Extended studies on the viability of Mycobacterium leprae outside the human body

Very little is known in leprosy regarding the transmission of the infection from the source to the susceptible host. One of the important factors which governs the transmission of the disease is the viability of Mycobacterium leprae outside the human body. In this study M. leprae obtained from untreated patients have been subjected to several adverse conditions. Their viability was verified by their multiplication in the footpads of normal mice. After drying in the shade the organisms were viable up to 5 months. On wet soil, they remained alive for 46 days. Kept in saline at room temperature, the organisms lived for 60 days. Surprisingly on exposure to direct sunlight for 3 hours a day the bacteria survived for 7 days. On refrigeration at 4 degrees C, the bacteria could be preserved for 60 days. On the other hand, keeping at -70 degrees C, the bacteria could be maintained in a living condition for only 28 days. On exposure to antiseptics like Savlon (R) and alcohol, the bacteria were rapidly killed. These results indicate the survival outside the human body of M. leprae under different environmental conditions in India where the disease is endemic. Transmission of infection by indirect contact and occurrence of new cases in the absences of any known source, are consistent with M. leprae being viable outside the human body for varying periods of time. The findings could also be pointers to understand the epidemiology of leprosy.