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Pigs and humans are sharing antibiotic-resistance genes

A new study has found evidence of zoonotic spread between hospital patients and Danish pigs. The researchers analyzed samples of the superbug Clostridioides difficile across 14 pig farms in Denmark and confirmed the circulation of multiple antibiotic-resistance genes between pigs and humans. The findings provide the latest evidence that animal to human (zoonotic) transmission is possible.

“Our finding of multiple and shared resistance genes indicate that C. difficile is a reservoir of antimicrobial resistance genes that can be exchanged between animals and humans,” said Dr. Semeh Bejaoui from the University of Copenhagen. 

“This alarming discovery suggests that resistance to antibiotics can spread more widely than previously thought, and confirms links in the resistance chain leading from farm animals to humans.”

C. difficile infects the human gut and is resistant to all but three antibiotics. Some strains contain genes that produce toxins that cause damaging inflammation, leading to life-threatening diarrhea. 

This bacteria strain is one of the biggest antibiotic resistance threats in the USA. In 2017, C.difficile caused an estimated 223,900 infections and 12,800 deaths at a healthcare cost of more than $1 billion.

A strain of C. difficile that can cause more serious disease is associated with a rising number of infections in young and healthy individuals. Farm animals have recently been identified as carriers of this strain. 

In this study, Danish scientists investigated the prevalence of C. difficile strains in pigs. They also looked at the potential for zoonotic spread of antimicrobial resistance genes by comparing the pig samples to Danish hospital patients.

Stool samples were collected from 514 pigs in two batches. Batch A included 330 samples from sows, piglets and slaughter pigs from fourteen farms in 2020. The 184 samples in batch B were collected during slaughtering in 2021.

The results showed that C. difficile was detected in 54 of the pigs sampled. Further analyses of 40 samples revealed that C. difficile was more common in piglets and sows than slaughter pigs. This could be because of the difference in age between piglets and adult pigs, as younger pigs are more susceptible to infection.

In total, thirteen sequence types found in animals matched those found in the human stool samples. All of the animal samples were positive for the toxin genes, and ten had an even greater capacity to cause disease.

Overall, 38 isolates from the animals contained at least one resistance gene. Resistance was predicted for seven classes of antibiotics that treat severe bacterial infections.

The findings demonstrate how overuse of antibiotics in human medicine and farms are now impacting the ability to cure bacterial infections

“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,” said Dr. Bejaoui. “This highlights the importance of adopting a more comprehensive approach, for the management of C. difficile infection, in order to consider all possible routes of dissemination.”

The researchers were unable to determine the cause or direction of transmission. This is because the strains found in human and animal samples were identical, suggesting that they could be shared between groups. 

The experts empathize the importance of deeper analysis to better understand how the bacteria is continuously exchanged between the human population and farm animals. 

The research was presented at this year’s European Congress of Clinical Microbiology & Infectious Diseases (ECCMID) in Lisbon, Portugal.

By Katherine Bucko, Staff Writer

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