Evidence from archaeological sites in the medieval English city of Winchester shows that English red squirrels once served as an important host for Mycobacterium leprae strains that caused leprosy in people according to a study published in the journal Current Biology.

Leprosy is one of the oldest recorded diseases in human history and is still prevalent to this day in Asia, Africa, and South America. “For thousands of years, humans were thought to be the only natural host of M. leprae until the discovery of M. leprae in several wild animals in recent decades, such as armadillos and, more recently, red squirrels and chimpanzees,” the authors write. While scientists have traced the evolutionary history of the mycobacterium that causes it, how it may have spread to people from animals in the past was not known beyond some hints that red squirrels in England may have served as a host.

In the new study, the researchers studied 25 human and 12 squirrel samples to look for M. leprae at two archaeological sites in Winchester. The city was well known for its leprosarium (a hospital for people with leprosy) and connections to the fur trade. In the Middle Ages, squirrel fur was widely used to trim and line garments. Many people also raised them as pets.

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, says a release. The study found 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.

“These findings on the natural reservoir of M. leprae indicated that M. laprae circulates in more wild animals than we suspected, and zoonotic infection may contribute to the epidemic of leprosy. Therefore, it is inevitable that leprosy epidemics can persist for a long time in the future, and we should remain vigilant against the spread of M. leprae between humans and wildlife,” they write.