On a US farrow-to-finish hog farm, epidemiologists found two species of bacteria carrying genetic material that confers antibiotic resistance, but the bacteria do not seem to have entered the human food supply.

This discover marks the first reported case on a US pig farm of bacteria harboring plasmids, or rings of DNA, that contained genes granting protection against carbapenem antibiotics, such as ceftiofur. Ceftiofur often is used at birth and again after surgically castrating male pigs.

“Ceftiofur is often a good choice because it is highly efficacious against many important pathogens and it is metabolized quickly so low risk of tissue residues compared to some other antibiotics,” study co-author Thomas E. Wittum, PhD, chair of the department of veterinary preventive medicine at Ohio State University, told Pig International. “But resistance problems can emerge on farms, and the more a single drug is used, the more likely that resistance to that drug will be a problem. If farms are regularly using ceftiofur to control disease outbreaks, they may need to discuss with their veterinarian options for rotating antibiotics based on appropriate diagnostics, to effectively treat disease problems without selecting for resistant organisms.”

Problems with plasmids

The presence of the carbapenem-resistance genes on plasmids is worrisome, noted Wittum and his co-authors in the journal Antimicrobial Agents and Chemotherapy. Since plasmids can be swapped between different bacteria species, resistance to ceftiofur and other antibiotics could spread more easily on plasmids than by reproduction of resistant organisms.

“This may just be a rare isolated occurrence, but we really don’t know for sure yet,” Wittum said. “We are going to continue surveillance efforts so we can better know the risks to pig health and to food safety.”

No antibiotic-resistant plasmids found in human food supply

However, none of these potentially dangerous, plasmid-carrying bacteria were found in harvest-ready pigs in the finishing barn, meaning that the bacteria were unlikely to have entered the human food supply. The researchers suggested that the absence of ceftiofur in the nursery and finishing barns may have removed the environmental niche that selected for carbapenem-resistance plasmids.


“Farmers obviously don’t want these highly resistant organisms entering the food supply, and they don’t want them impacting the health of their pigs,” said Wittum. “We don’t have all the answers yet, but it is important that producers work with their veterinarians to ensure that they are using antibiotics in ways that allow them to appropriately treat sick pigs while minimizing the antibiotic selection pressure that drives resistance.”

Carbapenem-resistant plasmids on US pig farms study design

Over a five month period, samples were collected from a 1,500 sow farm during four visits. The environmental and fecal samples were tested for signs of antibiotic resistance.

“We have national surveillance in place and screen samples from around the country that originate from a variety of sources,” said Wittum. “This farm happened to have contributed samples for a different project that went into our surveillance and we detected this resistance gene.”

During the first visit, two out of 30 environmental samples revealed bacteria, Escherichia coli and Proteus mirabilis, that harbored plasmids containing the gene blaIMP-27, which can confer carbapenem antibiotic resistance. Eleven out of 24 environmental samples from two farrowing rooms showed signs of the gene during the third visit. No signs of the gene were found in fecal samples.

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