Killer squirrels 'spread fatal disease to people in medieval England'

Image: Tony Nellis/SWNS)

Red squirrels spread a deadly disease to people in medieval England, reveals new research.

Evidence from archaeological sites in Winchester, Hampshire, shows that the nut-loving rodents once served as an important host for Mycobacterium leprae strains that causes leprosy in humans. Study senior author Professor Verena Schuenemann said: "With our genetic analysis we were able to identify red squirrels as the first ancient animal host of leprosy.

"The medieval red squirrel strain we recovered is more closely related to medieval human strains from the same city than to strains isolated from infected modern red squirrels. Overall, our results point to an independent circulation of M. leprae strains between humans and red squirrels during the Medieval Period."

Co-author Dr Sarah Inskip said: "Our findings highlight the importance of involving archaeological material, in particular animal remains, into studying the long-term zoonotic potential of this disease, as only a direct comparison of ancient human and animal strains allows reconstructions of potential transmission events across time."

Leprosy is one of the oldest recorded diseases in human history and is still prevalent today in Asia, Africa, and South America. Scientists have traced the evolutionary history of the mycobacterium that causes it, but didn't know how it may have spread to people from animals in the past beyond some hints that English red squirrels may have served as a host.

For the new study, published in the journal Current Biology, the research team studied 25 human and 12 squirrel samples to look for M. leprae at two archaeological sites in the historic city of Winchester. The city was known for its leprosarium - a hospital for people with leprosy - and connections to the fur trade. In the Middle Ages, squirrel fur was widely used to trim and line garments.

Many people at the time also kept squirrels, trapping them in the wild and raising them as pets. The research team sequenced and reconstructed four genomes representing medieval strains of M. leprae, including one from a red squirrel. An analysis to understand their relationships found that all of them belonged to a single branch on the M. leprae family tree.

The researchers also showed a close relationship between the squirrel strain and a newly constructed one isolated from the remains of a medieval human. They say that that the medieval squirrel strain is more closely related to human strains from medieval Winchester than to modern squirrel strains from England, indicating that the infection was circulating between people and animals in the Middle Ages in a way that hadn't been detected before.

Prof Schuenemann, of the University of Basel, Switzerland, said: "The history of leprosy is far more complex than previously thought. There has been no consideration of the role that animals might have played in the transmission and spread of the disease in the past, and as such, our understanding of leprosy's history is incomplete until these hosts are considered.

"This finding is relevant to today as animal hosts are still not considered, even though they may be significant in terms of understanding the disease's contemporary persistence despite attempts at eradication."

Dr Inskip, of the University of Leicester, added: "In the wake of Covid-19, animal hosts are now becoming a focus of attention for understanding disease appearance and persistence. Our research shows that there is a long history of zoonotic diseases, and they have had and continue to have a big impact on us."