Scientists have discovered human leprosy in British red squirrels, uncovering one leprosy-driving bacterial strain, in particular, that is similar to that responsible for outbreaks of the disease in medieval Europe. The researchers say their findings suggest squirrels have been a reservoir for these ancient bacteria for decades, though they stress that the chances of people catching the disease from the animals are low. Leprosy, now largely confined to developing nations, has been thought to be exclusively transmitted in humans, mainly because scientists have failed to culture the bacteria in animals. Here, as part of efforts to further understand incidences of dwindling red squirrel numbers in the U.K., Charlotte Avanzi and colleagues examined more than 100 red squirrel cadavers from the U.K., Ireland and Scotland with genetic screening and blood serum tests, looking for different strains of leprosy-driving bacteria. All 25 red squirrels tested from the U.K.'s Brownsea Island were infected with M. leprae, the oldest pathogen associated with leprosy. Finding M. leprae in squirrels in this region was unexpected, the authors say, because leprosy was eradicated from the British Isles several centuries ago, thus demonstrating that a pathogen can persist in the environment long after its clearance from the human reservoir. Some of the squirrels studied also harbored Mycobacterium lepromatosis, a recently discovered, severely debilitating form of leprosy. The squirrel M. lepromatosis strain from the British Isles diverged from a similar human strain previously found in Mexico about 27,000 years ago, the authors say. The high level of infection that Avanzi and colleagues uncovered in squirrels may be working against efforts to eradicate the disease in humans; therefore, it will be critical to track other occurrences of this stubborn disease in animals, the authors say. A Perspective by Timothy P. Stinear and Roland Brosch lends more insight into the implications of leprosy in animals.