Not all resistant microbes spring from misguided human medicine. Farms are some
of the most insidious sources of antibiotic resistance. As mentioned in Chapter
3, antibiotics are routinely fed in tiny amounts to farm animals -- not to fend
off disease, but to boost growth. And low-level use of antibiotics is a perfect
way to foster resistant organisms. In recent years, livestock industry experts
had estimated that 40 percent of antibiotics produced in the United States went
to farm animals. In 2001, however, a report from the Union of Concerned
Scientists, a Cambridge, Massachusetts-based environmental advocacy group,
raised this estimate to a whopping 70 percent. Public health experts have long
worried that farmers are squandering human life-saving drugs on animals that
are not even sick.
Take Campylobacter, the most common bacterial cause of foodborne
illness. In 1995, American poultry farmers began using a fairly new class of
drugs known as fluoroquinolones to treat respiratory infections in poultry. In
people, these broad-spectrum, low-toxicity drugs are some of the most prized
antibiotics today, because they are slow to breed resistance and are effective
against some of the hardest-to-treat infections. Almost immediately after
poultry farmers began dosing their birds with the medication, thousands of
people who ate undercooked chicken contaminated with fluoroquinolone-resistant
strains of campy themselves became infected with the drug-resistant bacteria.
Before the drug was used, no Campylobacter specimens cultured from
hospital patients had been resistant; today, nearly a fifth are, and the figure
is sure to rise.
Even more dangerous than drug-resistant Campylobacter is
resistant Salmonella, which is also present in poultry and meat. In
human medicine, fluoroquinolones are the preferred treatment for invasive, and
often life-threatening, Salmonella infections. Yet today, doctors are
resorting to higher and higher doses of fluoroquinolones to treat Salmonella
-- a possible prelude to full-blown resistance. And now there's a
frightening new wrinkle in treating the organism. In 1998, a 12-year-old
Nebraska boy picked up a Salmonella infection from his family's cattle
that was resistant to ceftriaxone -- one of the cephalosporin class of
antibiotics -- as well as a dozen other antibiotics. Fortunately, he survived
when doctors treated him with a combination of other drugs. But when this
unprecedented case was reported in 2000, it terrified public health officials.
Ceftriaxone is one of the few antibiotics that reliably kills most bacteria.
And it is the drug of choice for children whose Salmonella infections
have entered the bloodstream -- a condition that kills about 1,000 Americans
every year. Ceftriaxone is also the drug that doctors turn to when treating
young victims; because of worries about bone growth, quinolones are not
approved for children. Since 2000, more cases of ceftriaxone-resistant
Salmonella in people have turned up. "This Salmonella is so
multiresistant," says the CDC's David Bell, "there are no good drugs left
that are approved for children." To history-minded physicians, the situation
evokes futile attempts at the turn of the last century to treat typhoid fever,
another Salmonella infection. Extrapolating from subsequent studies of
patients, health officials calculate that tens of thousands of Salmonella
cases each year are ceftriaxone-resistant. The clinical problem also
touches on a moral quandary: ceftriaxone is not used as a growth promoter, but
rather to treat sick animals. "It portends a dilemma," says the CDC's Fred
Angulo. "Societally, what do you want to do: treat sick people or sick
animals?"
Another foodborne infection is VRE -- yes, the same bug that wreaks so much
havoc in critically ill hospital patients. In this country, VRE isn't primarily
foodborne; the organism is most often bred by massive vancomycin use in
hospitals. It's a different story in Europe. Soon after farmers there began
feeding avoparcin, a growth promoter related to vancomycin, to livestock in
1974, the animals developed vancomycin-resistant enterococci. (Because it may
be a carcinogen, avoparcin never received approval in the United States.) In
1986, France found its first human patient with VRE. Within a few years, the
bacterium spread throughout human intestinal tracts on the Continent. U.S.
public health experts believe that at least some of the VRE organisms in this
country may have come from Europe and then proliferated under the selective
influence of vancomycin in hospitals here.
