By Maryn McKenna - 31 January 2012
A paper released recently, by the University of Iowa team that is the lone US research group tracking “pig MRSA” ST398, caused a ripple. (It came out while I was at ScienceOnline and it’s taken me a while to catch up.) The paper compares the occurrence of MRSA, drug-resistant staph, on various cuts of retail pork from pigs that were raised either conventionally in confinement, with routine use of antibiotics, or in an alternative rearing scheme with no antibiotics. (NB: Not “organic,” despite what some headlines said; that’s a separate issue of USDA licensure.) The team found that both conventionally raised and antibiotic-free meat carried MRSA, both the human-associated kind and the pig-adapted kind.
The TL;DR over the past week has been: There’s just as much resistant bacteria on drug-free meat as there is on conventional meat, so why spend the money — or raise the alarm over farm antibiotic use?
My interpretation is a little more nuanced. But my takeaway is that, in its underlying data, the study proves what campaigners against ag antibiotic use keep saying: that once you use antibiotics indiscriminately and drive the emergence of resistant organisms, you have no way of predicting where that resistance DNA will end up.
A recap: The team, which is led by Tara Smith, PhD, collected 395 samples of pork (300 conventional, 95 raised without antibiotics) from supermarkets in Minnesota, Iowa, and the New Jersey suburbs of New York City.
- They found S. aureus, staph bacteria, on 256 of them (202 conventional, 54 alternative.)
- Of those isolates, 230 (183 conventional, 47 alternative) were MSSA or drug-sensitive staph.
- Of the remaining 26 (19 conventional, 7 alternative) MRSA or drug-resistant strains, 7 (5 conventional, 2 alternative) were of types usually associated with the livestock strain, which is usually called ST- or CC- 398.
For those keeping score at home, that’s an overall prevalence of MRSA on pork of of 6.6 percent, which is the highest ever found in the small number of such studies done in the United States. Breaking it down by type of farm, it’s (roughly; this is my math not theirs) 6.3 percent for the conventionally raised meat and 7.3 percent for the alternatively raised.
Readers who got this far in the paper said: Since there was MRSA, and livestock-associated MRSA, on both the conventionally and alternatively raised meat, there really isn’t any evidence for calling conventional farm-antibiotic use into question, and we can all go home.
I think, to the contrary, that there are several issues to unpick. For assistance, I’m reproducing one of the tables from the paper, which lists all the MRSA isolates, their source (conventional v. “RAW,” raised without antibiotics) and the drugs/drug families to which they were resistant. A quick guide to the far column, which types the samples by the results of a particular test: t034 and t011 fall under ST398; t002 and t008 are human, hospital or community, strains. The underlying assumption is that the human strains represent contamination during/after slaughter — though that may be incorrect, as pigs have been found recently carrying what we think of as human strains.
The first issue to raise is that the conventional samples have a crazy amount of strain diversity on them — one of the types, t8314, has never been seen before. What’s driving that?
The second issue to raise is that when the test results were linked back to the samples’ source, 6 of the 7 MRSA-positive RAW isolates turned out to be from the same supermarket chain, and four of them from the same store — including 4 of the 5 human-adapted ones. At that store, the meat arrived as large cuts and was broken down and packaged at the counter by store personnel, which raises the possibility of human contamination — maybe even by a single human — at point of sale.
But the biggest issue, to me, is what you see in the “Resistance profile column.” Cheat sheet: O stands for oxacillin, the lab equivalent of methicillin; CL is clindamycin, E is erythromycin, and T is tetracycline. It’s the T results that I think are worth looking at.
Here’s the issue. Historically, human-adapted MRSA has not been resistant to tetracycline, because tetracycline wasn’t prescribed for it; even now, tetracycline is not a first-choice drug. So, when MRSA ST398 arose in pigs in the Netherlands in 2004, and was resistant to tetracycline, that was a big red flag, because it was a change in the pattern — and because tetracycline is used in huge amounts in pigs, it was a pointer back to the involvement of farm drugs in the strain’s emergence. But in this table, there is a ton of T (tetracycline) resistance, in half of the alternative isolates and in almost all of the conventional ones, including the human types.
So what that says to me is not: There’s MRSA everywhere, and therefore no point in concentrating on farm-origin drug-resistance. To me, it suggests the opposite: A marker for farm-origin drug-resistance is present in the majority of isolates found in this study, even when found on animals raised without antibiotics, and even when presumed to be coming from humans.
Campaigners against antibiotic use on farms aver, and have good evidence to support, that once antibiotic resistance emerges on farms, it doesn’t stay on farms, but leaves them via animals, via manure, via wind, water and farmworkers — and that once that resistance DNA gets into the world, there is no way of tracking where it ends up. Rather than undermining that contention, this study seems to me to uphold it — because the indicator of farm-origin resistance is present in a number of isolates where it would have no obvious reason to be.
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