Red squirrels in the British Isles are infected with leprosy bacilli

Charlotte Avanzi, Jorge Del-Pozo, Andrej Benjak, Karen Stevenson, Victor R Simpson, Philippe Busso, Joyce McLuckie, Chloé Loiseau, Colin Lawton, Janne Schoening, Darren J Shaw, Jérémie Piton, Lucio Vera-Cabrera, Jesùs S Velarde-Felix, Fergal McDermott, Stephen V Gordon, Stewart T Cole, Anna L Meredith

Leprosy, caused by infection with Mycobacterium leprae or the recently discovered Mycobacterium lepromatosis, was once endemic in humans in the British Isles. Red squirrels in Great Britain (Sciurus vulgaris) have increasingly been observed with leprosy-like lesions on the head and limbs. Using genomics, histopathology, and serology, we found M. lepromatosis in squirrels from England, Ireland, and Scotland, and M. leprae in squirrels from Brownsea Island, England. Infection was detected in overtly diseased and seemingly healthy animals. Phylogenetic comparisons of British and Irish M. lepromatosis with two Mexican strains from humans show that they diverged from a common ancestor around 27,000 years ago, whereas the M. leprae strain is closest to one that circulated in Medieval England. Red squirrels are thus a reservoir for leprosy in the British Isles.