But while foodborne vancomycin-resistant enterococcus infections are uncommon
in the United States, a similar chain of events is starting to happen here with
another drug. For more than a quarter century, American poultry farmers have
used the growth promoter virginiamycin in chicken feed. In chickens, the drug
helped breed enterococci that are resistant to virginiamycin's human-use
cousin, Synercid. Synercid is the other "last-resort" antibiotic, approved for
humans in 1999. Yet as a frightening presentiment to the drug's potential
downfall, more than half of grocery-store chickens carry bacteria impervious to
this end-of-the-line human drug. People are picking up these resistant bacteria
in their meals. At any one time, at least 1 percent of the U.S. population is
carrying Synercid-resistant enterococci. Usually, these intestinal bacteria are
expelled as food moves through the intestines, never causing a problem. But in
the rare instance that such an individual enters the hospital -- say, for a hip
replacement -- and happens to be treated with Synercid, the resistant
enterococcal bacteria in the gut will go wild, threatening an infection that no
antibiotic can quell.
One of the most frightening and enigmatic foodborne pathogens is a
drug-resistant strain of Salmonella typhimurium. Known as Definitive
Type 104, or DT 104, it defies five important classes of drugs in the United
States; in Europe, where it surfaced in 1984, it thwarts seven. This monster
resistance has helped it spread in cattle, because in animals that receive any
one of these drugs, DT 104 gains an advantage. In the U.S., hundreds of
thousands of people suffer DT 104 infections annually. Raw milk is a common
culprit, the bacterium having infiltrated dairy herds. In 1997, for example,
more than 100 Californians became sick from DT 104 in two overlapping outbreaks
in Hispanic communities, where residents ate homemade Mexican-style cheese made
from unpasteurized milk and sold by street vendors and specialty markets.
When scientists tried to figure out where this renegade came from, they were
shocked. DT 104's resistance genes were a strange combination -- so strange,
they had never before been seen in Salmonella. Where they had
turned up was worlds away: in Asian aquaculture, where fish have been
regularly treated with antibiotics since the early 1980s. So how did they land
in the American heartland? One theory holds that some of those Asian fish may
have been ground up into fish meal, an international commodity often fed to
pigs and poultry. Or DT 104's resistance genes may have found their way into
animal breeding stock, perhaps through the rendered protein of other animals.
However it happened, DT 104 appeared more or less simultaneously around the
world in the 1980s, suggesting that the animals acquired these alien bacteria
en masse.
In 1969, Britain's Swann Committee concluded that antibiotics used in human
therapy or those that promote cross-resistance in people should be banned from
animal growth promotion. Unfortunately, livestock producers hew to the position
that whatever drugs they feed their animals are proprietary secrets. Besides,
say industry officials, they need antibiotics to produce safe and affordable
food. A 1999 report published by the Institute of Medicine and the National
Research Council questioned this claim. Using the livestock industry's own
estimates, the report calculated that if farmers quit using antibiotic growth
promoters, the added costs would be less than $10 per American consumer per
year. And a 2001 United States Department of Agriculture report showed that hog
farmers actually lose money by giving pigs antibiotics that promote growth;
while animals do fatten up more, the extra poundage expands overall supply and
drives down market prices.
Poultry and livestock aren't the only creatures being dosed with drugs. Salmon,
catfish, and trout on domestic fish farms get antibacterial drugs in the water.
Honeybees get antibiotics in their hives. And each year, an estimated 300,000
pounds of antibiotic pesticides drift down on fruit trees and other crops to
control or prevent bacterial infections such as fire blight. That disease is
caused by the pathogen Erwinia, a bacterial cousin of E. coli,
Salmonella, and Shigella. Erwinia now resists both streptomycin, an
old drug, and tetracycline. Researchers don't know if the fresh fruit
invitingly stacked in your supermarket is delivering drug-resistant genes to
your intestines. According to microbiologist Abigail Salyers, both the use of
untreated or partially treated water for irrigation or for washing vegetables,
or the use of manure as a fertilizer for vegetables and fruits could
contaminate food plants with antibiotic-resistant bacteria. Proving that no
good deed goes unpunished, a 1993 study found higher levels of
multidrug-resistant bacteria in intestines of vegetarians than in meat eaters.
Whether carnivore or vegetarian, you cannot avoid the aftermath of antibiotics
applied lower in the food chain.
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