Red squirrels in medieval England were carriers of leprosy.

DNA from the bacterium that causes leprosy has been found in human remains and a red squirrel excavated at a medieval British site. This makes the red squirrel the oldest known non-human host of infection, and suggests that the infection may have spread between rodents and humans at that time.

In 2016, scientists discovered that the red squirrel (vulgaris ciurus) carried the mycobacterium leprosy, bacteria that causes the chronic disease Hansen’s disease. Some of the strains were similar to strains that infected people in England more than 700 years ago.

“So we were hinting that maybe medieval red squirrels might have had it too,” he says. Sarah Inskip at the University of Leicester, UK.

To further investigate the remains, Inskip and his colleagues discovered 25 bodies at the site of a medieval leper hospital in Winchester, as well as nearby ruins that were home to at least one fur store during the 11th and 13th centuries.

While most of the human bones showed the characteristic lesions associated with leprosy, the squirrel bones showed signs of inflammation, which may be another sign of the disease.

By analyzing bone DNA, the research team discovered the following gene sequence of Mycobacterium leprae in 3 people and 1 red squirrel.

“In fact, leprosy was prevalent among squirrels in the Middle Ages,” Inskip said, making this species the earliest non-human species reported to carry leprosy.

DNA showed that the strains of Mycobacterium leprae found in medieval red squirrels were more closely related to those of the Three Medieval Peoples than those of modern red squirrels. This suggests that infectious diseases were probably passed back and forth between squirrels and people in medieval England.

“Medieval Winchester had a lot of opportunity for lore,” he says. Verena Schuenemann, a doctor from the University of Basel in Switzerland, also worked on this research. In addition to the hospital and the city’s famous fur trade, historical reports from the time suggest that people in the area often kept squirrels as pets, she says.

This finding also suggests that the leprosy strains found in modern squirrels may not necessarily be descendants of the strains found in this specimen. “Historically, there may have been multiple transmission events between humans and squirrels,” Inskip says.

Although a small number of red squirrels currently have leprosy, it is important to emphasize that the risk of transmission to humans is essentially zero, Schuenemann said.

“Leprosy has undoubtedly existed since ancient times; Mycobacterium leprae probably has a much more robust ecological history than previous modern observations have suggested.” Richard Truman previously worked at the U.S. Public Health Service. “It’s important to understand this better.”