Industrial farm workers carry drug-resistant bacteria associated with livestock, study finds

July 03, 2013
A new study found drug-resistant bacteria associated with livestock in the noses of industrial livestock workers but not in the noses of antibiotic-free livestock workers in North Carolina.

Dr. Chris Heaney

Dr. Chris Heaney
Dr. Steve Wing

Dr. Steve Wing
Dr. Jill Stewart

Dr. Jill Stewart

The research, led by Christopher D. Heaney, PhD, and begun while he was a W.K. Kellogg Health Scholar at Gillings School of Global Public Health, was published online July 2 in the journal PLOS ONE.

Gillings School co-authors include Steve Wing, PhD, associate professor of epidemiology; Jill Stewart, PhD, assistant professor of environmental sciences and engineering; and doctoral students Jessica Rinsky (epidemiology) and Maya Nadimpalli (environmental sciences and engineering).

The drug-resistant bacteria examined were Staphylococcus aureus, commonly known as Staph, which include the well-known bug MRSA (methicillin-resistant Staphylococcus aureus). New Staph strains, aptly called livestock-associated Staph, are emerging in people who have close contact with livestock animals. While all study participants had direct or indirect contact with livestock, only industrial workers carried antibiotic-resistant Staph with multiple genetic characteristics linked to livestock.

Many industrial livestock operations raise animals in large conferment buildings and use antibiotics, including non-therapeutically, in animals’ feed and water to promote their growth. Previous studies have detected strains of drug-resistant S. aureus from livestock, first among farm workers, and subsequently in hospital and community settings in Europe.

In the United States, such strains have been detected among industrial livestock operation workers in Iowa – and now in North Carolina – making scientists concerned that these bacteria could follow a similar trajectory into the community. North Carolina is a major livestock producer, ranking second behind Iowa in hog production in the United States.

S. aureus can cause a range of illnesses in humans, from minor to life-threatening skin, bloodstream, respiratory, urinary and surgical-site infections. Like most illnesses caused by bacteria, S. aureus infections are treated with antibiotics. According to the Centers for Disease Control and Prevention, some Staph are antibiotic-resistant, i.e., they cannot be killed by antibiotics. MRSA is a strain of Staph bacteria that is resistant to methicillin and certain first-line antibiotics called beta-lactams. Infections with drug-resistant strains, like MRSA, can be particularly difficult to treat.

The study was based on interviews and nose swabs that were collected and analyzed from individuals who worked at two different types of livestock operations in North Carolina. At industrial livestock operations, animals are grown in large confinement buildings using antibiotics. At antibiotic-free livestock operations, animals are grown without the use of antibiotics, typically outdoors on pasture. Researchers tested the S. aureus isolated from nose swabs for resistance to a range of antibiotics and for genetic markers considered to indicate that the bacteria may have come from livestock.

The authors note that the livestock-associated strains are present among workers at industrial livestock operations and are resistant to multiple antibiotics, including ones used to treat human infections.

Workers were not experiencing Staph infections at the time of the study, but when antibiotic-resistant bacteria do cause infections, they can be harder to treat. Researchers found that multidrug-resistant S. aureus were roughly twice as prevalent among individuals exposed to the industrial compared to the antibiotic-free livestock operation environment and tetracycline-resistant S. aureus were 19 times as prevalent among industrial compared to antibiotic-free livestock operation workers. Tetracycline is an antibiotic that has been used in industrial livestock production since the 1950s.

Other co-authors of the article are Lance B. Price, PhD, professor of environmental and occupational health at The George Washington University; Jesper Larsen, PhD, and Marc Stegger, PhD, of the Statens Serum Institute and Dothula Baron, executive director, and Devon Hall, project manager, of the Rural Empowerment Association for Community Help (REACH), in Warsaw, N.C. Heaney is now assistant professor of environmental health sciences and epidemiology at the Johns Hopkins Bloomberg School of Public Health.

“Livestock-associated methicillin and multidrug resistant Staphylococcus aureus is present among industrial, not antibiotic-free livestock operation workers in North Carolina” is available on the PLOS ONE website.




Gillings School of Global Public Health contact: David Pesci, director of communications, (919) 962-2600 or