MRSA Strain Pervasive in Pigs Can Jump to Humans

Researchers in the United Kingdom have determined that a highly antibiotic-resistant strain of the superbug MRSA that has emerged in livestock in the last 50 years can jump to human hosts.

According to a study published recently in the journal eLife, the CC398 strain has become the dominant type of MRSA [methicillin resistant Staphylococcus aureus] in European livestock and is also a growing cause of human MRSA infections, even among those who have not had direct contact with the animals.

“Historically high levels of antibiotic use may have led to the evolution of this highly antibiotic resistant strain of MRSA on pig farms,” says Dr. Gemma Murray, a lead author of the study, previously in the University of Cambridge’s Department of Veterinary Medicine and now at the Wellcome Sanger Institute. “We found that the antibiotic resistance in this livestock-associated MRSA is extremely stable—it has persisted over several decades, and also as the bacteria has spread across different livestock species.”

While livestock-associated CC398 is found across a broad range of livestock species, it is most commonly associated with pigs. Its rise has been particularly evident in Danish pig farms where the proportion of MRSA-positive herds has increased from less than 5% in 2008 to 90% in 2018. MRSA doesn’t cause disease in pigs but is considered one of the world’s greatest threats to human health by The World Health Organisation.

And though antibiotic use in European livestock is much lower than it has been in the past, the researchers say the impact on CC398 prevalence will be limited in pigs because it is so stable.

The success of CC398 in livestock and its ability to infect humans, resist antibiotics, and evade the human immune system is linked to three mobile genetic elements in the MRSA genome.

The researchers reconstructed the evolutionary history of two particular mobile genetic elements called Tn916 and SCCmec that confer antibiotic resistance in MRSA, and found they have persisted in a stable way in CC398 in pigs over decades. They also persist when CC398 jumps to humans—carrying with them high levels of resistance to antibiotics commonly used in farming.

In contrast, a third mobile genetic element called φSa3—which enables the CC398 strain of MRSA to evade the human immune system—was found to have frequently disappeared and reappeared over time, in both human-associated and livestock-associated CC398. This suggests that CC398 can rapidly adapt to human hosts.

“Cases of livestock-associated MRSA in humans are still only a small fraction of all MRSA cases in human populations, but the fact that they’re increasing is a worrying sign,” says Dr. Lucy Weinert in the University of Cambridge’s Department of Veterinary Medicine, senior author of the paper. “Understanding the emergence and success of CC398 in European livestock—and its capacity to infect humans—is vitally important in managing the risk it poses to public health.”

Although antibiotic use in European livestock is much lower than it has been in the past, the researchers say the impact on CC398 prevalence will be limited in pigs because it is so stable. The authors say a soon-to-be-enacted EU ban on zinc oxide by pig farmers may not help reduce the prevalence of CC398 because the genes conferring antibiotic resistance are not always linked to the genes that confer resistance to zinc treatment.