More MRSA Appearing in Meat

Nearly 65% of supermarket pork products labeled “antibiotic-free” contain some form of Staphylococcus aureus, and more than 6% harbor the drug-resistant strain known as MRSA.

That finding surprised the scientists who performed the research, said Tara Smith, PhD, an epidemiologist at the University of Iowa and a lead investigator on the study, which was published in the journal PLoS One

“We were surprised not only that the prevalence was so high but that there was no difference between conventional pork products and alternative products,” Dr. Smith said. The study included some 395 pork product samples from stores in Iowa, Minnesota, and New Jersey. “In our on-farm studies, we hadn’t seen any evidence of MRSA in antibiotic-free farms. It’s possible that’s because our on-farm studies were small; we only tested 18 farms, nine of each type.”

Dr. Smith pointed out that, once pigs leave individual farms, there are at least half a dozen other steps in the process during which pigs could become contaminated with MRSA—during transport, at multiple points within the processing plant, and being wrapped in store at butcher counters. “We would love to get into the processing plants to follow the meat through and really see what happens step by step, but it’s hard to get people to let us do that type of sampling.”

The question of MRSA in meats has been examined in a number of studies recently, including others from Dr. Smith and her team, but no one seems to be sure what it means for public health. And that’s the question Dr. Smith wants answered. “We’re now doing a risk analysis to try to determine what, if anything, is the risk of having MRSA on food. We know it’s on the meat, but how important is it in terms of human colonization?” The Centers for Disease Control and Prevention has never linked a MRSA case to contaminated food, but that doesn’t necessarily mean it hasn’t happened.

The new University of Iowa study, funded by the USDA, will follow 100 families at home, taking samples from them on a weekly basis to determine colonization and following up to track possible infections, while also examining the meat in the stores those families shop in. “We’ll also be looking at MRSA in their environment. At the end of a year of sampling, we hope to put that all together and determine if there’s a higher level of MRSA on the meat products some people encounter, and are those people more likely to be colonized?” The fully enrolled study is now receiving about 250 samples a week from the families involved, along with about 100 meat products.