mrsa
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A vet examines a pig at a farm in Denmark. Ninety per cent of all Danish herds now have MRSA, which is symptomless in pigs, up from 5 per cent in 2008
Fears are growing over a strain of MRSA common among farm animals that scientists say is capable of "rapidly adapting to human hosts".

The World Health Organisation lists the superbug as one of the biggest global threats to human health due to its resistance to antibiotic medicines.

The feared strain, called CC398 is a growing cause of human MRSA infections and first emerged in livestock around 50 years ago, most likely due to heavy and widespread use of antibiotics in pig farming.

New research has found that CC398 has maintained its antibiotic resistance over decades, across livestock and humans, and the team said they have found evidence of increasing numbers of people contracting the infection among both those who have direct contact with livestock, and, perhaps more concerningly, those who have not.

"Historically high levels of antibiotic use may have led to the evolution of this highly antibiotic resistant strain of MRSA on pig farms," said 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.

She added: "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."

Antibiotic use in European livestock is now "much lower" than it has been in the past, the researchers said, but they warned that ongoing reductions in antibiotic use on pig farms - due to recent policy changes - are likely to have a limited impact on the presence of this strain of MRSA in pigs because it is so stable.

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 per cent in 2008 to 90 per cent of all herds in 2018.

MRSA is symptomless in pigs, but it can be dangerous to humans. Normally it causes relatively mild infections on the skin, such as sores, boils or abscesses.

"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," said Dr Lucy Weinert in the University of Cambridge's Department of Veterinary Medicine, senior author of the paper.

"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," she said.

Intensification of farming, combined with high levels of antibiotic use in livestock, has led to particular concerns about livestock as reservoirs of antibiotic-resistant human infections.

MRSA - methicillin resistant Staphylococcus aureus - was first identified in human patients in 1960. It tends to infect people staying in hospital, and due to its resistance to antibiotics it is much harder to treat than other bacterial infections.

The research is published in the journal eLife.