antibiotics in animal feed & human infectious diseases

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Antibiotics in the Animals We Eat

Low-dose antibiotics in animal feed fuel drug-resistance in human infectious

diseases.

By Bonnie M. Marshall and Stuart B. Levy | April 1, 2012

Corbis, Jasper White

For as many decades as antibiotic resistance has thwarted the cure of

bacterial infections, scientists have pondered the origins of resistance

genes and how they became such a problem. Fingers were pointed squarely at

the overprescription of antibiotics in human medicine-and rightly so, as

early on, these drugs were liberally utilized for every imaginable malady

without concern for the possible consequences. Not long after their

discovery, however, these miracle drugs were applied not only to sick humans

and animals, but to healthy ones as well.

Nowhere is this practice more prevalent and controversial than in animal

husbandry, where animal feeds laced with small amounts of antibiotic are

provided over extended periods of rearing. Labeled as " growth promotion " and

employed primarily in large, concentrated feedlots for poultry, swine, and

cattle, this nontherapeutic application appeared to fatten the animals

faster, prevent rampant herd disease, and help bring healthy animals to

market more quickly.

While US farmers and other stakeholders have argued tenaciously for the

continuation of subtherapeutic dosing, Europeans adopted the " precautionary

principle, " instituting sequential bans on the practice beginning in the

mid-1990s. Arguments on both sides of this issue continue to the present

day, but evidence of the negative consequences of low-dose antibiotic

feeding has been mounting. Since 1976, several persuasive scientific studies

have illustrated how animals fed low-dose antibiotics not only propagate

resistant bacteria, but spread these resistant strains to farmers, their

families, community residents, and ultimately, hospitalized patients.

Particularly worrisome is the continued use in animals of antibiotics that

are close structural relatives of those that are used in human medicine. It

is feared that, in time, these drugs will lose potency as bacteria express

" cross-resistance " to the related drugs.

Some researchers have countered that the resistant bacterial strains found

in serious hospital infections bear little or no resemblance to the strains

found in farm animals. They argue that eliminating antibiotics on the farm

would harm animal health, result in economic loss, and have little or no

impact on reducing human morbidity and mortality. However, these rebuttals

overlook the inherently promiscuous nature of bacteria-in particular, the

transferable genetic elements they often carry (e.g., bacterial plasmids,

transposons, phages) that can readily share DNA segments bearing resistance

genes. They pass among strains, species, and even diverse bacterial genera,

rearranging and accumulating even more resistance genes. Tracking the

evolution of such complex bacterial exchanges from food animals to people

poses a daunting challenge, making definitive proof elusive. But we argue

that the preponderance of evidence, coupled with a diminishing pipeline of

new antibiotics and the appearance of multidrug-resistant " superbugs, "

warrants closer scrutiny of how and where we are using these

antimicrobials-and the adoption of stricter measures of control.

While still declining to issue an all-out ban on subtherapeutic feeding, the

US Food and Drug Administration has taken measured steps in the right

direction. First, in 2005, the agency prohibited the use of fluoroquinolones

in poultry, and just this January, it prohibited certain off-label uses of

cephalosporins in livestock generally. It is a matter of concern, however,

that the FDA does not address the ongoing use of penicillins and

tetracyclines as growth promoters. Thus, we are still a long way from the

steps needed to safeguard precious classes of drugs for effective treatment

of human disease.

Judicious surveillance in Europe in the decades following the bans on

antibiotic use in animals has shown that the emergence and spread of

resistant bacteria can be controlled and even reversed. Alternatives to

antibiotics in animal husbandry in these countries, including improving

hygiene practices and reducing overcrowding, need to be more fully explored

for implementation in the United States.

Bonnie M. Marshall is a senior research associate in the Center for

Adaptation Genetics and Drug Resistance at Tufts University School of

Medicine in Boston. Stuart B. Levy MD is the Center director, and a Tufts

University professor. Both are actively involved in the Alliance for the

Prudent Use of Antibiotics. For more information about their views on the

use of antibiotics in animal husbandry, see B.M. Marshall and S.B. Levy,

Clinical Microbiology Reviews, 24:718-33, 2011.

Source: TheScientist

http://the-scientist.com/2012/04/01/antibiotics-in-the-animals-we-eat/

Comment:

In just a few hundred years time, Evolutionary Psychologists will include

our current era and conditions in the 'EEA'

Posted by

Karl Stonjek