Aetiology
Blastomycosis is caused by Blastomyces dermatitidis.[4] It is a dimorphic fungus that grows as a white cottony mycelium that turns tan or brown at 25°C (77°F), and as a cream-to-tan, heaped, or wrinkled yeast at 37°C (98.6°F).[19] B dermatitidis exists as a saprophytic mould in the environment, and has been isolated from samples in such sites as a beaver pond and lodge, soil and organic debris from a fishing site along a river bank, and woodpiles.[15][16][20]
Other species within the Blastomyces genus have also been described, including B helicus, B gilchristii and B percursus.[21][22] However, their relative importance as pathogens are yet to be defined.
Pathophysiology
Infection with B dermatitidis usually occurs by inhalation.[4] In the lung, the conidia (spores) are either phagocytosed and eliminated by alveolar macrophages, neutrophils, or monocytes, or they convert to the yeast form, which is more resistant to phagocytosis.[23] This can lead to a self-limiting asymptomatic pulmonary infection or acute lobar pneumonia. From the lung, the yeast form can disseminate haematogenously, leading to extrapulmonary infection, most commonly in the skin and soft tissues, bones and joints, CNS, and urinary tract.[23] However, direct inoculation of the conidia into the skin can occur as well.[24]
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