'Environmental' sources of Mycobacterium leprae: issues and evidence.

Though Mycobacterium leprae is considered to be primarily a parasite of humans, there is a long history of studies, evidence and arguments which have indicated possible non-human sources of the agent. Thus different authors have suggested that M. leprae may be harboured in soil, – 3 in water, on plants, – 7 or in various animal species including amoeba, insects, fish, – 13 primates, and armadillos. The question of possible extra-human sources of M. leprae is an important one for leprosy epidemiology and control. If non-human sources exist, their recognition may help to explain patterns of infection and disease in human populations. Even more importantly, they would have implications for the control of the disease, and in particular for the possibility of its ‘elimination’ or even ultimate eradication. We consider here the nature and implications of the evidence for such extra-human sources. There have been two different sorts of observations motivating the search for extrahuman sources of M. leprae. One is the repeated observation of clinical leprosy in individuals with no apparent history of exposure to other known cases. – 20 The second is the observation that clinical leprosy clusters in particular areas, such as near water sources, which has led some authors to suggest that M. leprae may have an extra-human source in such environments. Neither of these lines of argument provides a strong case for extrahuman sources of M. leprae. The long incubation period of the disease, the inability to recall contacts and encounters years after the event, the fact that stigma leads to hiding of cases in many societies, and the well-recognised fact that multibacillary cases can go undetected for long periods mean that there are substantial opportunities for unrecognised, unremembered or unacknowledged source contacts. The apparent clustering of leprosy in particular environments may simply reflect that certain environments are associated with certain social groups, health conditions or behaviours which predispose to M. leprae transmission or the manifestation of leprosy disease. Or they may reflect environmental