Human health professionals are increasingly alarmed about the overuse and misuse of antibiotics in humans and the subsequent development of antibiotic resistance. Some even say the problem is fast becoming a life-threatening public health issue.

To this end, all of the factors leading to the increase in antibiotic resistance, including antibiotic use in agriculture, must be understood, says Brenda M. Afzal, RN, MS. She's community health consultant for the Environmental Health Education Center at the University of Maryland (UM) School of Nursing in Baltimore.

“To best protect the health of our patients and to understand the full range of causative factors,” she says, “we need to have the best data regarding the causes and associations of exposures to diseases.”

Caught in the discussion of antibiotic resistance is the prophylactic use of antimicrobials as growth promotants in food animals. Included in this scrutiny are ionophores (such as monensin, lasalocid, laidlomycin, salinomycin and narasin) — antimicrobial compounds fed to ruminant animals to improve feed efficiency.

Because of the complexity and high degree of specificity of ionophore resistance, it appears that ionophores don't contribute to the development of antibiotic resistance to important human drugs, says T.R. Callaway a member of the USDA Food and Feed Safety Research Unit, Southern Plains Agricultural Research Center, College Station, TX.

“Therefore, it appears that ionophores will continue to play a significant role in improving the efficiency of animal production in the future,” he says.

Callaway says these antimicrobials specifically target the ruminal bacterial population and alter the microbial ecology of the intestinal microbial consortium. This results in increased carbon and nitrogen retention by the animal, increasing production efficiency.

The feeding of ionophores to cattle decreases the feed needed for growth and increases feed efficiency, agrees Gary Weber, director of regulatory affairs for the National Cattlemen's Beef Association (NCBA).

NCBA maintains a strong stance that ionophores are not a concern for antibiotic resistance in cattle or humans. During the Cattle Industry Summer Conference in Dallas, TX, this year, NCBA members resolved to strongly urge the FDA and other appropriate agencies to reclassify polyether ionophores to reflect their true function as modifiers of rumen fermentation and coccidian prevention compound; and to discontinue classification of polyether ionophores as antibiotics.

No Government Policy?

In 1997, the World Health Organization called for a ban on using antibiotics to promote growth in animal agriculture. In 1998, the European Union banned adding human-use antibiotics to animal feed.

Proponents of controlling antibiotic use are dismayed that at this time, no U.S. government agency collects comprehensive data on antibiotic use for any purpose. In 1999, an interagency task force composed of the FDA Centers for Disease Control, USDA and seven other federal agencies, was created to develop and implement procedures to monitor antibiotic use in human medicine, agriculture, veterinary medicine and consumer products. However, no policies have yet been set in stone.

Ellen Silbergeld, PhD, program director of UM's human health and environment studies, isn't convinced that an antibiotic policy needs to be written by FDA. Silbergeld is a scientist studying the relationship between bacteria resistance and antibiotic use in animal agriculture. She says that while the exact scientific mechanism of growth is not fully understood, many of the same antibiotics used in animal agriculture are also used for human treatment.

She points to resistant bacterium, such as E. coli, Campylobacter and Salmonella, that can be passed to humans by ingestion of contaminated meat products or through direct and indirect exposure to animal feces via water, soil or crops.

Silbergeld says that agricultural uses are of particular concern for the following reasons:

  • Most antibiotics are not used for therapeutic purposes but to promote growth, at relatively low (i.e., subtoxic/subtherapeutic) doses.

  • Drugs of great clinical importance (or similar in action to clinically important drugs) are widely used in agriculture.

  • Drug residues are found in animal waste and in surface water near large scale animal production facilities.

FDA recognizes that a major issue related to the use of antimicrobials in food-producing animals is their sub-therapeutic use in livestock.

“While this use of antimicrobials is decreasing, it is still a widespread practice,” Jane E. Henney, Ph.D, said nearly three years ago. She was FDA Commissioner until January 2001, and is considered by many to be the engineer of FDA's efforts to have a continued supply of safe and effective antimicrobials available to protect the health of both humans and animals.

“We're taking a renewed look at this practice, and are focusing our efforts on those uses that seem to pose the greatest potential risk to public health,” she added. “As in all of our decision making, science will be used to ground and guide our actions.”

FDA's efforts continue, but it's still unclear the tack the current FDA administration will take, especially with regard to ionophores used in the cattle feeding industry. But with 92.9% of the nation's largest cattle feeding operations using ionophores, according to USDA's National Animal Health Monitoring System, this is certainly no small issue of importance to the cattle industry.