Red squirrels in the British Isles are infected with leprosy bacilli

Leprosy, caused by infection with Mycobacterium leprae or the recently discovered Mycobacterium lepromatosis, was once endemic in humans in the British Isles. UK red squirrels (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 showed 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.