subtherapeutic antibiotics," Beran says--which means, "if there are alternatives, consider using them."

As the "first authoritative U.S. report to explicitly acknowledge that use of antibiotics in farm animals poses a risk to human health," this is a "landmark" document, says Patricia B. Lieberman of the Center for Science in the Public Interest, an advocacy group in Washington, D.C. However, she finds its recommendations "soft," arguing that sufficient data already exist for the NRC/IOM panel to have justified "ending the subtherapeutic use of antibiotics."

Says Beran, "We didn't consider that tenable." Where subtherapeutic antibiotics have been phased down or out, data show that overt animal disease requiring antibiotic therapy sometimes increases, he adds.

Not in Sweden, observes Stuart B. Levy, director of the Center for Adaptation Genetics and Drug Resistance at Tuffs University in Boston. At a World Health Organization (WHO) meeting in Berlin on livestock and antibiotic resistance last October, Martin Wierup of the Swedish Animal Health Service in Johanneshov described how farmers coped with Sweden's 1986 ban on antibiotics to promote livestock growth.

While infectious outbreaks in the first year increased the need for antibiotic therapy, Levy says, use of the drugs fell thereafter. Total antibiotic use for food animals in Sweden is now 55 percent lower than before the ban, according to a report by Wierup to be published this month in the Alliance for the Prudent Use of Antibiotics Newsletter.

Though Frederick J. Angulo of the Centers for Disease Control and Prevention in Atlanta would also prefer that "all growth-promoting use of antibiotics be terminated," he would be willing to accept the more limited recommendation that came out of the Berlin meeting--a ban on the subtherapeutic treatment of livestock with antibiotics prescribed for people or with closely related drugs.