They are becoming more of a rare find in the UK since the grey squirrel was introduced, but red squirrels are harboring a secret that scientists have only just discovered: they carry leprosy-carrying bacteria.

Researchers wanted to pinpoint the reason for the red variety of squirrel’s decline in the British Isles and performed DNA tests on more than 100 animals to see if they could find any evidence as to why. All the specimens were collected from Scotland, England and Ireland but the ones found on Brownsea Island, located on the southern coast were found to have the Mycobacterium leprae strain of leprosy.

The squirrels all had lesions on the skin and swellings – symptoms that were seen in humans with leprosy. Leprosy has not been found in Britain since the 1500s – a disease that is spread from human to human but is now found to have been hidden in red squirrels for six centuries.

“That for me was a real gobsmacker,” says Stewart Cole who directs the Global Health Institute at the Swiss Federal Institute of Technology. “The very same strain that would cause disease in humans back in the Middle Ages was still present in the squirrels. That’s awesome.”

There is no evidence that leprosy is being passed on to humans but it has opened up new information about the way leprosy works. Up until now it was only thought to be found in humans and is only caught when coming into contact with an infected person. The question now asks, how did squirrels come to be infected? The researchers believe  “humans may have been infected through contact with red squirrels bearing M. leprae, as these animals were prized for their fur and meat in former times.”

Details of the study were published in the journal Science.