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Drug-Resistant Bacteria in U.S. Meat Processing Plants

Drug-Resistant Bacteria in U.S. Meat

October 17, 2001 By Amy Norton

NEW YORK (Reuters Health) - Three new studies add to the growing concern over the human health effects of routinely giving antibiotics to animals destined to enter the food supply.

Two of the studies uncovered significant amounts of drug-resistant bacteria in chicken and meat taken from US supermarket shelves. The third demonstrated that such bacteria can persist in the intestinal tract days after a person ingests them.

Researchers say the findings bolster the arguments of public health experts who want to limit the use of antibiotics in livestock. The drugs are used to treat sick animals, but in the US they are also routinely given to boost the nutritional benefits of animal feed and promote growth in food animals.

The concern with this practice is that the needless use of antibiotics gives a survival advantage to drug-resistant strains of the bacteria behind foodborne illnesses and other infections. Many health experts worry that food animals are providing a "reservoir" of drug-resistant bacteria that could be transmitted to humans. And the new studies add even more weight to these concerns, according to researchers.

"They are adding nails to the coffin," Dr. Sherwood L. Gorbach told Reuters Health in an interview.

Gorbach, a researcher at Tufts University School of Medicine in Boston, Massachusetts, wrote an editorial published with the three reports in the October 18th issue of The New England Journal of Medicine.

In one study, public health officials tested chicken samples from supermarket shelves in parts of Oregon, Georgia, Maryland and Minnesota. They found that at least 17% of chickens from each area had Enterococcus faecium bacteria that were resistant to an antibiotic combination called quinupristin-dalfopristin.

E. faecium is notoriously resistant to antibiotics, and illnesses caused by the bacteria--which include infections of the blood and urinary tract--are a growing problem in US hospitals. The quinupristin-dalfopristin combination was approved in the US in 1999 for the treatment of E. faecium infections that do not respond to the old standby antibiotic vancomycin.

The chicken in this study likely developed bacteria resistant to the drug combination because the animals had been routinely fed antibiotics in the same class, according to researchers led by Dr. L. Clifford McDonald of the Centers for Disease Control and Prevention in Atlanta, Georgia.

That drug, called virginiamycin, has been used in the US since 1974 to promote growth in farm animals.

Similarly, another research team found that of 200 ground meat samples bought in the Washington, DC, area, 20% contained various strains of Salmonella bacteria, most of which were resistant to at least one antibiotic. Among the strains isolated was a particularly virulent, resistant strain known to be a major cause of salmonella outbreaks. The meat samples included beef, chicken, turkey and pork.

"These findings provide support for the adoption of guidelines for the prudent use of antibiotics in food animals," Dr. David G. White, of the Food and Drug Administration's Center for Veterinary Medicine in Laurel, Maryland, and his colleagues.

Still, the immediate threat to human health posed by drug-resistant bacteria in meat is not fully clear. Besides studying chicken samples, McDonald's team analyzed 334 human stool samples submitted to the study areas' health departments. Only three showed drug-resistant E. faecium, and the researchers suggest this means that virginiamycin use in animals "has not yet had a substantial influence" on human health.

However, the third study suggests that drug-resistant E. faecium from animal products does live in the human digestive tract for up to 2 weeks after ingestion. Danish researchers had healthy volunteers consume milk laced with safe amounts of the bacteria, then collected stool samples to track what happened to the bacteria once ingested. They found traces of drug-resistant E. faecium in samples from 8 of 12 volunteers 6 days after ingestion and in one volunteer 14 days afterward.

This, Gorbach explained in an interview, shows that E. faecium bacteria in the food supply "don't just pass through...they establish temporary residence."

This residence itself is not enough to cause illness, a co-author on the study, Dr. Niels Frimodt-Moller, told Reuters Health. But if, for instance, a person receives antibiotics in a hospital, these drug-resistant bacteria may "overgrow" in the intestines, spread to the skin and other body areas and possibly contaminate hospital equipment such as catheters.

Taken together, these studies provide the "smoking gun" that argues for a ban on using antibiotics to promote growth in livestock, according to Gorbach.

Europe has issued such a ban, and, Gorbach noted, the US Food and Drug Administration is considering the move.

Health experts who advocate limiting antibiotic use want the drugs to be used only against specific pathogens in sick animals, by order of a veterinarian.

For their part, consumers can prevent the transmission of foodborne bacteria by properly handling raw meat and thoroughly cooking it before eating. However, Gorbach said, that is easier said than done--since, for instance, traces of bacteria from uncooked meat can readily be left on kitchen surfaces.

"Most consumers," he noted, "aren't microbiologists."

SOURCE: The New England Journal of Medicine 2001;345:1147-1154, 1155-



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