A bit of medical advice: Stay away from medieval squirrels. That’s what a study in Current Biology seems to suggest, anyway, after showing that red squirrels hosted strains of the leprosy bacteria Mycobacterium leprae in England’s Middle Ages.

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

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Squirrel Spillover

Humans have a long history with leprosy, a devastating disease associated with nerve damage, paralysis, and disfigurement. Though treatable today, the limited leprosy treatments of the Middle Ages meant that the disease spread widely, despite being surprisingly difficult to transmit. As many as 10 to 40 percent of medieval Europeans showed signs of M. leprae infections at the time of their deaths, depending on when and where they were buried.

While scientists have studied the historical symptoms and the human-to-human spread of leprosy, also dubbed Hansen’s disease, they’ve struggled to determine whether M. leprae historically transferred between animal and human hosts. Though hints have been found that the bacteria have set up shop in squirrels as well as humans, for instance, those hints have been broadly understudied.

Hoping to pin down the truth of this squirrel-human transmission, Schuenemann and a team of researchers turned to the medieval city of Winchester in England. Their analysis of 12 squirrel samples and 25 human samples from the medieval city revealed four separate strains of M. leprae, including one strain from squirrels and three strains from humans. Their subsequent analysis of the four strains found that the infection bounced between red squirrels and humans in Winchester’s medieval hustle and bustle.

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

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Hopping From Squirrel to Human

Recreating the genomes of the four medieval strains of M. leprae and reconstructing their genetic relationships, the team found that the strains belonged to the same familial branch of M. leprae bacteria. The team also found that the squirrel strain from the Middle Ages shared more similarities with the human strains from the Middle Ages than modern squirrel strains. This suggests that the infection once spread between medieval squirrels and humans.

“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,” said Sarah Inskip, another study author and a researcher in the school of archaeology and ancient history at the University of Leicester, in a statement.

According to the team, wild squirrels were trapped throughout the Middle Ages and harvested as a source of fur or housed as household pets. In particular, Winchester had a high population of furriers and fur traders, as well as a high prevalence of leprosy in its leprosarium, making it the perfect place to study spillovers of the disease.

“With our genetic analysis, we were able to identify red squirrels as the first ancient animal host of leprosy,” said Verena Schuenemann, a senior study author and a professor of archeological sciences at the University of Basel, in a statement. “Our results point to an independent circulation of M. leprae strains between humans and red squirrels during the Medieval Period.”

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Sam Walters is a journalist covering archaeology, paleontology, ecology and evolution for Discover, along with an assortment of other topics. Before joining the Discover team as an assistant editor in 2022, Sam studied journalism at Northwestern University in Evanston, Illinois.