Transmission is by contact with traumatised skin, biting flies, or inhalation.
The causative agent, Histoplasma farciminosum, is a dimorphic fungal soil saprophyte. Diagnosis is made by identification of the organism in smears of lesion material, by histopathology, and by isolation of the fungus. Organisms are usually surrounded by a 'halo' when stained with Gram stain or haematoxylin and eosin. Assays reported for detection of antibody include fluorescent antibody, enzyme-linked immunosorbent assay, and haemagglutination inhibition tests. In addition, a skin hypersensitivity test has been described.