POSTED 7 APRIL 2005
This
drug makes you grow faster
Some studies show that farm animals grow faster if their feed contains a slight, steady dose of antibiotics. Although nobody is sure why this happens, most American farm animals are fed these "sub-therapeutic" doses in commercial feed, and it is often the same med used to treat bacterial illnesses in people. (For studies questioning whether such feeding works, see "Growth performance..." and " Performance and..." in the bibliography.)
Constant, low-level doses kill all susceptible bacteria, leaving only the few bugs with the genetic tools to defeat the chemical. That promotes the evolution and growth of antibiotic resistant bacteria.
And that is undermining a critical public-health advance of the 20th century -- antibiotics. Some of the problem stems from overprescribing of human antibiotics, but a lot of animals are also eating a lot of antibiotics. According to the Union of Concerned Scientists, "An estimated 70 percent of antibiotics and related drugs produced in this country are used for non-therapeutic purposes such as accelerating animal growth and compensating for overcrowded and unsanitary conditions on large-scale confinement facilities known as 'factory farms.' This translates to about 25 million pounds of antibiotics and related drugs fed every year to livestock for non-therapeutic purposes -- almost eight times the amount given to humans to treat disease."
Hog heaven? A typical pig "finishing" operation in the
United States. Pigs
grow to maturity in these giant barns. Courtesy Kellogg
Schwab, Johns Hopkins University.
Fear on the farm?
In a recent study (see "Airborne Multidrug-Resistant..." in the bibliography), bacteria in the air of a giant hog barn, where the animals eat low-dose antibiotics, were massively resistant to multiple antibiotics. In December 2003 and January, 2004, Amy Chapin, Kellogg Schwab and colleagues at the Johns Hopkins Bloomberg School of Public Health collected air samples inside two hog houses, population 1,500 each.
They found bacteria in three genuses -- Enterococcus, Staphylococcus and Streptococcus. All of the bacteria are opportunists, as Chapin, a recent doctoral graduate, explained: Depending on the situation, they can coexist with us, or turn pathogenic. They can behave, or they can injure or kill.
Most commercial
animal feed is laced with low-level antibiotics used to prevent disease and
speed animal growth. Photo: USDA
When the researchers tested the bacteria for resistance to four antibiotics (erythromycin, clindamycin, virginiamycin and tetracycline) commonly mixed in hog feed, 98 percent of the strains were resistant to at least two of the drugs. All of these meds (or their human-drug counterparts) are used to treat human bacterial infections.
Vanquished by vancomycin
But all of the bacteria succumbed to vancomycin, a last-ditch antibiotic used to treat serious infections when other antibiotics fail, but which is not legal for pig feed in the United States. Because the bacteria resisted only the drugs that can be used in pig feed, the authors concluded that low-level antibiotic feeding had caused the antibiotic resistance.
While Chapin sampled air only inside the barns, Shawn Gibbs of the University of Texas Health Science Center at Houston has found antibiotic resistant bacteria downwind of two Texas hog barns. Tests showed that Staphylococcus aureus, several Salmonella species and fecal coliforms were resistant to the antibiotics ampicillin, erythromycin, oxytetracycline, penicillin, tetracycline and tylosin (see "Airborne Antibiotic Resistant ..." in the bibliography) In addition, researchers have found antibiotic resistant bacteria in packaged pork, and in the manure spread by large pig operations, says Chapin.
Playing with fire
As we've said, medically useful antibiotics are losing their effectiveness as bugs continue to evolve and acquire resistance, and the hog barns seem to be playing a role, Chapin says. For example, she says, quinupristin, or dalfopristin, a relative of virginiamycin (one of the antibiotics she tested), is a "very important antibiotic in human medicine," used to cure bacterial infections when common antibiotics fail.
"We know virginiamycin is used in swine, we don't know the extent, but the increased resistance among bacteria to this very important class of antibiotics" is ... "alarming," Chapin says. "We had a hunch that we would find some level of antibiotic resistance in the airborne bacteria, but we were very surprised at how dramatic the results were -- 98 percent expressing high-level multi-drug resistance."
Before
tuberculosis was tretated with antibiotic, it was deadly. Now that so many
bugs resist antibiotics, it's again becoming deadly. This X-ray of the chest
reveals that this patient has far-advanced tuberculosis. Photo: CDC
Part of the danger concerns the exposure of farm workers -- who seldom if ever wear masks -- to resistant bacteria. But even if workers are not falling ill or spreading resistant bacteria, studies also find antibiotic resistant bacteria spreading from large animal facilities in manure, groundwater, surface water and the air. "We are all part of one large ecological community and when we interact in a number of environments," Chapin says, "from our homes to hospitals -- we could be sharing or transferring bacteria and their resistance genes."
To Chapin and many other critics, the best solution is to change farm practice. "I think we can reduce or eliminate the non-therapeutic use of antibiotics in swine production in the United States. Perhaps public health officials, extension agents, growers, the animal production industry could work together to address this. The Europeans have shown they can eliminate non-therapeutic antibiotic use and still have a productive industry."
We contacted the National Pork Producers Council for comment. They referred us to Paul Sundberg, scientific advisor to the National Pork Board, but he did not return our call. When we searched the Council's website for "antibiotic," the term was not found.
Is industry concerned? Here's what we found when we searched "antibiotic" at
the National Pork Producers Council.
What about the air in your house?
