I’m spending the first part of this week at the International Conference for Emerging Infectious Diseases, a biennial scary-disease nerdgasm that is sponsored by the Centers for Disease Control and Prevention, the American Society for Microbiology and other worthy organizations, and in this iteration has drawn about 1,600 participants from more than 50 countries. (You can see the program here. I had planned to live-blog it, but the hotel was just renovated and apparently they built a Faraday cage into it, since connectivity is a painful trickle.)
On Monday’s agenda, among other intriguing talks: Two updates on MRSA, methicillin-resistant S. aureus, among farmers in two states.
A quick recap: MRSA, drug-resistant staph, was first a hospital infection (starting in 1961), then spread into a broad “community MRSA” epidemic in people who have no connection to hospitals or healthcare (since about 1996) — and then sparked a third epidemic of “livestock-associated” MRSA, slightly different from the previous two and first identified (in 2004) in the families of Dutch pig-farmers. LA-MRSA — or more familiarly “pig MRSA,” though swine agriculture understandably dislikes the term — spread through the European Union, crossed to Canada in 2007, and in 2009 was identified in Iowa pigs and pig farmers by the research team led by Tara C. Smith, Ph.D., at the University of Iowa.
In Europe and Canada, LA-MRSA has been identified not only in farm families but also in retail meat, in hospital patients and healthcare workers, and in people who have no known connection to agriculture. How big a problem it might pose in the United States is disputed. The Netherlands, where it arose, unlike the United States has almost no community MRSA epidemic; LA-MRSA may have burgeoned there precisely because that ecological niche wasn’t otherwise occupied.
No one can say at this point how big it might get in the United States, in part because few teams other than Smith’s are looking for it. But it’s important to keep track because, in the back-and-forth arguments over the influence of agricultural antibiotic use on resistance, LA-MRSA represents something unusual: an organism whose resistance to tetracycline indicates farm-drug exposure. (For an explanation of that, try this recent post.)
There were two presentations at ICEID Monday on LA-MRSA, which also goes by the designation ST398 for the result it usually returns in the molecular test MLST, multi-locus sequence typing. Both presentations demonstrate how perplexing the bug continues to be.
The first, a poster whose first author is Lynda Odofin of Yale University, reported the results of two rounds of sampling on farms in Connecticut. The first round, in 2009, visited 35 swine farms and found pigs and workers carrying MRSA, without showing symptoms, on five of the 35. In the second round, the researchers went back to those five farms, and also to nine farms that were negative in the first study and were intended to function as controls in this second round.
But things got complicated. In the second round, two of the five originally positive farms remained positive — that is, the researchers could still find MRSA on them — but three were negative. On the other hand, of the nine control farms that were all originally negative — with no MRSA evident in workers or pigs — five now harbored MRSA. Making things yet more complex, the MRSA types in the pigs on those farms were largely human strains. Most of them were a type associated with hospital MRSA, but one farm exhibited a type associated with community MRSA. Two also harbored a staph strain that falls under “pig MRSA” — but only one was MRSA, and the other was methicillin-susceptible, or MSSA.
If there are lessons in those findings (Odofin was not available to ask), they are, first, that MRSA persists on some farms year to year, but also that strains are moving back and forth between pigs and humans in complex ways.
The second presentation, a slide session by Tara Smith, represented a glimpse inside an ongoing study of human MRSA and MSSA, dubbed SIRI for “Staph in Rural Iowa,” which so far has recruited 1,293 adults and 53 children in 32 counties in Iowa. (The study subjects are already participants in a separate ongoing study of pesticide-using farmers called the Agricultural Health Study.) The participants raise swine, cattle, chickens or some mix of those, mostly on smaller farms — that is, mostly not on the very large confinement farms whose antibiotic use is believed to drive the evolution of resistant organisms.
In their participants, Smith said, the Iowa team has so far found carriage, without symptoms, of staph that is resistant to tetracycline, and also staph that is multi-drug resistant, though not necessarily to methicillin and other beta-lactam antibiotics — making it both resistant and yet technically MSSA. (Similar to these meat samples.) Farmers raising cattle were somewhat more likely to be carrying resistant staph, and those raising swine were three to four times more likely to be.
The strains are still being analyzed, she said, but so far the most common type of MRSA they have found is the community strain; farmers are also carrying hospital-, community- and livestock-associated types that are MSSA but still resistant to several antibiotics. As part of the study, they are following up with farmers to see if the strains they are carrying cause infections. So far, five farmers have reported back, two of them with repeat infections; six of the infections have been MSSA and one MRSA. One sample from an infection has been analyzed so far and found to be ST398, the livestock strain, and while not MRSA, resistant to three antibiotics: tetracycline, levaquin, and the combination trimethoprim-sulfamethoxazole.
With much more work to go, the team can say this much, Smith said: “We are seeing livestock-associated staph in this cohort, we are seeing it in spouses as well, and we are seeing that carriage of staph turn into infections.”
- Odofin L, Smith TC, Hanson B et al. Persistence of Methicillin-Resistant Staphylococcus aureus (MRSA) in Connecticut Swine Farms. 8th International Conference on Emerging Infectious Diseases, Atlanta, Ga. March 12, 2012.
- Smith TC. Prevalence and Molecular Epidemiology of S. aureus in a Cohort of Rural Iowans. 8th International Conference on Emerging Infectious Diseases, Atlanta, Ga. March 12, 2012.
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