July 18, 1998 Livestock's role in antibiotic resistance
By J. Raloff
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 subtherapeutic doses to promote weight gain in apparently healthy livestock.
In the past year, several strains of pathogenic bacteria with resistance to nearly all 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 livestockespecially growth-promoting, subtherapeutic applicationsas 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 sayswhich 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 Tufts 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 meetinga ban on the subtherapeutic treatment of livestock with antibiotics prescribed for people or with closely related drugs.
From Science News, Vol. 154, No. 3, July 18, 1998, p. 39.
Copyright © 1998 by Science Service.
1998. The use of drugs in food animals: Benefits and risks. National Research Council and the Institute of Medicine Report. July. Washington, D.C.
1997. The medical impact of the use of antimicrobials in food animals. WHO Meeting Report. October. Berlin. (Available at http://www.who.ch/emc/diseases/zoo/zoo97_4.html)
Wierup, M. 1998. Preventive methods replace antibiotic growth promoters: Ten years experience from Sweden. Alliance for the Prudent Use of Antibiotics Newsletter 16(July).
1998. Major gaps in research on antibiotic resistance need filling. WHO Press Release (June 9). (Available at http://www.who.ch/inf-pr-1998/en/pr98-46.html)
1997. Foodborne diseases possibly 350 times more frequent than reported. WHO Press Release (Aug. 13). (Available at http://www.who.ch/inf-pr-1997/en/pr97-58.html)
1997. Monitoring antimicrobial resistance in humans and animals in Europe. EuroSurveillance 2(March). (Available at http://www.b3e.jussieu.fr/ceses/EuroSurv/v2n3/en9-223.html)
Glynn, M.K., et al. 1998. Emergence of multidrug-resistant Salmonella enterica serotype typhimurium DT104 infections in the United States. New England Journal of Medicine 338(May 7):1333.
Levy, S.B. 1998. Multidrug resistance A sign of the times. New England Journal of Medicine 338(May 7):1376.
Lieberman, P.B., and M.G. Wootan. Protecting the crown jewels of medicine: A strategic plan to preserve the effectiveness of antibiotics. Center for Science in the Public Interest Report (May 28).
Raloff, J. 1998. Drugged waters. Science News 153(March 21):187.
Seachrist, L. 1996. Infections making a deadly comeback. Science News 149(Jan. 20):38.
Seppa, N. 1998. Survey finds wide resistance to TB drugs. Science News 153(June 6):359.
______. 1998. Ulcer bacteriums drug resistance unmasked. Science News 153(April 25):262.
______. 1997. A new gap in the antibiotic arsenal. Science News 151(June 7):348.
Smaglik, P. 1997. Proliferation of pills. Science News 151(May 17):310.
Frederick J. Angulo
Centers for Disease Control and Prevention
Mailstop A38, 1600 Clifton Road
Atlanta, GA 30333
George W. Beran
Iowa State University
2134 Veterinary Medicine
Ames, IA 50011
Stuart B. Levy
Center for Adaptation Genetics and Drug Resistance
Tufts Medical School
136 Harrison Avenue
Boston, MA 02111
Patricia B. Lieberman
Center for Science in the Public Interest
1875 Connecticut Avenue, N.W.
Washington, DC 20009-5728
Alliance for the Prudent Use of Antibiotics
P.O. Box 1372
Boston, MA 02117
copyright 1998 ScienceService