The acid-fast bacilli are longer and broader than in bovine tuberculosis. Group 1 photochromigen mycobacteria are present in brackish, stagnant water. The patient likely acquired her infection from watertroughs, which she cleaned for her horses. Primary inoculation atypical mycobacterial infection is an occupational hazard for salt-water fishermen. Outbreaks have been associated with swimming pool exposure.
Aquariums are now a common source of infection.
A nodule develops at the site of inoculation that often suppurates or that may ulcerate. The nodule may have a purulent vegetative surface. Sporotrichoid may develop along the lymphatic vessels extending up the extremity. The histopathologic features are characteristic. The pseudoepithiomatous hyperplasia may be even greater than observed here simulating carcinoma.