Antibiotic use as a livestock growth promoter increases the risk of human antibiotic resistance, a Marshfield Clinic researcher and his colleagues have found.

Results of the nearly $1.4 million three-year study, funded by the Centers for Disease Control and Prevention, Atlanta, Georgia, are in the November 1 issue of The Journal of Infectious Diseases.

Edward Belongia, M.D., Marshfield Clinic Research Foundation, Marshfield, Wis., and his colleagues examined poultry exposure as a risk factor for antibiotic resistance in Enterococcus faecium, a gut bacterium that is increasingly the cause of infections in hospitals. The investigation team focused on use of a growth-promoting antibiotic, called virginiamycin, in poultry.

Virginiamycin is closely related to quinupristin-dalfopristin, an antibiotic licensed to treat patients with serious, antibiotic-resistant infections. The drug is prescribed under the brand name Synercid. According to Belongia, "There is a relative lack of data on the impact of antibiotic use in livestock and its relationship to antibiotic resistance in humans, but there is a fair amount of indirect evidence suggesting that antibiotic use could pose a risk to human health."

"We've known for a long time that resistant bacteria can be found on retail poultry products, but our study is one of the first to show an association between human carriage of antibiotic resistance genes and eating poultry or handling raw poultry.

"These results indicate that virginiamycin use in poultry leads to transfer of antibiotic resistance genes to human gut bacteria through the food supply and they provide additional evidence that use of growth promoters in animals may have long-term consequences for human health. The Food and Drug Administration (FDA) can use this information to improve its risk assessment procedures."

The importance of this issue was illustrated by a recent FDA Veterinary Medicine Advisory Committee meeting about an application to license a broad spectrum antibiotic, called cefquinome, for use in cattle. Belongia spoke at the hearings, representing the Infectious Diseases Society of America.

"There was a great deal of concern that this antibiotic could promote resistance to cephalosporin drugs that are essential for many patients with serious or life-threatening infections," Belongia said, "and at the end of the day the FDA committee recommended against the drug. Our study focused on a different drug in a different type of animal, but this is a timely example of the controversy regarding the appropriate use of antibiotics in food-producing animals.

"We need to have drugs to treat sick animals," he added, "but we should not be using antibiotics to promote growth."

Working with Belongia, as principal investigator, were members of the Marshfield Enterococcal Study Group - Amy L. Kieke, Ph.D., Mark A. Borchardt, Ph.D., Burney A. Kieke, Susan K. Spencer and Mary F. Vandermause; and Minnesota Department of Health, St. Paul, Minnesota - Kirk E. Smith and Selina L. Jawahir. Amy Kieke was the first author on the published paper. Borchardt directed laboratory activities to detect antibiotic resistance and resistance genes. Belongia and colleagues posed the question: Does exposing poultry to virginiamycin lead to Synercid-resistant E. faecium in humans?

They isolated E. faecium in stool samples from 105 newly-hospitalized patients and 65 healthy vegetarians, as well as in 77 samples of conventional retail poultry and 23 antibiotic-free poultry meat samples.

After exposure to virginiamycin, E. faecium from conventional poultry and from patients who consumed poultry became resistant to Synercid more often than E. faecium from vegetarians or from antibiotic-free poultry. Some of the resistance was attributed to a specific gene and both the gene and resistance were associated with touching raw poultry meat and frequent poultry consumption.

Laboratory tests showed the bacteria isolated from patients and vegetarians had no pre-existing resistance to Synercid. Resistance was rare among antibiotic-free poultry but a majority of bacterial isolates from conventional poultry samples were resistant.

From Marshfield Clinic