Antibiotic-Resistant Bacteria Surround Big Swine Farms — In China as Well as the U.S.

I suspect we think of large-scale confinement agriculture as a uniquely American issue. Possibly that’s because growth-promoter antibiotic use, which makes meat-raising efficient, originated in the United States; more likely, it’s because some of the largest firms in that sector — Smithfield and Tyson, for example — are US-based. But public and private research efforts (including the US Department of Agriculture, the United Nations’ Food and Agriculture Organization and the Pew Charitable Trusts) have documented that intensive livestock-raising is increasing in emerging economies such as India and China; as incomes rise, demand for meat does too.

A paper published Monday in the Proceedings of the National Academy of Sciences demonstrates that the unintended consequences of confinement agriculture are occurring in those countries as well. A multi-national team of researchers from Michigan State University and two campuses of the Chinese Academy of Sciences found — well, I can’t put it better than their paper’s title does: “Diverse and abundant antibiotic resistance genes in Chinese swine farms.”

If you’ve followed news about food in China (at this blog or elsewhere), you’ll have seen that regulation of food safety is failing under the twin pressures of needing to produce a lot of protein and wanting to make a lot of money. (I think of food in China as being where the United States was before Upton Sinclair came along.) This lack of regulation is as true for agricultural antibiotic use as it is for other aspects of food production. China is both the largest producer and the largest consumer of antibiotics in the world, and it is putting almost half of its annual production into agriculture: about 96 million kilograms, which by my math (using the newest ADUFA numbers in my last post) works out to about 7 times what the US is using each year.

As in the US, there is no requirement to report how or in what species agricultural antibiotics are used, and no organized effort to keep track of the potential effects. Which to me makes it especially heartening that the research published Monday originated with the Chinese researchers, who for several years now have been publishing studies of environmental conditions around the growing number of big confinement farms in the country.

For this paper, the team went to three farms, in three parts of China, that are large by current Chinese standards but not by American ones: Each one raised about 10,000 pigs per year. (By US EPA definitions, a large CAFO, “confined animal feeding operation,” starts at 10,000 and goes up from there.) At each farm, they took samples of fresh pig manure; manure as it was being composted for use as fertilizer; and farm soil where the composted fertilizer had been applied. For controls, they also took soil samples from a virgin forest elsewhere in China, and manure from pigs in the United States which had never been fed antibiotics.

The analysis of the samples was the responsibility of the American side of the effort. James Tiedje and colleagues at Michigan State applied high-capacity quantitative PCR to look for any antibiotic-resistance genes present in the samples, a technique that allowed them to cast a much wider net than traditional culture-based methods. They found 149 unique resistance genes, present anywhere from 192 times to 28,000 times more frequently than in the control samples, and an equally high presence of transposases, enzymes that enable the movement of those resistance genes from one bacterium to another.

To quote from the paper: “The diverse set of resistance genes detected potentially confer resistance to all major classes of antibiotics, including antibiotics critically important for human medicine.”

An important concern, emphasized by the authors, is that resistance genes tend to group together and can be transferred between bacteria in clusters — a process enhanced by the influence of heavy metals such as zinc, copper and arsenic that are also being used on these farms. As a result, resistance to a drug can appear even in an area where the drug has not been used and therefore hasn’t exerted evolutionary pressure on bacteria. For instance, resistance to particular drug classes — aminoglycosides in one case and amphenicols in another — appeared around farms where those drug classes had not been used. The authors also found some genes that influence resistance to vancomycin, one of the very last-ditch drugs, but say they are less concerned about those because the emergence of true vancomycin resistance requires the activity of a number of genes in concert.

Their summation:

The diversity and abundance of (antibiotic resistance genes) reported in this study is alarming and clearly indicates that unmonitored use of antibiotics and metals on swine farms has expanded the diversity and abundance of the antibiotic resistance reservoir in the farm environment. The coenrichment of ARGs and transposases further exacerbates the risks of transfer of ARGs from livestock animals to human-associated bacteria, and then spread among human populations.

If you’ve been following this issue for a while, you’ll recognize that the last clause in that summation has always been the sticking point for the agriculture industry: not whether antibiotic resistance emerges on farms, but whether it moves off farms to affect people with no farm connection. (Generally, industry says No, it does not.)  I asked Tiedje — who was many times zones to the west and kindly got up early to talk — his thoughts about this disagreement. He said:

It is a question of probability. The larger the concentration of antibiotic resistance genes, and the more they are distributed in the environment, the more there is an increase in the probability this will lead to serious problems.  I am not in favor of waiting for this to happen, when we know it is going to happen at some stage; it is better to manage it up front as best we can. It is extremely difficult to find new, effective, safe antibiotics. So we have go to protect the ones we have.

One final point: I asked Tiedje if it was possible to draw comparisons between the use of antibiotics on these Chinese farms, and the drugs’ use on American CAFO. I expected to hear that, with China’s food production so uncontrolled, its antibiotic use would be too, and the farms’ drug use would be off the charts, a cautionary tale that we could learn from. He corrected me, though.

“We consider the Chinese farms that we chose to represent reasonable, standard practice for many places; they are not the high-concentration outliers,” he said. “We know the antibiotic usage on these farms — on average, per pig — is not that much different from the US.” Which is not good news — because it means, if this research is accurate, that this diversity and volume of antibiotic resistance genes, and their potential for movement away from the farm and into the rest of the environment, will be present around US CAFOs as well.

Cite: Zhu YG, Johnson TA, Su JQ et al. “Diverse and abundant antibiotic resistance genes in Chinese swine farms.” PNAS, Feb. 11, 2013.

(Addendum: For more about swine agriculture in China, a good resource is the Institute for Agriculture and Trade Policy’s 2011 report, Feeding China’s Pigs.)




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