The skin may be infected by mammalian tubercle bacilli as in the lesions acquired by pathologists and butchers, or the ulceration of lips, external genitalia, or anus from tuberculosis of the associated organs ; by attenuated mammalian tubercle bacilli in lupus vulgaris ; by the artificially attenuated Mycobacteria boris of Calmette and Guerin from the site of a BCG vaccination; by Mycobacteria leprae; and by Mycobacteria fortuitum to produce a small abscess.
Moreover, chronic skin ulcers may be colonized by commensal mycobacteria which can be isolated when appropriate methods are used. There are two species of mycobacteria — Mycobacteria ulcerans and Mycobacteria marinum—which are exclusively skin pathogens. They remain strictly localized, multiplying only in the cool, super-ficial tissues, and they give rise to chronic skin ulcers. The regional lymph nodes are not enlarged and there is no systemic disturbance.
Infection with Mycobacteria ulcerans occurs in Victoria and Queensland, Australia, in Uganda (Buruli ulcer), the Congo, Nigeria, Malaya and Mexico; and foci of infection, generally near rivers and lakes, are probably widely distributed through-out the tropics. Although it is apparently a strictly human parasite, there is as yet no evidence of its spread from man to man ; nor of carriage by insects although biting flies are prevalent in areas where infection with Mycobacteria Ulcerans is found. The organism may be introduced into the skin of an exposed part, usually an arm or leg, by some slight injury such as the prick of a thorn or an insect bite. In the course of a few weeks an area of induration develops which breaks down, and the ulceration spreads slowly under the skin and into the deeper tissues.
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