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Pigs are spreading antibiotic-resistant bacteria to humans

Antibiotic-resistant strains of staph bacteria are spreading between pigs, farmworkers, their families, and communities, according to researchers at the Johns Hopkins Bloomberg School of Public Health. The study, which was focused on pig farms in rural eastern North Carolina, raises new concerns about an emerging public health threat.

Staphylococcus aureus is commonly found in soil and water. It lives on the skin and in the upper respiratory tract of animals like pigs, and also humans. Staph can cause problems ranging from minor skin infections to life-threatening issues like sepsis.

The findings provide evidence that multidrug-resistant S. aureus strains are capable of spreading in and around factory farm communities. 

The study was focused on bacteria samples collected from pigs and humans in the top pig-producing counties of North Carolina. The team used DNA sequencing to analyze relationships among these bacterial strains.

The analysis revealed that the strains were very closely related, which provides evidence for transmission between pigs and people. According to the researchers, most of the strains carried genes that provide resistance to multiple antibiotics.

“We found that these livestock-associated S. aureus strains had many genes that confer resistance to antimicrobial drugs commonly used in the U.S. industrialized pig production system,” said study first author Dr. Pranay Randad.

“These findings warrant future investigations into the transmission dynamics in nearby communities and disease burden associated with these strains in the United States,” said  study senior author Dr. Christopher Heaney.

Experts have speculated that S. aureus and other bacteria are transmitted from humans to pigs on factory farms. Once they are contained within the pigs, the bacteria have an opportunity to evolve antibiotic resistance. 

Pigs raised on factory farms are routinely given antibiotics to prevent outbreaks in their dense concentrations. The drug-resistant bacterial strains may then be transmitted back to humans, becoming a potentially serious source of disease.

Dr. Heaney and colleagues have been collecting S. aureus isolates from pigs and farmworkers at factory-scale pig farms in North Carolina. In recent years, their research confirmed that livestock-associated strains of S. aureus, many of which are antibiotic-resistant, can be found in the pigs, farmworkers, their family members, and residents living nearby.

For the current study, the team sequenced the entire genomes of 49 of the S. aureus isolates. The DNA analysis showed that all of the isolates, whether from humans or pigs, belonged to a group of S. aureus strains known as clonal complex 9 (CC9).

“This CC9 is a novel and emerging subpopulation of S. aureus that not many people have been studying, apart from a few reports in Asia,” said Dr. Randad.

The researchers discovered that the CC9 isolates from North Carolina were closely related, which implies that there has been recent transmission between pigs and people. Furthermore, nearly all of the isolates that were involved in transmission between pigs and humans were multidrug resistant. This suggests that diseases caused by these isolates would be hard to treat.

“In other countries, such as in Europe, we see a high level of coordinated research on this topic from a public health perspective, with open access to collect bacterial isolates from pigs raised on factory farms, but so far in the U.S. not as much is being done,” said Dr. Randad.

The study is published in the journal Emerging Infectious Diseases.

By Chrissy Sexton, Staff Writer

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