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Member Research and Reports

UNC-Johns Hopkins Study: Hog Workers Carry Drug-Resistant Bacteria Even After Leaving the Farm

A new study suggests that nearly half of workers who care for animals in large industrial hog farming operations may be carrying home livestock-associated bacteria in their noses, and that this potentially harmful bacteria remains with them up to four days after exposure.

hogs_tim geers flickr

[Photo: Commercially farmed hogs (Photo by Tim Geers)]

The study was led by Dr. Christopher D. Heaney, UNC Gillings School of Global Public Health alumnus and assistant professor in the departments of environmental health sciences and epidemiology at the Johns Hopkins Bloomberg School of Public Health.

Researchers had believed that livestock-associated bacteria would clear from the noses of hog workers quickly – within a day or two. However, this small study of hog workers in North Carolina, reported online September 8 in the journal Occupational and Environmental Medicine, suggests it can stick around longer. Much of the Staphylococcus aureus bacteria the workers carried was antibiotic-resistant, likely due to antibiotics being used to promote hog growth as well as to treat sick animals. The longer the bacteria remain in workersnoses, the researchers say, the greater the potential the bacteria will be spread to hog workersfamilies, their communities, and even into hospitals, where the bacteria have been associated with an increased risk of staph infections.

In Europe, children of livestock workers have been treated for infections caused by a new livestock-associated strain of MRSA (methicillin-resistant Staphylococcus aureus) unlike strains found more widely in communities and hospitals. This suggests the children were exposed to MRSA strains through direct contact with the livestock or through family members who worked on livestock farms. Evidence of persistent carriage of this new livestock-associated strain and its drug resistance has led to restrictions on the non-therapeutic use of antibiotics in livestock overseas.

“Before this study, we didnt know much about the persistence of livestock-associated strains among workers in the United States whose primary full-time jobs involve working inside large industrial hog-confinement facilities,” says study author Dr. Christopher D. Heaney, Gillings School alumnus and assistant professor in the departments of environmental health sciences and epidemiology at the Johns Hopkins Bloomberg School of Public Health. “Now we need to better understand not only how persistence of this drug-resistant bacteria may impact the health of the workers themselves, but whether there are broader public health implications.”

Though it is hard to determine how many people work in the hog industry, census data from 2007 suggest that there are roughly 292,000 livestock workers in the U.S. In North Carolina, where the study was conducted, 6,400 workers are employed at the 938 hog operations that reported hired labor.

The study, done in conjunction with researchers from the University of North Carolina-Chapel Hill and community organizers from the Rural Empowerment Association for Community Help (REACH), involved 22 hog workers in North Carolina.

Between June and August 2012, researchers recruited industrial hog workers to be studied for two weeks. In the first week, the goal was for workers to have at least a 24-hour stretch off from work. During that week, each participant collected nasal swabs in the morning before going to work and again in the evening, whether they worked that day or not.

On day 14, study participants took two more nasal swabs. The longest time spent away from the farm was four days, with an average of two days among workers. Researchers later analyzed 327 separate nose swabs to see what kind of Staph bacteria they found, whether the strains were traditionally found in livestock or humans and whether the bacteria were drug-resistant.

Eighty-six percent of the hog workers – 19 of them – carried at least one type of Staphylococcus aureus at some point during the study period, while 16 of them (72.7 percent) carried the livestock-associated strain at some point. In contrast, only about one-third of the general population carry a strain of Staphylococcus aureus associated with humans.

However, 10 of the 22 workers (45.5 percent) were determined to be persistent carriers of livestock-associated Staph, meaning they had these strains in their noses all or all but one of the times they provided samples, even after leaving work at the animal confinements. Six of them persistently carried the multi-drug resistant kind of S. aureus, while one persistently carried MRSA.

Researchers found that the bacteria were present in workers’ noses even after four days away from the hog operation.

Garden-variety staph are common bacteria that can live without consequence in the human body. When they do cause infection, most are not life-threatening and appear as mild infections on the skin, like sores or boils. Staph also can cause more serious skin infections or can infect surgical wounds, the bloodstream, the lungs or the urinary tract. Strains of staph such as MRSA, which are resistant to some antibiotics, can be the most damaging because they can be so hard to treat.

MRSA is particularly dangerous in hospitals, where the bacteria are hard to get rid of, and hospital patients are the most vulnerable.

Dr. Heaney and colleagues are conducting more research to see whether hog workers with persistent drug-resistant bacteria are spreading it to their family members and communities.

“Were trying to figure out whether this is mainly a workplace hazard associated with hog farming or whether it is a threat to public health at large,” he said. “To do that, we need to learn more not just about how long workers carry bacteria in their noses, but how it relates to the risk of infection and other health outcomes in workers, their families and communities.”

Funding for the study was provided by the North Carolina Occupational Safety and Health Education and Research Center; the Centers for Disease Control and Preventions National Institute for Occupational Health and Safety; the Johns Hopkins Center for a Livable Future and a grant from the National Science Foundation.

In addition to Dr. Heaney, co-authors on the study, “Persistence of livestock-associated antibiotic-resistant Staphylococcus aureus among industrial hog operation workers in North Carolina over 14 days,” include Gillings School researchers Ms. Maya Nadimpalli, Dr. Jill Stewart and Ms. Elizabeth Pierce (environmental sciences and engineering), and Ms. Jessica L. Rinsky, Dr. Steve Wing, Ms. Jean Strelitz, and Ms. Lauren Harduar-Morano (epidemiology); Johns Hopkins researchers Drs. Keeve E. Nachman, Dave Love and Nora Pisanic; Mr. Devon Hall (REACH); and Dr. Jesper Larsen (Statens Serum Institute, Copenhagen).