"Pig MRSA" Carried by Workers from North Carolina Intensive Hog Farms

Pig farms from the air. Maryn McKenna, Creative Commons License

I saved this post until today to allow everyone to get their holiday hot dogs guilt-free. Now that’s over: An important study has just been published which makes a close connection between the emergence of antibiotic-resistant bacteria, and the use of antibiotics on large-scale conventional hog farms. Bonus: It involves the resistant bacterium MRSA ST398 (known in shorthand as “pig MRSA”), which is widespread in Europe but up to this point has been found mainly in only one state in the US, Iowa. With this paper, the count rises to two(see Update): The study subjects in this paper are hog-farm workers in eastern North Carolina.

A quick explanation of why this is important: “Pig MRSA” is a particular strain of drug-resistant staph that is slightly different from the hospital and community (sports, gym) varieties. It was first spotted in the Netherlands in 2004, in the toddler daughter of pig farmers and in the family’s pigs. Since then, it has spread widely across Europe, not just in agriculture, but in healthcare and in everyday life, and has also been found widely in retail meat.

The question of whether livestock production’s use of antibiotics causes antibiotic-resistant bacteria to move into the wider world is much argued-over, and pig MRSA, or ST398 to be polite, is crucial to that dispute. That’s because, unlike most resistant bacteria, it has a genetic signature that makes an inarguable link back to farm drug use. More on that below. (If you want more, here’s an archive of my posts on ST398; the story of its emergence in 2004 and what happened afterward is told in my 2010 book SUPERBUG.)

Now, the study. Quick summary, with more unpacking to follow: Researchers checked livestock-farm workers in North Carolina to see whether they were carrying staph, and also drug-resistant staph. The workers formed two groups: one group worked at conventional hog operations, which routinely use antibiotics, and the other group at antibiotic-free farms. Both groups carried staph and also drug-resistant staph, which would be expected; about 30 percent of the population carries sensitive staph and about 4 percent carries the drug-resistant form. But, the key difference: Workers from the conventional, antibiotic-using farms were many times more likely to carry staph with the specific signature of farm-drug use.

That illuminates a potential occupational risk to the workers — and it also suggests that the workers could be a channel for that farm-influenced bacterium to move off the farm.

Studies like this are hard to do; in the US, large-scale agriculture has mostly declined to participate, for reasons that are not hard to imagine. When you look at the “About the authors” box on this paper, you get a sense of the collaboration it took to get this study done. The researchers come from University of North Carolina, Chapel Hill, close to some of the most intensive pig-farming in the country; Johns Hopkins University, where there’s a significant focus on antibiotic misuse; George Washington University, new home of Lance Price, probably the foremost researcher in the US in antibiotic-resistance genomics; and the Statens Serum Institute in Denmark, the government-supported research center in the country that has done the most to control agricultural antibiotic use. Crucially, the last partner in the research was the Rural Empowerment Association for Community Help, a farmworkers’-rights nonprofit in North Carolina.

Here’s what the collaborators did. They recruited farm workers whose jobs were on either conventional or antibiotic-free farms, and some of their household members, and checked them to see whether they were carrying drug-sensitive or drug-resistant staph. Staph’s preferred habitat on our bodies is just inside the nostrils, so they took nasal swabs from the participants, cultured bacteria from the swabs, and then did a range of analyses on the bacteria that reproduced: a culture-based susceptibility assay, and also PCR, molecular typing and whole genome-sequencing for identification and to confirm resistance factors as well.

Here is the core of the results:

  • 41 of 99 (41.4 percent) conventional-farm workers and 42 of 105 (40 percent) antibiotic free-farm workers carried staph bacteria in their nostrils.
  • 13 of the 41 staph-positive conventional-farm workers carried ST398, the pig-MRSA strain.
  • 1 of the 42 antibiotic free-farm workers did.

The group takes a number of other cuts through the data, regarding who was carrying “classic” MRSA, MRSA that has acquired resistance to additional families of drugs, and MRSA of different clonal complexes. I’ll put the key table below. The important point is that workers on antibiotic-using farms come off worse in every category. An additional issue: In this analysis, some staph strains are tetracycline-resistant even when they are not MRSA or what this team calls MDRSA, multi-drug resistant staph. Tetracycline resistance is the hallmark of pig MRSA; it is what allowed ST398 to be identified in the first place in the Netherlands, because tetracycline was being used only in pigs and not in humans.  So if tetracycline resistance is spreading through staph populations on hog farms, that reinforces the claim that agricultural use fosters the emergence of resistance.

The entire study is worth reading, because in the discussion of results the authors provide a granular analysis of the data, along with candor regarding where they may have fallen short. (They also appropriately acknowledge Tara Smith, who was the first and until now the only researcher to identify ST398 in US farm workers.) They end with this:

The present study would be strengthened by sampling animals and production facilities; however, we did not have access during the course of the study. Access to livestock operations by the public health research community in Europe and other countries has allowed a robust body of evidence to develop there regarding the extent of exposure to S. aureus, MRSA, and MDRSA among (conventional-farm) workers as well as the potential dissemination of livestock-associated strains into the broader community. Though our study may be viewed as small, to the best of our knowledge, it is one of the largest studies on this topic in the United States to date.

Imagine how much more we would know about this difficult problem if the barriers they hint at did not exist.

*Update: I missed that Tara Smith and colleagues published an important paper on ST398 in May. (Was I traveling? Probably.) I will double-check with them — and try to write it up too as it deserves attention — but it looks as though they found MRSA ST398 primarily in Iowa (as their original 2009 paper did), but also in Illinois, and a single isolate in Ohio. So it’s not correct to say North Carolina is the second state, as I did above. It’s one of a few.


Rinsky JL, Nadimpalli M, Wing S et al. Livestock-Associated Methicillin and Multidrug Resistant Staphylococcus aureus Is Present among Industrial, Not Antibiotic-Free Livestock Operation Workers in North Carolina.  PLoS ONE 8(7): e67641. doi:10.1371/journal.pone.0067641. E-pub July 2, 2013.


(Personal note: This is the first post I’ve written at a new-to-me standing-up desk. With the change in perspective, there  may be typos. Let me know about them, but try to be kind. – m.)


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