Did Red Squirrels Spread Leprosy in Medieval England?

In a scientific first, an international team of researchers from the United Kingdom, Switzerland and the United States has found evidence to suggest that at least one species of animal was passing leprosy on to humans in medieval England. The animal in question is the red squirrel, a creature that lived in even closer proximity to people in the Middle Ages than it does today.

While contact with armadillos has been identified as a risk for leprosy in the modern age, never before has a medieval animal been linked to leprosy outbreaks experienced by human populations in more ancient times.

Studies have shown that modern red squirrels do carry the bacteria that causes leprosy. But there is no evidence of squirrels passing leprosy to people in the modern age, which is why this new discovery is so intriguing.

Squirrels were often kept as pets, as depicted in this Holbein painting, but they may have been carrying the deadly leprosy bacteria (Hans Holbein the Younger / Public Domain)

“The finding of leprosy in modern squirrels was surprising and then it’s incredible that we found it in the medieval period,” study co-author Sarah Inskip, an archaeologist from the University of Leicester, told the BBC. “It really goes against the narrative that it was a human disease specifically.”

The Truth was Hidden in the Bones

In an article just published in the journal Current Biology, the archaeologists and genetic experts involved in the new study explain how they were able to establish a link between medieval leprosy and contact with squirrels.

The researchers examined human and red squirrel bones that have been recovered over the years during excavations in and around the city of Winchester in southern England, choosing samples that showed signs of damage consistent with the ravages of leprosy. DNA fragments were extracted and analyzed, and the researchers ultimately found the genetic fingerprints of  Mycrobacterium leprae (the bacteria that causes leprosy) in 25 human bone samples and 12 that belonged to squirrels.

The human bones were recovered during excavations of burials of leprosy victims who died at a medieval hospital in Winchester, while the squirrel bones were found in an ancient burial pit used by furriers. All the bones were dated to a period covering the 10th through the 13th centuries.

A comparative analysis of the particular strain of  M. leprae identified in the medieval red squirrels showed it to be surprisingly similar to the strain found in humans who lived in England in the Middle Ages, making it clear that there was some kind of connection.

“Vair” fur made from squirrel pelts is so common a medieval material that it appears as a heraldic device, but the use of this fur may have passed on leprosy (Kürschner / Public Domain)

In their  Current Biology article, the study authors note that “the medieval squirrel strain is more closely related to some medieval human strains from Winchester than to modern red squirrel strains from England.” This discovery strongly indicates that leprosy in squirrels has never been a self-contained illness, and that it must have passed back and forth between humans and red squirrels quite readily in the Middle Ages (but not now, which explains why the leprosy found in modern squirrels diverges more from that found in people). 

Medieval English People Liked Squirrels a Lot

Leprosy is a truly ancient disease that was around in England several hundred years before the Middle Ages, so the researchers involved in this new study are not claiming that the disease somehow originated in red squirrels before being transmitted to humans. It seems likely that humans passed the disease on to squirrels first at some point in the past, but that doesn’t reveal much about patterns of transmission in medieval England specifically. 

It is impossible to know which gave the disease to the other first at that particular place in that particular time period. In other words, although the disease was undoubtedly transmitted in both directions once it had gained a foothold in both populations.

But the main question remains: if red squirrels were giving leprosy to humans, and vice versa, in medieval times but not today, why? Well, in the Middle Ages, the “relationship” between humans and red squirrels was closer and even more intimate than it is in modern times.

Squirrel fur was frequently and routinely added to clothing of all types, and there was a thriving squirrel fur industry in the city of Winchester during that time period. But rather than coveting their fur many people in medieval England actually kept red squirrels as pets, and this might have been the easiest way for leprosy to pass between the two species. Leprosy generally doesn’t transmit from one person to another unless there is close and prolonged contact, and this rule would presumably apply to transmission between animals and people as well. 

Are Humans Still Contracting Leprosy from Animals?

While more common in the past than it is today, leprosy is far from extinct. Also known as Hansen’s disease, leprosy remains disturbingly prevalent in parts of Africa, Asia and South America, with over 200,000 new cases being diagnosed in as many as 120 different countries annually.

The disease primarily attacks the skin and the peripheral nervous system, and if left untreated it may cause permanent injuries or disabilities.

The newly discovered connection between leprosy in humans and squirrels in medieval times could mean that animal transmission is a factor in modern outbreaks of the disease. If squirrels and armadillos carry it than some other animals may as well, and human contact with these creatures could be a hidden factor explaining leprosy’s persistence.

“The history of leprosy is far more complex than previously thought,” acknowledged University of Zurich paleogeneticist Verena Schünemann, the new study’s lead author. “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.”

The findings of this new study do guarantee that the question of animal-to-human transmission will be researched more deeply in the future, which could have ramifications for both the historical understanding of leprosy and the medical understanding of it today.

Top image: Red squirrels may have been spreading deadly leprosy amongst the population of medieval England. Source: Fraser by Peter Trimming / CC BY-SA 2.0.

By Nathan Falde