MRSA and swine: collision course

Blogging on Peer-Reviewed Research Both Mike and Revere have new posts up documenting swine as a new threat to human health (beyond the pork chops and bacon), via carriage of methicillin-resistant Staphylococcus aureus in these animals. Several papers have been published recently documenting high rates of MRSA carriage in swine in the Netherlands, and also have documented transmission of this bacterium from swine to humans. However, even more worrisome to me than the Dutch publications is a new one out in Veterinary Microbiology, showing high rates of MRSA in Canadian swine--and guess where we import about 9 million hogs from every year?

More after the jump...

The current study looked at pigs and pig farmers in 20 farms in Ontario, Canada. They found the bug on almost half of the farms they investigated, and in a quarter of the swine--much higher than the 1% or so of people in the U.S. who are estimated to carry it. Additionally, 20% of their (admittedly small sample of 25) pig farmers were also colonized with MRSA, and humans carrying MRSA were only found on farms where pigs were also MRSA-positive.

To sample farmers, the investigators took only nasal swabs (Staph aureus is typically carried in the nose), but another recent paper suggests that the use of nasal swabs combined with throat swabs captures additional carriers (in their study, almost 13% of those who were positive for Staph aureus carried it in the throat alone--so even higher levels of farmers may be colonized than this study detected).

As expected if the bacterium was moving between man and swine, the isolates taken from the farmers were indistinguishable from those taken from the animals on their farms--so either humans are spreading these bacteria to swine, or (more likely given these data), swine are spreading them to humans.

Finally, percentages of colonized swine varied widely on the farms they sampled. They note:

On many farms, most or all tested pigs were colonized, however on some farms only a small number of pigs were colonized. It is unclear whether there are differences in management on farms associated with this variation. It would be interesting to re-test low prevalence farms to determine whether the prevalence of MRSA colonization has increased, as it is possible that the low prevalence could indicate recent introduction of MRSA.

As Mike notes, MRSA is a commensal bacterium in pigs (and typically, in humans), and doesn't typically cause disease. Therefore, it can spread throughout the population fairly quickly and easily--which it apparently has. This is one thing that makes MRSA such an insidious pathogen: it can spread among us and we don't even know it, until it causes a serious or deadly infection in our community.

It's still uncertain why swine are so heavily infected in these recent studies. Prior research had shown that MRSA could be present in cattle, horses, dogs, cats, and other animals, but it hasn't seemed to spread the way these strains have in the swine investigated. (And indeed, in several of those animals, the direction of transmission seemed to go from humans to animal, instead of the other way around). In the Netherlands, antibiotic use on farms has been implicated, because they have incredible infection control procedures and MRSA is rare even in hospitals. They did find that a common human clone of MRSA was present in some of these pigs, suggesting initial transfer from human to pig, spread through farms, and then transfer back to humans from the infected animals. The question then becomes, even if antibiotic use in animals didn't generate the original mutation, did it help the resistant strains spread through the population?

Revere suggests it's the pig's revenge; Mike notes that the research is interesting and disturbing. I concur with both.

[Edited to add: Alex Koppelman at Salon has an excellent review of MRSA in pigs and swine for more information...]


Khanna et al.. 2007. Methicillin resistant Staphylococcus aureus colonization in pigs and pig farmers. Veterinary Microbiology, in press. Link.

Mertz et al. 2007. Throat swabs are necessary to reliably detect carriers of Staphylococcus aureus. Clinical Infectious Diseases. 45:475-7. Link.


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For those who, like me, wondered why on earth the US is importing so many living hogs, there's a nice explanation here:

I love me some pancetta same as the next girl, but globalized factory farming is bad news for everyone.

Any thoughts about what we average Americans can do about it, Dr. Smith?

Perhaps a silver lining to the falling value of the dollar will be a decline in the number of hog imports.

Any thoughts about what we average Americans can do about it...?

Visit your local farmer's market. Chances are excellent that what you'll find there will be very fresh, extremely good, probably local, and (potentially) less expensive than in that glitzy supermarket around the corner.

Very interesting. Antibiotics are all over in cattle farming too bc cows didnt evolve to eat corn and it messes up their system. Bad thing is, demand for beef and pork keeps going up. Even faster in other countires than USA so its a world wide problem.

My thoughts, go with SOY!! Just not the raw stuff you need to cook it or you can't get the proteins so well.

I find it mystifying that you are alarmed by Canadian porcine imports because there are higher rates of MRSA found in Canadian pig farmers compared to the low carrier rates in the general US population. Aren't you comparing two different populations? For this comparison to be worth anything, you should be comparing Canadian pig farmers to US pig farmers and the general Canadian population to the general US population. Given the highly integrated livestock market in North America, I would be very surprised if there was a statistical difference between MRSA carrier rates in US pig farmers vs Canadian. I personally think the value of these studies lies not in alarmist "Canadian farmers are bad" rhetoric (which is unsupported by the data you've cited at best, and politically motivated at worst), but in an examination of factory farming and its role in antibiotic resistance.

Dr. Smith,
The students in my undergrad microbiology class
(all nursing students) have been swabbing nose, throat, skin, vagina (or perianally, males) for S. aureus for 12 semesters.

The data from the Swiss (throat colonization) is correct.

Every semester we isolate S. aureus from all sites in
1 or 2 students; nose and throat from some; just nose; or
just the throat. 5-10% of these isolates are MRSA.
Interestingly, in some students, there is one strain of
MRSA in the nose, and different strain of MRSA in the throat.

How nasal mupirocin to "eliminate carriage" is going
to clear up tonsil colonization is not obvious.......

none of the students are hog farmers, BTW.
although some could stand to lose a few pounds.....

Beach Bum, should be comparing Canadian pig farmers to US pig farmers and the general Canadian population to the general US population.

Indeed, and I'm not saying at all that "Canadian farmers are bad." I'm saying that it's confirmed there are high rates of colonization there, at least in the farms studied--and I certainly would rather compare apples to apples regarding US swine and farmers as well, but that data isn't available right now. Previous studies and some unpublished data I'm aware of have shown very low levels or no detectable carriage, but none of those (that I've seen) are very recent. I'd argue that it's imperative that some of these studies are done, in fact, so we can track the movement of MRSA amongst farms, including across country borders.

Additionally, one thing I didn't discuss in the post is that other studies have shown MRSA in other household pets and veterinarians, so that's another means of potential introduction of a strain into livestock, from which it can spread.