Farm Animals Breed Antibiotic Resistance

You may already have heard Tuesday’s interview about India being a breeding ground for deadly drug resistant bacteria. But India isn’t the only source of these lethal bacteria. They are present all over the world.

These bugs and their genes are spreading so rapidly around the world that some scientists regard the resistance genes as a new kind of environmental contaminant.

You’ve surely heard of MRSA, or Methicillin-Resistant Staphylococcus aureus which is a serious public health problem here in the US. There’s also multi-drug resistant E. coli and Klebsiella. For more examples, head over to the US Center for Disease Control and Prevention.

I wanted to bring up one source of drug resistance that didn’t get addressed in Tuesday’s interview—industrial animal farms.

But first, here’s a quick reminder of how drug resistance evolves: some microbes are naturally resistant to antimicrobial substances. (Here’s a cool study on the abundance of drug resistance genes in the ancient Alaskan environment.) Excess antibiotic in an environment selects for antibiotic resistance.

In other words, the antibiotic kills the sensitive bugs, and allows the resistant ones to thrive. The key thing here is that bacteria of various kinds often exchange genetic material with each other, through mobile chunks of DNA. So, a single drug resistance gene can spread like wildfire through a range of both benign and harmful microbes. And that’s exactly what has happened in the last century as we have ramped up our use and abuse of antibiotics.

In countries like the US, agriculture is the biggest user of antibiotics.

Low doses of antibiotics work as a growth promoter in animals, thus reducing costs for farmers. But when farms are large, they require large amounts of antibiotics.

According to this investigative story in the Washington Post, of the 35 million pounds of antibiotic used in the US in 2008, 70 percent “went to pigs, chickens and cows.”

Worldwide, that number is about 50 percent.

For decades now, scientists have been documenting how antibiotic resistance can evolve on farms and then spread to people. As early as 1976, Stuart Levy of Tufts University showed that highly drug resistant E. coli on farms can pass easily to farm workers in a matter of a few weeks.

Later, he also showed how those resistant bacteria can spread throughout the community. Many studies in the years since have further proven this linkage. (Click here for more information.)

One recent study that caught my eye was published earlier this year in the journal mBio. In addition to illustrating that microbes and their drug resistance genes can pass between humans and livestock, it also illustrates how these genes are spreading around the globe.

The study involved a recently evolved strain of MRSA called CC398. For about a decade, this strain has been spreading through poultry and pig farms around the world. It has also infected people, most of whom are associated with farms. In parts of the Netherlands, where this bug was first detected, it causes up to 25 percent of all MRSA infections.

Lance Price, of the Translational Genomics Research Institute in Phoenix, and his international team of researchers collected 89 samples of the strain from livestock and people in 19 countries, including the US, China, Germany, Peru, Poland, and French Guyana. Then, they sequenced the entire genomes of the samples and used the sequences to “reconstruct the evolutionary history of this organism,” says Price.

The evolutionary tree of CC398 revealed that it started off as a methicillin sensitive bacterium in humans. Once it jumped to farm animals, it rapidly evolved resistance and did so multiple times on multiple farms.

The new drug resistant form, called livestock-acquired MRSA then jumped back into farm workers. (Read more about the study here.)

Price thinks the original microbe may have come from China. “The longest branches (of the evolutionary tree) are from China,” he says. The spread of the drug resistant form throughout the world probably happened through meat and poultry trade, he adds.

“It’s no big surprise that we’re seeing this,” says Price. “I’m sure we’re going to see this multiple times in the future, if we’re going to continue to use antibiotics in these filthy CAFOs (Concentrated Animal Feeding Operations).”

However several countries are already trying to reduce antibiotic use on farms.

Sweden was the first country to ban the use of antibiotics as growth promoters. Since then, Denmark, the Netherlands and most recently, the EU has also managed to restrict use of antibiotics on farms. And most recently, the US FDA announced a rule that will require farmers to have prescriptions in order to use antibiotics.

Even if we succeed in reducing the load of antibiotics in our farms, there’s no saying that drug resistance will plummet in the near future. Once the resistance genes have spread, they’re likely to stay in the environment for a while. But at least we can reduce the emergence of new super bugs and their concomitant spread throughout the world.

In the meantime, we also need “a global initiative” to develop new antibiotics, says Joakim Larsson of Gothenburg University. Larsson studies how the pharmaceutical industry contributes to antibiotic resistance in different parts of the world. “You really need a revolution in the antibiotic field. And no one has really shown that there’s any revolution in sight.”

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