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Those are among the conclusions of three studies published in today's New England Journal of Medicine. However, the new research also suggests the interaction among animals, people and microbes may not be as simple and predictable as previously believed.
Antibiotics are routinely given to chickens, pigs and cattle to prevent illness and to promote growth. The drugs are put in feed or water in concentrations below that used to treat infections. The practice, while commonplace, is controversial because it encourages the emergence of antibiotic-resistant microbes.
In 1998, the European Union prohibited the use of antibiotics as animal growth-promoters if the drugs are similar to ones used in human medicine. Numerous groups are pushing for a similar ban in the United States.
In the first study, researchers at the Food and Drug Administration (FDA) and the University of Maryland looked for salmonella bacteria in meat bought at three supermarkets in the Washington area in the summer of 1998. About 50 samples each of chicken, turkey, beef and pork were studied.
Salmonella enterica, a microbe that can cause an intestinal infection with vomiting and diarrhea, was found in 41 of 200 samples. Four of the samples carried two different strains of the microbe. The bacterium was present more often in chicken (35 percent of samples) and turkey (24 percent) than in pork (16 percent) and beef (6 percent).
More than three-quarters of the bacteria isolated were resistant to at least one antibiotic, and 53 percent were resistant to at least three. About 16 percent of the isolated bacteria were resistant to ceftriaxone, an intravenous antibiotic that is the treatment of choice for severe cases of salmonella infection in children.
The samples were taken soon after the U.S. Department of Agriculture's Hazard Analysis and Critical Control Point (HACCP) system began in January 1998. The program was designed to significantly reduce bacterial contamination of meat, and has generally done so.
Last year, 26 percent of ground turkey samples studied by government inspectors tested positive for salmonella; the bacterium was found in 14 percent of ground chicken and 3.3 percent of ground beef. There was less contamination in each category than is reported in the just-published study, suggesting it may be somewhat out of date.
In the second report in the medical journal, researchers at the Centers for Disease Control and Prevention (CDC) and several other institutions looked at the frequency of resistance to a particular antibiotic by a microbe found in chicken meat and in human feces. Since 1974, many chicken growers have routinely given their birds virginiamycin, an antibiotic that is very similar to quinupristin-dalfopristin, which was recently approved for human use and is sold under the trade name Synercid.
From July 1998 to July 1999, the researchers sampled about 400 chickens from about 25 stores in four states. Resistant strains of the intestinal microbe Enterococcus faecium were found in 58 percent of the chicken samples. At the same time, about 300 samples of human feces collected from patients at clinics were also tested. Only 1 percent contained drug-resistant E. faecium.
This suggests there is very little of the resistant bacterium in the human population, despite a quarter-century of virginiamycin's use in poultry agriculture.
In the third study, researchers in Denmark fed drug-resistant intestinal bacteria to 18 healthy volunteers to see whether the microbes would show up in their feces. Four different strains of E. faecium were used, all obtained from recently slaughtered chickens or swine.
The drug-resistant microbes showed up in all the volunteers for five days after they consumed them. By the the 12th day of the study, however, only one person carried the ingested microbe, and by the 35th day, none did. This suggests that bacteria that develop antibiotic resistance in animals can survive in the human intestinal tract -- an observation made by other studies. However, at least in this case, they didn't seem to stay long.
Those who want to ban antibiotics in animal feed and those who back their continued use found support in the studies.
Sherwood L. Gorbach, of Tufts University School of Medicine, in an editorial in the journal, said the studies provided "the proverbial smoking gun" on the human hazards of antibiotic use in animals. He called for the banning of use of the drugs in animals, except to treat illness.
A spokesman for the National Chicken Council, Richard Lobb, pointed to the CDC study as evidence there's little to fear.
"They were looking for resistant E. faecium in humans and they wanted to attribute this to virginiamycin in chickens, and they did not find it. The actual data they found . . . does not seem to support the type of restrictions that are mentioned in the editorial," Lobb said.
The FDA is convinced there are risks posed by at least some agricultural use of antibiotics.
A year ago, the agency proposed prohibiting the fluoroquinolone family of antibiotics (which includes ciprofloxacin, or Cipro) from use in chicken farming. Of the two drug companies making animal feed containing the antibiotic, one, Abbott Laboratories, agreed to withdraw its product. The other, Bayer, refused. The FDA is trying to force it to do so.
A microbe called campylobacter is the most common cause of food-borne intestinal illness, and the fraction of cases that are fluoroquinolone-resistant is rising. About 10,000 Americans ill enough with the microbe to warrant antibiotic treatment have a resistant strain of it.
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