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Antibiotic-resistant bacteria are common
in American meat, and the microbes survive in the human intestine
for a week or more, where they could potentially be the source
of drug-resistant infections in people.
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.
Three new studies suggest the interaction
among animals, people and microbes may not be as simple and
predictable as previously believed.
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 found that at least 17% of chickens
from chicken samples from supermarket shelves in parts of
Oregon, Georgia, Maryland and Minnesota 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.
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.
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 residence itself is not enough to
cause illness. 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.
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.
NEJM
October 18, 2001;345:1147-1154, 1155-1160, 1161-1166, 1202-1203
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