Workers at industrial livestock operations are much more likely than workers at antibiotic-free livestock companies to carry livestock-associated strains of Staphylococcus aureus, as well as multidrug-resistant S. aureus, according to a new study from researchers at Johns Hopkins University’s Bloomberg School of Public Health.
The researchers recruited workers at swine and poultry plants in North Carolina for the study; they also studied two individuals from each worker’s household. Both sets of workers had a similar prevalence of S. aureus and methicillin-resistant S. aureus (MRSA)—about 40 percent and 7 percent, respectively. But 37 percent of the industrial operation workers carried multidrug-resistant strains of SA, compared with just 19 percent of antibiotic-free operation workers.
The researchers were particularly interested in a strain of livestock-associated MRSA and MDRSA (tetracycline-resistant, CC398, scn-negative), which had been associated with a European outbreak of MRSA several years ago. This strain, which had principally been found in livestock workers, ultimately spread into community and hospital settings, says one of the study’s authors, Christopher Heaney, PhD, an assistant professor of environmental health sciences and epidemiology. “Screening programs at hospitals in the Netherlands and Denmark found that a large proportion of these MRSA and MDRSA cases were this livestock-associated sequence type 398.”
In the North Carolina study, SA CC398 was found only among the workers—not anyone else in the household—and almost exclusively among workers in industrial livestock operations. (Only one of the antibiotic-free livestock workers carried this strain.) The industrial workers were also the only ones carrying strains of SA that are resistant to tetracycline.
“We think this study contributes to the body of work that raises questions about antimicrobial uses in livestock production,” Dr. Heaney says.