Researchers from the University of Iowa College of Public Health and the Institute for Agriculture and Trade Policy (IATP) tested 395 fresh pork samples from 36 retail stores in Iowa, Minnesota and New Jersey. They found that 68.4% (256) of samples were contaminated with S. aureus—a bacteria which can cause serious blood, skin and lung infections in humans, while 6.6% were contaminated with MRSA, a deadly bacteria that causes nearly 19,000 deaths a year in the US.
Within the MRSA strains, there was a high level of antibiotic resistance, with 76.9% resistant to two or more antibiotics and 38.5% resistant to three or more antibiotics tested.
The study, which was published in online science journal PloS ONE, represents the biggest screening of raw meat products for MRSA carried out so far in the US. The prevalence of MRSA found in the study was much higher than previous, smaller studies, have indicated. A screening of 165 meat samples last year indicated that 1.2% of pork was infected with the bacteria.
IATP’s MD David Wallinga said that the results suggested that the prevalence of MRSA in the US was "on the same order as Europe", although while European studies had commonly found the MRSA ST 398 strain, or “livestock associated” MRSA, the MRSA found in US pork was generally not the ST 398 strain, so it was unclear exactly where the infection was coming from.
He suggested that MRSA could be on the increase because of the use of antibiotics in pork production.
"MRSA is getting more prevalent outside the hospital in general. In Europe, there have been studies strongly implicating community based MRSA infections with industrial livestock production, and with raising pigs on antibiotics in particular. That connection has been more equivocal in the US," he said.
"We don’t know exactly why it is prevalent in the US, and seemingly newly so, although it wasn’t being closed tracked in the past. This is exactly why Americans deserve to have our regulators and food safety agencies doing much, much more to routinely monitor MRSA in the food supply and to chase down how and why it is getting there."
Interestingly, the study found no statistically significant difference in MRSA prevalence on conventional pork versus “alternative” pork which had been “raised without antibiotics” or “raised without antibiotic growth promotants”, free of antibiotics. This contradicts findings from farms, which have to date found that MRSA is more prevalent on conventional farms than “alternative” pig farms.
The report on the study suggested that this could be because the link between on-farm antibiotic use and MRSA on meat products had been “obscured” by contamination of meat with MRSA post-slaughter. They also pointed out that there was no third-party verification of “raised without antibiotics” and “raised without antibiotic growth promotants” meat in the US.
Wallinga said that until proven otherwise, the industry should act as routine antibiotic use in livestock production drives contamination of food with bacteria resistant to multiple antibiotics, including but not limited to MRSA.
"Hog producers ought to be finding every possible way to avoid antibiotic use, starting in the adult or finisher pigs, and aiming to eliminate such use altogether over the next several years," he said.