Leprosy, a disease that can cause nerve damage, lesions, and the loss of smell and eyesight, has afflicted people for millennia—and we’re not alone. It’s also plagued squirrels since at least the Middle Ages, according to a new study. After analyzing squirrel bones dating to medieval England, researchers found that the furry critters carried a strain of a leprosy-causing bacterium strikingly similar to the one many Brits harbored centuries ago. The finding suggests the disease once bounced back and forth between humans and squirrels, which could help scientists understand how leprosy persists today.
“It’s one of the first times we can actually identify pathogens in ancient animal remains to a conclusive level,” says Maria Spyrou, a genomicist at the University of Tübingen who was not involved in the study, published today in Current Biology. Such archaeological evidence could help scientists understand “why diseases emerge in some populations, why they decline, and perhaps ways to eradicate them,” she adds.
Leprosy, which can be caused by two bacterial species—Mycobacterium leprae and M. lepromatosis—is on the decline but still afflicts more than 200,000 people each year, mostly in Asia, Africa, and South America. It is spread mostly through prolonged contact with an infected person. The disease was long believed to occur only in humans, but since the 1970s, researchers have discovered M. leprae in nine-banded armadillos in North and South America—which occasionally pass it on to people—and in a few nonhuman primate species.
In 2016, scientists also discovered that red squirrels (Sciurus vulgaris) in England carry a strain of M. leprae that’s closely related to the one that circulated among medieval English humans. But how the disease might have jumped between the two species through history remained uncertain. “It’s in humans, and then it’s in squirrels much later on, but we don’t know anything about what’s happening in between,” says Sarah Inskip, an osteoarchaeologist at the University of Leicester and an author of the new study.
Medieval squirrels, the team reasoned, might have answers. Middle Ages England was squirrel crazy: People kept the arboreal rodents as pets and used their furs to line and trim clothing. Inskip and her colleagues zeroed in on the medieval city of Winchester, which had not only skinners, tailors, and furriers, but also a hospital for leprosy patients. “It was a really smart way of [looking for] more cases,” Spyrou says.
After sorting through remains previous archaeologists had gathered from a pit in Winchester, the team identified 12 red squirrel bones from between the 10th and 13th centuries that appeared swollen, rough, or damaged from infection or inflammation, the markers of leprosy. By crushing these bones and plucking fragments of DNA from the resulting powder, they managed to isolate and reconstruct a genome of an M. leprae strain—the first ever recovered from archaeological remains of a nonhuman animal.
“I was amazed we actually got it,” Inskip says. Archaeology tends to prioritize human remains, so the tiny bones the team had to work with were not all well-preserved or well-labeled—nor was isolating pathogen DNA within them an easy task. “It was a bit like looking for a needle in a haystack.”
To the team’s surprise, the medieval squirrel strain bore an even closer relation to medieval human strains gathered from the site of the leprosy hospital than to those taken from modern red squirrels. This indicates leprosy circulated between squirrels and humans in England during the Middle Ages, possibly “ping-ponging” between the two, Inskip says—though how frequently remains uncertain. It’s also unclear where and how the squirrels first picked up the disease.
The study shows why it’s important to consider the history and ecology of diseases, not just how they infect people today, says Elizabeth Uhl, a veterinary pathologist at the University of Georgia who was not part of the study. “If you want the whole story, you have to get the pieces from all the different disciplines,” she says.
Although there’s no evidence of red squirrels infecting people in modern times, defining the contexts in which leprosy bounced between species in the past, Uhl explains, can help us anticipate future risks—both with squirrels and other nonhuman carriers. Targeting only human infections might miss our relationships with these animals, she says, allowing the disease to persist.
