After inoculation into the skin, M. ulcerans proliferates and elaborates a toxin that causes necrosis of the dermis, panniculus, and deep fascia. Early lesions are closed, but as the necrosis spreads, the overlying dermis and epidermis eventually ulcerate, with undermined edges and a necrotic slough in the base of the ulcer. Histopathologic sections reveal a contiguous coagulation necrosis of the deep dermis and panniculus, with destruction of nerves, appendages, and blood vessels.
Interstitial edema is noted.
Clumps of extracellular acid-fast bacilli are plentiful and are frequently limited to the base of the ulcer and adjacent necrotic subcutaneous tissue. Bone is sometimes involved, and specific osteomyelitis is well-known. Histologically there may be local and regional lymphadenitis with invasion by M. ulcerans. In active lesions, inflammatory cells are conspicuously few, presumably as a result of the immunosuppressive activity of the toxin. With healing, there is a granulomatous response, and the ulcerated area is eventually replace by a depressed scar.