Some recent laboratory findings on Mycobacterium leprae. Implications for the therapy, epidemiology and control of leprosy.

From changes in the morphology and viability of Mycobacterium leprae in infected mice, some authors have concluded that the majority of smear-positive human lepromatous patients become non-infectious within 3-4 months of the beginning of regular treatment with sulfones, even at dosages equivalent to only one-hundredth of the conventional daily dose-namely, 1 mg daily. These very low dosages have, however, not been advocated because of the risk that resistant strains of Myco. leprae might develop. The laboratory findings have already been applied to human leprosy by some leprologists; if they were more widely adopted, leprosy programmes could be seriously affected and the consequences could be serious in endemic areas. Contrary to the results of laboratory tests, the shortcomings of clinical sulfone therapy are evident all over the world from the long times required for lepromatous cases to become bacteriologically negative, from the high proportion of relapses and from chemoprophylaxis trials in child contacts. Final proof of the relationship between the morphological index, i.e., the proportion of solidly staining bacilli present, and contagiousness can come only from prolonged and well-planned epidemiological studies. It appears that leprosy may develop in appreciable numbers of child contacts exposed to index cases already bacteriologically negative, with or without prophylactic dapsone treatment.The results of controlled BCG trials now in progress are not consistent although laboratory trials in mice have indicated that BCG vaccination confers a high degree of protection. However, the results that have been obtained in the mouse, an unnatural host, may not be obtainable in man.It is considered premature to apply laboratory findings to human leprosy before clinical and epidemiological studies have been made in man.