Kiera Butler

Kiera Butler

Senior Editor

A senior editor at Mother Jones, Kiera covers health, food, and the environment. She is the author of the new book Raise: What 4-H Teaches 7 Million Kids—and How Its Lessons Could Change Food and Farming Forever (University of California Press).


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Antibiotics Are No Longer Making Pigs Bigger

For decades, it's been thought that low, regular doses of antibiotics help livestock grow big—thus increasing meat producers' profits. So common is the practice of lacing farm animals' feed with the drugs that an astonishing four-fifths of all antibiotics in the United States now go to livestock.

But a new meta-analysis by two Princeton researchers shows that antibiotics aren't as effective at promoting growth as they used to be. Studies from 1950-1985 suggested that antibiotics increased weight of young pigs by an average of about 17 older pigs by 4 percent. But similar studies since 2000 found much less dramatic results: 1 percent increase for young pigs and no measurable increase for older pigs.

No one knows why the drugs have become less effective—and in fact, there's no consensus on how exactly antibiotics increased growth in animals to begin with. One theory is that the drugs fight low-level infections, which allows the animal to use its energy for growing instead of warding off germs. The authors of the new analysis theorize that as hygiene at livestock operations improve, the rate of infections might be decreasing, thus negating the need for antibiotics.

Another (scarier) possibility: Bugs that cause common animal infections are becoming resistant to the antibiotics. The consequences of antibiotic resistance, of course, go far beyond pigs' rates of growth. As my colleague Tom Philpott has reported, superbugs can jump from animals to humans. Antibiotic-resistant infections already kill 700,000 people every year worldwide. A recent UK report predicted that number will rise to 10 million by 2050.

"If the benefits of [antibiotics for animal growth] have diminished, then it becomes reasonable to be cautious and avoid the potential public health costs," write the new report's authors. "Antibiotics are not needed to promote growth, but they are essential to treat infectious diseases and maintain animal health." 

This week, McDonald's pledged to phase out serving chicken raised on antibiotics that can also be used to treat humans. To understand the giant implications this has for the meat industry, consider my colleague Tom Philpott's previous reporting on the topic. For starters, the livestock industry uses an astounding four-fifths of all antibiotics consumed in the United States. Mostly, these drugs are used not to treat infections but to promote growth in animals.

There is evidence that livestock antibiotic use contributes to antibiotic resistance, lessening the effectiveness of drugs that are medically important to  humans. And scientists have observed so-called "superbugs" migrating from farms to outside communities. It's a major problem—indeed, scientists predict that antibiotic failure will kill 20 million people by 2050. And yet, despite all this, the government still allows livestock producers to dose their animals with antibiotics.

McDonald's chicken move is a tacit acknowledgement that antibiotics are a precious resource. And considering that the chain serves 68 million people a day in practically every nation on Earth, it sends a powerful message indeed.

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