On the age of leprosy.

Leprosy is a chronic infection of the skin and nerves caused by Mycobacterium leprae and the newly discovered Mycobacterium lepromatosis. Human leprosy has been documented for millennia in ancient cultures. Recent genomic studies of worldwide M. leprae strains have further traced it along global human dispersals during the past ∼ 100,000 years. Because leprosy bacilli are strictly intracellular, we wonder how long humans have been affected by this disease-causing parasite. Based on recently published data on M. leprae genomes, M. lepromatosis discovery, leprosy bacilli evolution, and human evolution, it is most likely that the leprosy bacilli started parasitic evolution in humans or early hominids millions of years ago. This makes leprosy the oldest human-specific infection. The unique adaptive evolution has likely molded the indolent growth and evasion from human immune defense that may explain leprosy pathogenesis. Accordingly, leprosy can be viewed as a natural consequence of a long parasitism. The burden of leprosy may have affected minor selection on human genetic polymorphisms.