Red squirrel and human leprosy link found at English medieval archaeological site

New evidence from medieval archaeological sites shows that English red squirrels once served as an important host for Mycobacterium leprae (M. leprae) strains also responsible for leprosy in humans.

Researchers from the University of Leicester and then University of Basel studied 25 human and 12 squirrel samples from Winchester, Hampshire to look for genetic evidence of M. leprae.

The city is well-known for its medieval leprosarium (a hospital for people with leprosy) and for its connections to the fur trade during the Middle Ages.

During this period, squirrel fur was often used to trim and line garments. Many people also kept squirrels trapped as kits in the wild and kept them as pets.

Leprosy is a chronic infectious disease spread through close contact over months with someone or something who has untreated leprosy.

The disease is one of the oldest in recorded human history as evidenced by the archaeological skeletons of sufferers and is still prevalent in Asia, Africa and South America.

According to the World Health Organisation, 200,000 people a year contract leprosy across the world.

The finding of red squirrels as hosts  may have implications for understanding the spread and persistence of leprosy today and answering why it has not yet been successfully eradicated.

Although scientists have traced the evolutionary process of M.  leprae, they did not know, until now, how it may have spread to humans from animals beyond some hints that red squirrels in England may have served as the host.

Researchers sequenced and reconstructed four genomes representing medieval strains of M. leprae, including one from a red squirrel from a fur traders pit.

Analysis found that all of them belonged to a single branch of the M. leprae family tree. It also showed a close relationship between the medieval squirrel strain and a newly constructed one isolated from the remains of a medieval person.

Researchers report 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.

According to Dr Verena Schuenemann, University of Basel: “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.”

Dr Sarah Inskip, University of Leicester, 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.”

The full findings have been published in Current Biology (3 May, 2024).