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.