Bacteria resistant to a family of vital antibiotics have been discovered on a pig farm in the US.

‘Superbugs’ resistant to carbapenems were discovered by researchers from Ohio State University. The bacteria have a transmissible gene, allowing them to pass on resistance to “critically important” last resort treatments.

Carbapenems aren’t used in livestock farming, though bacteria resistant to the antibiotics was detected on European farms in 2014, which led the industry group Responsible Use of Medicines in Agriculture (RUMA) to claim that resistance must have developed from human medicine (resistant bacteria have been found in hospitals). However, the Ohio researchers said a related antibiotic approved for farm use in the US might have led to the development of resistance.

Although farmers don’t use carbapenems, they do use other beta-lactams - the class of antibiotics to which these belong. Carbapenem-resistant bacteria have been found in waste water and soil, and environmental sources of antibiotic resistance (for example, drugs passed through humans and released into the environment) remain little-studied, and poorly understood.

Researchers found resistant bacteria in piglets, but not in slaughter-ready pigs on a 1,500-sow farm. They hypothesised that the resistant bacteria have developed as a result of the use of beta-lactam antibiotic ceftiofur, which is used on newborn piglets and males at castration. However, when pigs are sent for finishing the resistant bacteria must compete with other microbes, which could be why they didn’t find it in older pigs.

Even so, they recommended monitoring the resistance to ensure it doesn’t spread. Prof Thomas Whittum, who led the study, said, “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.” Prof Whittum highlighted practices like administering Ceftiofur, to all piglets in farrowing barns rather than just those that happen to sicken (an FDA-approved application known as "disease control”) as potentially unsustainable.

The development is another worrying step, coming after the UK government-commissioned O’Neill report on antimicrobial resistance (AMR). The review, published in May, warns that resistant bacteria, currently responsible for several hundred thousand deaths per year, could kill 10 million annually by mid-century unless concerted effort is made to tackle sources of resistance and preserve vital treatments for human medicine.

Although Defra celebrated a fall in sales of antibiotics for use on farms last month, campaigners have called on the government to crack down further on their use, arguing that the burden per animal is deceptively high in the UK, as most British livestock are relatively extensively farmed, and require fewer treatments than intensively farmed animals like pigs and some poultry. Talk of a bilateral trade deal between the UK and US could raise further questions about antibiotics use, where antibiotics are still used as growth promoters in livestock farming.