Study: Industrial food-animal workers test positive for livestock-associated MRSA

In a recent study comparing workers at industrial livestock operations and those employed at antibiotic-free livestock operations, researchers found that industrial workers were much more likely to carry livestock-associated strains of drug-resistant Staphylococcus aureus, more commonly and scarily known as MRSA.

First, it's important to note that both groups of workers had a similar prevalence of S. aureus and methicillin-resistant S. Aureus (MRSA); however, it was overwhelmingly workers at industrial livestock operations, sometimes known as concentrated animal feeding operations or CAFOs, whose nasal swabs tested positive for livestock-associated MRSA and multidrug-resistant S. aureus (MDRSA). The study, which was published earlier this month in PLOS ONE, also takes care to note that "despite current understanding of livestock-associated MRSA as a relatively rare cause of human infection in the United States (which may be limited due to a lack of systematic national surveillance), there is a public health concern about potential broad dissemination of drug-resistant S. aureus to the general public."

Christopher Heaney, a corresponding author of the study and an assistant professor of environmental health sciences and epidemiology at Johns Hopkins University, said the idea for the study stemmed from environmental health work already being conducted within North Carolina communities that are home to high densities of industrial livestock operations. Heaney said it was a chance to study the emergence of a drug-resistant bacteria in the U.S., where little is currently known about the relationship between livestock workers and antibiotic-resistant S. aureus.

"The science is emerging in Europe, where they're seeing livestock-associated drug-resistant S. aureus and it appears to be having an impact beyond just the workers," he told me. "In the United States, we have a limited understanding of the potential health implications of livestock-associated strains of S. aureus. In other parts of the world, there's been a greater degree of scientific discovery around the emergence of this novel strain."

Knowing that food-animal production is a documented source of MRSA and MDRSA, Heaney and his colleagues set out to gain a clearer picture of the genetic characteristics of S. aureus among industrial and antibiotic-free livestock workers and their household members. Study authors Heaney, Jessica Rinsky, Maya Nadimpalli, Steve Wing, Devon Hall, Dothula Baron, Lance Price, Jesper Larsen, Marc Stegger and Jill Stewart write:

The practices of intensive confinement and administration of antibiotics to animals — including non-therapeutically for growth promotion — are commonly used in industrial food-animal production and provide a reservoir for the selection of novel, antibiotic-resistant bacteria that can be exchanged between animals and humans....Of particular interest is S. aureus CC398, first reported among pig farmers in France in 2005 and later among pigs and pig handlers across Europe and North America. Although human infections from livestock-associated S. aureus CC398 and MRSA CC398 appear to be rare at this time in the United States, spread of MRSA CC398 from livestock to humans has increased the burden of MRSA infections in many European countries, including Denmark, Germany, and the Netherlands.

To conduct the study, researchers recruited North Carolina livestock workers employed at swine or poultry operations as well as up to two individuals in the same household as the worker. In addition to collecting information on participants' work responsibilities, livestock contact and medical history, researchers also took nasal swab samples. The results: S. aureus CC398 was found only among workers and mainly among workers in industrial livestock operations — in fact, the livestock-associated strain was 13 times as prevalent among industrial workers, Heaney said. Also, only industrial workers were carrying MRSA and MDRSA isolates resistant to tetracycline, an antibiotic used to fight a number of bacterial infections.

The study findings suggest that while both groups of workers are being exposed to MRSA and MDRSA, workers at industrial operations "may be exposed to a larger or more sustained livestock reservoir of MRSA and MDRSA, or that this livestock-associated MRSA and MDRSA are transmitted more frequently between animals and humans" in industrial as opposed to antibiotic-free settings. It's also important to note that among industrial livestock participants, more than 80 percent identified as Hispanic, while nearly 80 percent of antibiotic-free livestock workers identified as non-Hispanic.

"What was remarkable about our study is that while everyone had either direct or indirect contact with livestock, we observed drug-resistant S. aureus strains with multiple characteristics of livestock association only in the industrial group," Heaney said. "That was unexpected."

In terms of whether additional occupational protections are needed among industrial livestock workers, Heaney said more studies are needed to say definitively — "right now, we just don't know." Similarly, he said more research is also needed to determine the health risks that livestock-associated strains pose to people and workers.

"The bottom line is we still need to conduct further studies," Heaney told me. "What this means beyond this population we still don't know, so we need to take the next steps. But certainly if we follow the scientific discovery and the story of emergence of livestock-associated MRSA in Europe...it all contributes to concern and questions around antibiotic use in livestock production."

To read the full study, click here. To learn more about the public health hazards of industrial livestock operations, visit www.cdc.gov/nceh/ehs/Topics/CAFO.htm.

Kim Krisberg is a freelance public health writer living in Austin, Texas, and has been writing about public health for more than a decade.