Medieval Squirrels Linked to Spread of Leprosy in Humans

Squirrels in England carried leprosy bacteria as early as the Middle Ages. An international team of researchers has revealed a link between the pathogens found in the animals and people from a medieval leprosarium.

Skin spots, deformed noses, ulcers: leprosy, is an infectious disease that can bring about some serious symptoms. The bacterium responsible, Mycobacterium leprae, which still infects around 200,000 people each year, also has a long history in Europe. The international research group used archaeological findings to identify red squirrels (Sciurus vulgaris) as hosts for M. leprae in medieval England. The researchers also discovered that the leprosy bacteria in medieval squirrels were closely related to those isolated from medieval human skeletons from the same region. The results are published in the journal Current Biology.


“With our genetic analysis we were able to identify red squirrels as the first ancient animal host of leprosy,” says senior author Verena Schuenemann of the University of Basel in Switzerland. “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.”

“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,” adds Sarah Inskip of the University of Leicester, UK, a co-author on the study.


There were certainly a number of points of contact between humans and squirrels during the Middle Ages. One key aspect was fur trade, which provided the highly sought-after squirrel fur for the upper echelons of society. Especially in the 11th and 12th centuries, for example, entire coats made of squirrel fur were produced for the various royal families. Furthermore, squirrels were also kept as pets, in royal courts as well as nunneries.

For their study, the researchers focused on the city of Winchester in southern England. The material necessary for the genetic analysis originates from two different archaeological sites within the city. Twenty-five human remains were extracted from the location of a former leprosarium, a hospital for people with leprosy. The researchers were able to examine twelve samples of medieval squirrels thanks to hand and foot bones found at a former skinner’s shop. “We carried out the genetic analyses on the squirrels’ tiny hand and foot bones, which weigh between 20 and 30 milligrams. That is not a lot of material,” explains Christian Urban of the University of Zurich.

The researchers 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. They also showed a close relationship between the squirrel strain and a newly constructed one isolated from the remains of a medieval person. They 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.

“The history of leprosy is far more complex than previously thought,” Schuenemann said. “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.”


“In the wake of COVID-19, animal hosts are now becoming a focus of attention for understanding disease appearance and persistence,” Inskip adds. “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.”

The article, “Ancient Mycobacterium leprae genome reveals medieval English red squirrels as animal leprosy host,” by Christian Urban, Alette A. Blom, Charlotte Avanzi, Kathleen Walker-Meikle, Alaine K. Warren, Katie White-Iribhogbe, Ross Turle, Phil Marter, Heidi Dawson-Hobbis, Simon Roffey, Sarah A. Inskip and Verena J. Schuenemann, appears in Current Biology. Click here to read it.

See also: Medieval Fur Trade May Have Led to Spread of Leprosy

See also: Spread of leprosy tracked to early medieval Britain, researchers find

Top Image: Lady with pet squirrel, British Library MS Add. 42130.