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Livestock kick a drug habit
September 2005

"Just say no to drugs" was the message sent to chicken farmers in July of 2005 when the FDA banned the use of the antibiotic Baytril in poultry production. Citing concerns for human health, the FDA will no longer allow poultry producers to give their chickens, turkeys, and ducks Baytril-laced water to treat and prevent respiratory infections in the birds. That move reinforced the actions of McDonald's, Wendy's, and other fast food giants that have, in recent years, refused to buy chicken treated with Baytril and other selected drugs. Even the pork industry is getting in on the act. In August, Smithfield Foods Inc., the company likely to have supplied that glazed ham for your Sunday supper, announced that it would stop treating its pigs with selected antibiotics for growth-promotion purposes.

Where's the evolution?
But how does using an antibiotic on chickens and pigs affect human health, and what does this all have to do with evolution? At issue is the evolution of antibiotic resistant bacteria.

When a farmer treats a chicken flock with the antibiotic Baytril, it kills most of the bacteria responsible for the respiratory infection — but it also kills many of the campylobacter bacteria that naturally live in the chickens' guts. Ever take an antibiotic for strep throat and wind up with an upset stomach? You've done the same thing — killed most of your naturally-occurring gut bacteria!

Here's the problem: not all of the campylobacter are killed, and the few that survive probably carry a mutation that makes them resistant to Baytril. These resistant campylobacter then pass that mutation on to their offspring as they multiply. Hence, natural selection causes the evolution of Baytril-resistant campylobacter bacteria. If campylobacter get into your body (perhaps through contaminated chicken meat), you may wind up with food-poisoning. With normal campylobacter, you could just take the antibiotic Cipro to clear up the infection — but since Baytril and Cipro are similar antibiotics, Baytril-resistant bacteria are also likely to be Cipro-resistant...and, voila, you end up with a terrible case of food-poisoning and no useful drug to treat it.

The potential ramifications become even more frightening when you consider the fact that bacteria have the unusual ability to pass genes back and forth between species in a process called horizontal transfer. Cipro is one of the few antibiotics used to fight the anthrax bacterium. If a campylobacter passed its Cipro-resistance gene on to an anthrax bacterium, we could end up facing a frightening "super-bug."

The evolution of antibiotic resistant bacteria is a serious concern — and it is not just limited to Baytril. Many human antibiotics have sister-drugs that are freely used on livestock in copious amounts. The presence of these antibiotics sets the stage for the evolution of resistant bacteria in any environment: in the animals themselves or in the soil and water contaminated by the antibiotic.

Read more about it

News and journal articles:

• A typical news item on this topic from the Associated Press

• An excellent summary of the FDA's decision and the science behind it originally published in the Wall Street Journal

• An article covering the debate over uses of antibiotics on livestock from PBS

• A scientific perspective on the issue of antibiotic resistance, originally published in Scientific American in 1998

• Another case of antibiotic resistance in the news from Reuters Health

• Background information on how natural selection works

• Review of mutations and their role in producing genetic variation

• A research profile of a scientist working to slow the evolution of antibiotic resistance

• A set of examples showing how evolutionary theory helps us understand and treat disease

Discussion and extension questions:

• What role did evolution play in the decision to ban the use of Baytril in poultry?

• Why do antibiotic resistant bacteria in chickens threaten human health?

• Compare bacterial evolution to human evolution. How are they similar, and how are they different?

• Compare and contrast the role of natural selection in producing antibiotic resistant bacteria and in producing a dangerous strain of the avian flu (see Evolution and the avian flu).

• What strategies can help prevent the evolution of antibiotic resistant bacteria?

• Teach about natural selection. In this classroom activity for grades 9-12, students experience one mechanism for evolution through a simulation that models the principles of natural selection and helps answer the question: How might biological change have occurred and been reinforced over time?

• Teach about another application of evolutionary theory in medicine. In this classroom activity for grades 9-12, students learn why evolution is at the heart of a world health threat by investigating the increasing problem of antibiotic resistance in menacing diseases such as tuberculosis.

• Citing human threat, U.S. bans a poultry drug. (2005, July 29). The New York Times, p. A17.

• FDA bans some antibiotics in poultry. Talk of the nation/Science Friday. Ira Flatow (anchor). National Public Radio. (2005, August 19).

• Pork Barrel. (2005, August 11). Nature, 436, 775.