It is caused by Mycobacterium leprae affects the skin & peripheral nerves. The causative organism is less acid-fast than tubercle bacillus.
Grossly– These are characterized by development of nodules or masses formed in the skin, particularly on the face, hands & feet.
-It is slow communicable disease & the incubation period between first exposure & appearance of sign varies from 2-20 years. The infectivity from direct contact patient, who shed numerous bacilli from damaged skin or nasal secretions or may be through maternal-fetal transmission.
Microscopically- In the dermis, there is proliferation of macrophages with foamy changes, particularly around blood vessels, nerves, these are called lepra cells.
– Lepra cells are heavily laden with acid-fast bacilli staining, seen as globular masses, arranged in parallel fashion like “cigarette in pack”.
– This dermal infiltrate of lepra cells characteristically does not encroach upon the basal layer of epidermis & separated from epidermis by a uninvolved area- “clear-zone”.
– The epidermis overlying the lesion is thinned out, flat or even ulcerate.