Prof. J.M. Boopal Raj, MD,DD
Professor - Dermatology
& Prof. S. Anandan, MD,DD
Professor & Head-Dermatology
Sri Ramachandra Medical College & Research Institute (Deemed University)

Leprosy is a chronic infectious disease of low infectivity cause by Mycobacterium leprae.


Characteristics of the etiological agent which influences epidemiology:

Average generation time of the bacillus is 12-13 days

Though an obligatory intracellular organism, it is viable outside the human host for about 24-48 hours and a maximum of 7 days.

The life span of the organism is about 6 months.

Boiling and autoclaving kills the bacillus.

Its susceptibility to air, cold, water, drying, sunlight and disinfectants is uncertain.

a) Sources of infection:

  • Nasal secretions from the lepromatous


  • Discharge from ulcers in the skin occurring over the lesions in a lepromatous patient.

  • Ulcers in the skin occurring over borderline lesions during reactional states.

  • Trophic ulcers are the sources of infection since the ulceration is primarily due to the sensory loss over the skin and not due to bacillary inversion of the skin.

  • Leprosy bacilli are secreted in the milk of lepromatous mothers, whether they produce infection, immunity or tolerance in the child is uncertain.

b) Mode of transmission

The traditional view is that the infection is transmitted by a prolonged and intimate skin to skin contact. However in a number of cases this mode cannot be explained; whether it can be through insect bites or through the alimentary or respiratory tract by ingestion or inhalation is being investigated.A few cases of transplacental transmission have been reported.

c) Incubation period

The bacillus has a very long generation time; the average circulation period is 2-4 years.

d) Factors influencing susceptibility

  • Age: Children are more susceptible than adults.

  • Individual immunity : May be determined by certain genetic factors which influence the susceptibility of an individual

  • Nutritional factors may play a role.

  • Climate : Leprosy is prevalent in tropical and subtropical climates.