Methicillin-resistant Staphylococcus aureus (MRSA) may be more prevalent in retail pork products than previously thought, according to a study published in the Public Library of Science’s PLoS ONE in January.

More than six percent of 395 pork samples, taken from 36 grocery stores in Iowa, Minnesota, and New Jersey, were contaminated with MRSA, which is significantly higher than previous studies. More than 64 percent of samples tested positive for Staphylococcus aureus, or staph bacteria. The results also did not show a significant difference in MRSA contamination between conventional meats and alternative, or antibiotic-free meats.

MRSA is a dangerous type of staph bacteria that is carried by less than 2 percent of people, according to the Centers for Disease Control and Prevention. In 2005, around 94,000 people developed their first serious MRSA infection and of that population 19,000 people died. Peer reviewed journals estimate that around 86 percent of these infections are health-care associated and 14 percent are community associated. Though MRSA has been found in food products in previous studies, CDC has never linked a MRSA infection to eating a contaminated food product.

S. aureus, carried by around a third of the population, is estimated to cause around 185,000 cases of foodborne illnesses annually. The bacteria can also cause serious blood, skin and lung infections.

According to one of the study’s authors Tara Smith, an epidemiologist at the University of Iowa, the molecular typing from the samples doesn’t point to a clear source of contamination.

“In the simplest analysis of these findings, these molecular types (a combination of “human” and “pig” strains) suggests that MRSA on raw pork products may be coming both from farms and from food handlers,” said Smith. “However, in real life, it’s not quite so straightforward … the source of contamination and relative contributions of live pigs versus human meat handlers currently isn’t certain.”

Of the MRSA strains found, nearly 77 percent were resistant to two or more antibiotics and 38 percent were resistant to three or more antibiotics.

Researchers note that finding a certain livestock strain of MRSA, MRSA ST398, in farm environments is of concern, in part because of the strain’s potential to transfer from animals to humans, via the food supply.

“In North America, MRSA ST398 has been detected in pigs and farmers in Canada and the United States (U.S.), and human ST398 infections have been demonstrated in Canada,” notes the study. “As not all patients diagnosed with ST398 infections had known contact with livestock, the possibility of acquisition of ST398 via handling of contaminated pork products was suggested.”

The majority of pork samples testing positive for S. aureus also raises questions about what other pathogens might be present, according to Smith.

“[The study] suggests that we need processing plants and packing companies to work with us to determine where products are being contaminated–because while there may be arguments about the public health importance of MRSA on meats (or lack thereof), it’s very likely that if S. aureus are ending up on meat products, other pathogens are as well,” said Smith.

Smith’s recommendation? “Don’t assume that any meat product is contamination-free, and always use good food handling/cooking practices when dealing with raw meats.”