Antibiotic-resistant strains of bacteria are nothing new on U.S. farms, so why are some people so concerned about the recent discovery, on an American pig farm, of a gene that confers resistance to a vital class of antibiotics?

In a study published this week in the journal Antimicrobial Agents and Chemotherapy, researchers from Ohio State University looked at pigs raised a farrow-to-finish — meaning they spend their entire lives on the farm — and confirmed the presence in a small number of pigs of a gene that confers drug resistance to a class of antibiotics known as carbapenems.

Carbapenems are typically reserved for patients exhibiting resistance to multiple antibiotics. For example, last May the Centers for Disease Control and Prevention revealed that a Pennsylvania woman had been found to carry a bacteria resistant to almost all traditional antibiotics; she was lucky that her infection was susceptible to treatment with carbapenems.

None of the pigs found carrying the resistance gene were ill, and researchers say there is no evidence that any of this meat made its way into the human food supply.

However, the authors of the study explain that there is still concern about the mere presence of the gene because carbapenem-resistant infections are “especially threatening because they approach pan-resistance,” meaning they may be all but untreatable with the drugs currently available.

Enterobacter is a commonly found bacteria, and can be a source of infection. Carbapenem-resistant enterobacter is believed to have a high mortality rate because of the difficulty in treating the infection.

Carbapenem antibiotics — because of their importance as a “last-resort” drug in treating humans — are also not approved for use in livestock intended for food use. Additionally, researchers note that the operators of this farm have bred all their own swine for five decades, so there hasn’t been any outside DNA brought in. So how did these pigs develop resistance to a class of drugs they shouldn’t be exposed to?

“It’s a surprise that they would show up in livestock,” says study co-author Thomas Wittum, PhD, about the presence of the “bla IMP-27” gene plasmid, which confers this resistance on the animals. A plasmid is a small piece of independent DNA that can move easily from one bacterium to another, within an animal, from animal to animal, and even across species.

While carbapenems aren’t used on U.S. farms, carbapenem relatives in the larger β-lactam family of antibiotics can be used. The researchers note that one of these drugs, Ceftiofur, is generally given to all pigs after birth, and then again to the male pigs after they are castrated. They believe it’s possible that the use of this related drug could be leading to the development of the carbapenem resistance.

The IMP-27 plasmid has a wide range of hosts, according to the researchers, raising concerns that the resistance could transfer to humans.

Dr. David Wallinga, MD, at the Natural Resources Defense Council, (who was not involved with this study) explains that livestock farms can act like a “petri dish” for drug-resistant pathogens.

“The last terrible shoe may have just dropped when it comes to drug-resistant infections. This is just one more warning that doctors may soon have nothing left in their toolkit to save patients when these bugs strike,” says Wallinga. “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.”

In a statement, study co-author Wittum says it’s likely time for farmers to re-evaluate their use of antibiotics that are related to carbapenems.

“We may need to examine some of the practices of farms, and evaluate whether they are really appropriate, and whether the benefits outweigh the risks,” explains Wittum, suggesting that farmers reconsider giving Ceftiofur to all piglets at birth, reserving the drug for only those piglets that are actually ill.

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