But isn’t the industry saying that there’s no definitive evidence linking MRSA in pigs to MRSA in humans?
It depends on your standards of evidence. MRSA ST398 has been found colonizing pig farms and pig farmers in the US, Canada, and in the European Union. You can argue about how prevalent it is — it’s easy to cast doubt on whether it is common, because not very many studies have been done. But you can’t argue that it is there.
The argument over whether ST398 in pigs is causing MRSA disease in humans is more subtle. It definitely has in the Netherlands, though not often so far. Has it done so in the US? No one has done the microbiology to tell. We know there is a lot of community-strain MRSA everywhere in the country, much more than most people realize. It is so common that many physicians essentially now diagnose it by sight; if they order tests, the tests will be for drug sensitivity, because that helps the patient, not for strain identification, because that is primarily useful to public health authorities. If there were ST398 cases hiding within that larger epidemic of USA300, it would take a fair amount of science to find out, and someone has to pay for that science to be done.
A related argument is, Well, MRSA colonization may be widespread, but as long as it doesn’t cause illness, that’s not a problem. Again, I disagree. The presence of any resistant organism is a concern, because bacteria are promiscuous: They swap genetic information, including resistance factors, through an array of strategies evolved over millions of generations. It is not sensible to dismiss a resistant organism to which both animals and humans are vulnerable, and which is already present in animals with which humans share space, because humans are not being made obviously sick by it today.
There’s a lot packed into the last two paragraphs, but one thing that the various agricultural lobbies have never stated (for obvious reasons) is what it would take to convince them that there is transmission of either resistant strains or resistance genes from the farm to clinic. I’ve never heard this. What I’ve heard–to the point where you can just fill in the blanks with a particular antibiotic and bacterium–is something like this:
Whenever we start to see resistance to a particular drug in an agricultural setting, there’s the response (or some variant thereof) that resistance to that drug is mostly clinical. What they neglect to add is the phrase “at the present time.” Which is the whole fucking point. If we see a troubling rise in an MRSA clone in pigs (more like an epidemic sweep), then that should be cause for action. Of course, the ag lobby will argue that we should wait until resistance becomes relatively common–at which point, they will agree that it’s a problem. Then they argue that changing antibiotic use is moot because resistance to the drug of interest is so common it won’t make a difference. Nice denialist tautology you have there.