To improve meat safety, the federal government for years has allowed ranchers and farmers to feed antibiotics to beef, cattle, and poultry to control bacteria deemed harmful.
That practice, however, is coming under increasing criticism from some consumer groups and public-health organizations. Overreliance on antibiotics down on the farm, they say, may be contributing to a larger public-health problem: the excessive use of antibiotics in medicine and consumer products in general.
The trend, they say, is giving rise to bacteria that resist the antibiotics used to treat humans for the diseases associated with the microscopic organisms. Some of these antibiotics also are used as food and water additives to ward off illness and boost the meat yield in livestock.
"In certain cases, there are fewer and fewer antimicrobials available to treat serious diseases in humans," says Sharon Thompson, an associate director in the US Food and Drug Administration's Center for Veterinary Medicine in Rockville, Md.
At issue, researchers say, is not the use of antibiotics to combat specific disease outbreaks among livestock. Instead, the concern centers around farmers' routine use of antibiotics. Livestock use accounts for roughly half of the 25,000 tons of antibiotics produced in the US each year, according to a report last month from the Institute of Medicine. It adds that 40 to 80 percent of the antibiotics applied on the farm are unnecessary.
So far, evidence that the use of antibiotics in agriculture is having significant adverse off-farm effects is inconclusive, according to the Animal Health Institute, a pharmaceutical-industry organization based in Washington D.C.
Speaking at a veterinary meeting in College Park, Md., earlier this year, Richard Carnevale AIH vice president noted that while antibiotic resistance in general is a problem, "how much of it is really due to animal drugs and what is the medical impact?... The problem is complex," and "the risks of using animal drugs must be put in context with the real risk factors associated with food-borne illness."
Still, the AIH acknowledges that available studies "are suggestive" that "injudicious use" of antibiotics on the farm can lead to the development of treatment-resistant strains of unwanted bacteria.
Nor is the concern confined to the US. Earlier this month, the World Health Organization, based in Geneva, held a meeting to look at the impact of using a class of antibiotics known as fluoroquinolones to treat farm animals as well as people. Several countries approving the compound for use in agriculture noted that food-borne bacteria have become less susceptible to the antibiotic. WHO outlined an ambitious research agenda for members to pursue to assess the antibiotic's effects and to develop alternative treatments.
Antibiotics, or antimicrobials, are compounds formulated to kill or slow the growth of bacteria. Initially, they were derived from natural sources. With advances in molecular biology, however, pharmaceutical companies now add man-made components to an antibiotic or design and make the compounds synthetically.
Under this high-tech assault, bacteria evolve defensive mechanisms of their own, explains Rodney Dietert, professor of immunotoxicology at Cornell University's College of Veterinary Medicine in Ithaca, N.Y. While an antibiotic may weaken or kill most of its target bacteria, some survive and continue to reproduce, conferring their resistance on later generations. Moreover, bacteria of one species can readily exchange genetic material with bacteria of another species. Thus, they transmit antibiotic resistance among potentially harmful species.
Finally, he says, "when you find bacteria that resists one antibiotic, you often find that it can resist two or three others as well. It's quite onerous."
How this bacterial "arms race" plays out in agriculture is the subject of a National Research Council study due out next month. Although the results are still being hammered out, the NRC says that it will be the first "consensus" report that will try to determine how significant the problem truly is.
Even if the farm connection is uncertain for now, the costs to society of trying to deal with antibiotic-resistant bacteria are not. Those costs run from $4 billion to $5 billion a year, according to the Institute of Medicine in Washington.
Cornell's Dr. Dietert says that in his view, the routine applications he finds troubling aren't necessary, if farmers use good animal-husbandry practices.
Unless regulations governing the use of antibiotics on and off the farm are tightened, "the crown jewels of modern medicine will turn to dust," says Patricia Lieberman, a physiologist and staff scientist with the Center for Science in the Public Interest, a Washington-based consumer group. Emerging evidence regarding farm-introduced resistance to antibiotics may only be "the tip of the iceberg," she says.