Underscoring the potential public health threats widespread agricultural practices pose, bacteria resistant to a class of last-resort antibiotics known as carbapenems has for the first time been found on a U.S. pig farm.

The findings were published Monday in Antimicrobial Agents and Chemotherapy, a journal of the American Society for Microbiology, and come just months after another antibiotic used as a last resort—colistin—was found in a pig in the U.S.

The Centers for Disease Control and Prevention has said Carbapenem-resistant Enterobacteriaceae (CREs), which are carried by a mobile piece of independent DNA called a plasmid, present an urgent public heath threat.

"It's a surprise that they would show up in livestock," said corresponding author, Thomas Wittum, professor and chair of Veterinary Preventive Medicine at Ohio State University.

An accompanying press statement notes that "[c]arbapenems are a subset of β-lactam antibiotics. Β-lactam antibiotics which are not carbapenems are legal for use on farms in the U.S."

For the study, researchers took environmental and fecal samples during four visits over a five-month period from a 1,500-sow, farrow-to-finish pig farm which "followed typical U.S. production practices." Piglets in the farrowing area are given a dose of Ceftiofur—a β-lactam antibiotic—with males given a second dose.

The researchers said they were surprised at finding the few samples of the resistant gene, as it's a closed barn—no new animals had been introduced for five decades. So how did it get there? "We don't know," Wittum told HealthDay News.

Wittum added that the researchers found "no evidence the pigs carried the gene into the [human] food supply."

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The authors conclude:

The implication of our finding is that there is a real risk that CRE may disseminate in food animal populations and eventually contaminate fresh retail meat products. Food-borne transmission may then produce a reservoir of mobile carbapenemase genes in the enteric flora of consumers.

The Natural Resources Defense Council (NRDC) said the findings were troubling.

"The last terrible shoe may have just dropped when it comes to drug-resistant infections," said Dr. David Wallinga, senior health officer at NRDC.

"This is just one more warning that doctors may soon have nothing left in their toolkit to save patients when these bugs strike. Our overuse of antibiotics in livestock is creating reservoirs for the spread of resistance—and this study strongly suggests resistance to carbapenems is no exception. To save our miracle drugs, we have got to stop wasting them on animals that aren't sick," Wallinga added.

In September, when world leaders gathered for a historic high-level meeting at the United Nations General Assembly to lay out how to fight so-called "superbugs"—Dr. Margaret Chan, Director-General of the World Health Organization, said, "Antimicrobial resistance poses a fundamental threat to human health, development, and security. The commitments made today must now be translated into swift, effective, lifesaving actions across the human, animal and environmental health sectors."

"We are running out of time," she warned.