Drug-resistant bacteria in US meat: studies

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Drug-resistant bacteria in US meat: studies

By Amy Norton

NEW YORK, Oct 17 (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, land 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 Mc 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. G. White, of the Food

and Drug Administration's Center for Veterinary Medicine in Laurel,

land, 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, Mc'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, 1155-1160,

1161-1166, 1202-1203.