A type of MRSA found in humans originated in cattle at least 40 years ago, new research has found.

Scientists in Edinburgh say they have found evidence livestock were the original source of an MRSA strain now widespread in humans.

Researchers studied the genetic make-up of more than 40 strains of a bacterium called Staphylococcus aureus that can build up antibiotic resistance to develop into MRSA.

At least two genetic "subtypes" of the bacterium, which have become endemic in humans, have been traced back to cattle by a team at the Roslin Institute at Edinburgh University.

They say the most likely scenario is the bug crossed from cattle to people through direct contact - perhaps by working with farm animals.

After switching to human hosts, the Staphylococcus aureus bacterium became resistant to methicillin antibiotics and developed into methicillin-resistant Staphylococcus aureus, or MRSA.

It is hoped the research will help scientists prevent further strains from jumping from livestock.

Lead researcher, Professor Ross Fitzgerald, said: "Human infections caused by bacteria being transmitted directly from livestock are well known to occur.

"However this is the first clear genetic evidence of subtypes of Staphylococcus aureus which jumped from cattle and developed the capacity to transmit widely among human populations."

The study is published in the journal mBio and also involved Cambridge University, National Food Institute Denmark, Public Health England and the Statens Serum Institute, Denmark.