Leprosy: cause, transmission, and a new theory of pathogenesis.

Leprosy is generally accepted as being caused by Mycobacterium leprae, an acid-fast organism often present in great numbers in certain forms of leprosy. However, it has not been possible to confirm with scientifically acceptable evidence that this entity is the cause of leprosy; laboratory cultivation, an essential factor in the proof, has not been accomplished with the acid-fast bodies seen in leprotic tissue. The mechanisms of transmission of the disease also remain conjectural; prolonged, close contact and transmission by nasal droplet have both been proposed, and, while the latter fits the pattern of disease, both remain unproved. It is proposed that the causative agent of leprosy is not a difficult-to-transmit agent but, rather, an organism that has evolved a highly efficient state of parasitism in stable types of populations and that everyone in the population harbors the leprosy parasite at some time. The majority of the population incubate subclinical infections at various levels; clinical leprosy arises from within the pool of subclinical infection in the endemic population rather than by transmission from an index case. A theory of complete infection of endemic populations is consistent with the rate of development and distribution of positive lepromin reactions among healthy persons in endemic regions and provides an explanation for the difficulty in controlling leprosy in endemic populations by isolation of patients or by therapy for clinical cases.