Leprosy in the Middle Ages: New Insights on Transmission Pathways through Squirrels

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A lady plays with a pet squirrel, wearing a belled collar, in the early 14th cen
A lady plays with a pet squirrel, wearing a belled collar, in the early 14th century Luttrell Psalter. (Image: British Library Board Ms Add. MS 42130 f. 33r)

Researchers at the University of Basel and the University of Zurich have been able to prove that British squirrels carried leprosy bacteria as early as the Middle Ages. Further results revealed a link between the pathogens found in the medieval rodents and those in the local human population during that period.

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 especially in the Global South, also has a long history in Europe. The international research group led by paleogeneticist Professor Verena Schünemann (University of Basel, formerly University of Zurich) 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 were published in the journal "Current Biology".

From squirrels to humans or vice versa?

"This similarity shows us that leprosy bacteria were probably transmitted between animals and humans at that time," says Schünemann. However, she stresses that, based on current knowledge, it is not clear how this took place. "We don’t know whether the squirrels infected humans or whether humans were the ones to introduce the disease to the animals," says Schünemann.

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 11 and 12 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.

Genetic analysis from 20 milligrams

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. The human remains were extracted from the location of a former leprosarium, a care facility specifically for people suffering from leprosy. The researchers were able to examine the 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, first author of the study.

For the researchers, the results are particularly important for predicting leprosy in the future. Because to this day, it is not completely clear how the disease spreads. "Our One Health approach prioritizes finding out more about the role animals played in the spread of diseases in the past", says Schünemann. "A direct comparison between ancient animal and human strains enables us to reconstruct potential transmission events over time and helps to form conclusions about the long-term zoonotic potential of the disease", she adds.

The results are therefore relevant for today, as animals still receive very little attention as hosts of leprosy, even though they may be important for understanding the current persistence of the disease despite all’attempts to eradicate it.

Original publication

Christian Urban et al.
Ancient Mycobacteriumleprae genome reveals medieval English red squirrels as animal leprosy host
Current Biology (2024) doi: 10.1016/j.cub.2024.04.006