Leprosy passed between humans and red squirrels in medieval England, research suggests, supporting the theory that the fur trade could have played a role in the spread of the disease. Leprosy is one of the oldest infectious diseases recorded in humans and is typically caused by the bacterium Mycobacterium leprae. While most cases now occur in south-east Asia, and can be treated with antibiotics, leprosy was common in medieval England and caused sickness and disfigurement in rich and poor people alike. Previous research revealed that people in medieval England, Denmark and Sweden had a similar strain of leprosy to that found today in red squirrels in the south of England, with one theory being that the trading of squirrel furs, imported from Viking Scandinavia, could have been a factor in spreading the disease. Now experts say the theory has been given a boost, with genetic analysis revealing that red squirrels in medieval England experienced a very similar strain of the disease to humans living at the time. “This is the first time that we found an animal host of leprosy in the archaeological record, which is really exciting,” said Dr Sarah Inskip from the University of Leicester, who co-authored the research. Writing in the journal Current Biology, Inskip and an international team of colleagues report how they studied strains of leprosy found in samples from three people who lived in Winchester between 900 and 600 years ago, and a squirrel whose bones were found in a furrier pit in the city dating to between 1,000 and 900 years ago. The team focused on Winchester because it was an important city in the medieval period and had a leper hospital and myriad skinners involved in the preparation and selling of fur-lined clothes – meaning it was possible to obtain squirrel and human remains from the time. The team extracted and analysed DNA from the samples, revealing a very similar strain of leprosy was present in all. “In fact, the strains that are in the archaeological squirrels and the archaeological humans from Winchester are more closely related than the strain that’s in the medieval squirrels [and] the strain that is in modern squirrels,” said Inskip. The team say the results suggest there was transmission of the disease between humans and squirrels. However, the findings are based on just a handful of samples, and the results cannot shed light on whether humans initially caught leprosy from red squirrels or vice versa. Inskip said that even in the latter case, the animals could pass the disease back to humans, noting that while humans first gave the disease to armadillos in the Americas, they can now catch it from the animals. “We know that it can ping-pong backwards and forwards,” she said. Inskip added that there were a number of possibilities for how transmission may have occurred. “One mechanism would be the fur trade,” she said. Indeed the study reports that in 1384 alone, 377,200 squirrel skins were imported to England from Scandinavia and other places. However squirrels were also widely kept as pets, offering another route of contact with humans. “Both mechanisms are possible. And they’re not mutually exclusive either,” said Inskip. Inskip said the study also had implications for people suffering from leprosy today. “Maybe we need to go and look at the animals that are around these communities,” she said. “Because perhaps it’s possible that some of these animals maybe have the bacteria and that’s maybe why the disease is hanging around.”