Researchers studying the superbug Clostridioides difficile, which is considered one of the world’s major antibiotic resistance threats, have now found evidence of its genes in pigs and humans. This means the transmission of the pathogen is possible on a wider scale and that the genes that resist antibiotics can spread through an animal vector to humans.

Clostridioides difficile (C. difficile) is a bacterium that causes gut infection, triggering symptoms such as diarrhea and inflammation of the colon. It’s resistant to several antibiotics and some trains have genes that can cause extreme damage. It can be life-threatening for elderly patients who are getting antibiotics for other reasons.

It’s usually described as one of the world’s most significant antibiotic resistance threats. In 2017, there were an estimated 223,900 cases in hospitalized patients and 12,800 deaths in the US, according to the Centers for Disease Control and Prevention (CDC). In Canada, over 20,600 adults reported C. difficile infection between 2009 and 2015. The problem is only getting worse as the pathogen is slowly becoming more resistant to treatments against it.

Experts have warned repeatedly over antibiotics being overused on farm animals, which is one of the main causes of antibiotic resistance. In 2021, the UN released a statement with the Global Leaders Group on Antimicrobial Resistance asking to reduce the use of antimicrobials in food production and farm animals. The world is “rapidly heaving towards a tipping point,” the UN said back then.

“Our finding of multiple and shared resistance genes indicates that C. difficile is a reservoir of antimicrobial resistance genes that can be exchanged between animals and humans”, Semeh Bejaoui, study author, said in a statement. “This alarming discovery suggests that resistance to antibiotics can spread more widely than previously thought.”

A problematic superbug

C. difficile lives in people’s intestines as part of the balance of the digestive system, but its growth is controlled by other bacteria. The dangerous side of it comes because of antibiotics. When we take antibiotics, it destroys other bacteria in the gut as well as the infection it was targeting — including some useful bacteria. This can make drug-resistant C. difficile grow out of control and pose serious health problems.

Researchers at Copenhagen University and Denmark’s Statens Serum Institute wanted to identify whether strains of C. difficile known to have antibiotic-resisting genes, as well as toxin-producing ones, were present in pigs and humans — which would indicate that zoonotic transmission assists C. difficile in evolving into more dangerous forms.

They looked at samples of C. difficile obtained from pig farms in 2020 and 2021 in Denmark. Out of 514 pigs, 54 had the pathogen. It was more common in piglets and sows than in slaughter pigs. The researchers used genetic sequencing to isolate strains that had toxin-producing and drug-resistant genes. They found that 38 of the samples had at least one resistance gene.

Then, they compared the results from the pigs to samples from human patients who had a C.difficile infection in the same time period. They found that 13 sequence types matched between the pigs and the humans, with an animal-associated strain, ST11, being the most common. In 16 cases, the ST11 strain was identical in humans and animals.

“The overuse of antibiotics in human medicine and as cheap production tools on farms is undoing our ability to cure bacterial infections,” Bejaoui said in a statement. “This study provides more evidence on the evolutionary pressure connected with the use of antimicrobials in animal husbandry, which selects for dangerously resistant human pathogens.”

The study was presented at the European Congress of Clinical Microbiology & Infectious Diseases in Lisbon