Medieval Squirrels and Humans May Have Spread Leprosy Back and Forth

Red squirrels in medieval England had leprosy, making the rodents the earliest known animal hosts of the disease, according to a new analysis of archaeological remains published last week in the journal Current Biology.

Scientists found genetic similarities between the leprosy strains found in medieval squirrels and humans, suggesting the disease may have passed between the species.

“This is the first time that we found an animal host of leprosy in the archaeological record, which is really exciting,” Sarah Inskip, a co-author of the new study and archaeologist at the University of Leicester in England, tells the Guardian’s Nicola Davis.

“It really goes against the narrative that it was a human disease, specifically,” she says to BBC News’ Nia Price.

Leprosy is one of the oldest recorded diseases in human history. Infections are mainly caused by the bacteria Mycobacterium leprae, as well as M. lepromatosis. It does not spread easily between people, requiring prolonged, close contact, per the Centers for Disease Control and Prevention (CDC). Infections can cause skin lesions, nerve damage, blindness and hair loss, among other symptoms.

Today, leprosy is very rare in the United States, but more than 200,000 cases occur worldwide each year, largely in Asia, Africa and South America. Scientists once thought only humans could catch it, until they discovered infected armadillos in the Americas in the 1970s, reports the Atlantic’s Katherine J. Wu. Since then, researchers have also identified the disease in wild chimpanzees in West Africa and in British red squirrels.

Modern red squirrels have a strain of leprosy closely related to a medieval human strain. “So, we had an inkling that maybe medieval red squirrels have had it too,” Inskip says to New Scientist’s Chen Ly.

In medieval times, the lives of humans and squirrels were much more intertwined than they are today. Squirrel fur was the most commonly used fur for trimming and lining garments in the High and Late Middle Ages, and “staggering quantities” of squirrel skins were transported for trade, according to the study. The rodents were also widely kept as pets and would sit on people’s laps and shoulders.

The study authors zeroed in on archaeological remains from medieval Winchester, England, because leprosy had many opportunities to jump between squirrels and humans in that area. The city was well-connected to the fur trade, and on a street known as “skinners row,” people would prepare and sell garments with squirrel fur. Winchester also had a health care center where people with leprosy were cared for.

“It was a really smart way of [looking for] more cases,” Maria Spyrou, a genomicist at the University of Tübingen in Germany who did not contribute to the findings, tells Science’s Sean Cummings.

The researchers examined 25 samples of human remains and 12 squirrel samples from two sites in Winchester and were able to reconstruct genomes of M. leprae found in three humans and one squirrel. The genome of leprosy found in the medieval squirrel was more closely related to the medieval human strains than to strains found in modern red squirrels, suggesting leprosy was spreading between humans and squirrels at the time.

Still, it wasn’t clear whether squirrels were first infected by humans, other animals or the environment, the study authors write. The findings also don’t reveal whether humans first caught leprosy from squirrels, or if it was the other way around. Leprosy could have “ping-ponged” between both species, per the Atlantic, as they transmitted the disease back and forth.

“It may be that there has been more than one transmission event between humans and squirrels over history,” Inskip tells New Scientist.

Finding pathogens in archaeological remains could help researchers understand how certain diseases arise in different populations or probe ways to eradicate them, Spyrou tells Science.

“I think [the research] highlights that we still have a lot of work to do to understand the transmission of this ancient disease better, in our efforts to try and reduce the impact globally,” Stephen Walker, who studies leprosy at the London School of Hygiene and Tropical Medicine and was not involved in the research, says to BBC News.

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Will Sullivan is a science writer based in Washington, D.C. His work has appeared in Inside Science and NOVA Next.