Leprosy is one of the oldest and most devastating human diseases. Remains from 4,000 years ago show the disease has been with us for a long time, but we’re still learning how it propagated, especially between humans and animals. According to a new study, at least in medieval England, red squirrels might have played a role in this.

Medieval diseases

In the 10th and 11th centuries, England was a rough place. Many people didn’t own their land and worked for others, barely making ends meet. Even for people who owned land, life was tough. Then, of course, there were the diseases.

Just one out of ten people lived to the age of 50, and diseases were a big part of that. Diseases like dysentery, malaria, diphtheria, flu, typhoid, smallpox and leprosy ravaged the population.

Leprosy is an infection caused by a bacterium called Mycobacterium leprae. Nowadays, leprosy is curable with multidrug therapy, but during the Middle Ages, there was no cure.

The disease is not highly contagious, but if you did develop it, you were stuck with it — and with the stigma around it. People with leprosy were often isolated in a leprosarium, a care facility specifically for sufferers of this disease. Little did medieval Britons know, some leprosy could be coming from squirrels.

Researchers in Switzerland used archaeological findings to identify red squirrels (Sciurus vulgaris) as hosts for M. leprae in medieval England. They focused on the city of Winchester, in southern England, comparing the pathogens found in people to those found in squirrels. Overall, they analyzed 25 human and 12 squirrel samples from two archaeological sites.

“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.

Humans and squirrels exchanged diseases

The genetic analysis of the pathogens shows strong similarities, suggesting that the same strain of bacteria jumped from squirrels to humans. However, it’s not impossible for the reverse to have happened — or even for the bacteria to travel both ways, from squirrels to humans and back to squirrels.

“This similarity shows us that leprosy bacteria were probably transmitted between animals and humans at that time,” says Verena Schünemann from the University of Basel (formerly University of Zurich). 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.”

Humans wouldn’t have exactly gone around hugging squirrels, but there were definitely points where they would have come together. In particular, researchers point out fur trade as one potential point of contact. Squirrel fur was a highly sought-after commodity for the upper echelons of British society. And, in some royal courts or estates, squirrels would have also been kept as pets.

Some animals may still carry leprosy

This is more than just a curiosity from medieval times. The findings are still important today. Leprosy still occurs in more than 120 countries, with more than 200,000 new cases reported every year. But, despite its persistence, leprosy hasn’t received as much attention. By understanding how animals transmitted leprosy in the past, we can better understand how the pathogen travels in the present.

“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.

“Although zoonotic diseases have become increasingly prominent in modern health agendas, historical zoonoses have received scant attention, despite the potential importance of earlier transmission events in shaping past and present health landscapes. Essential in the evolution and persistence of zoonotic pathogens are animal hosts, which have been largely omitted from most studies on past diseases due to the lack of pathological and genetic research on zooarchaeological remains,” the researchers write in the study.

Leprosy has been curable since the 20th century. But if left untreated, it can cause permanent damage to people’s nerves, limbs, skin, and eyes. It’s also still associated with major stigma, although large-scale efforts are underway to fight this.

As for red squirrels, they have their own plight. In the UK and Europe, the squirrel parapoxvirus has been ravaging red squirrel populations. This pathogen is often carried by the larger grey squirrels, which can host the virus but don’t appear to be affected by it. In parts of the UK, conservation programs are in place, attempting to protect the red squirrel population. Leprosy also still affects squirrels, but it is not considered a major threat to them.

The study was published in Current Biology.