Histoplasma capsulatum

A dimorphic fungus species of worldwide distribution that causes histoplasmosis in humans and other mammals; its ascomycetous state is Ajellomyces capsulatum. The organism's natural habitat is soil fertilized with bird and bat droppings, where it grows as a mold, fragments of which, following inhalation, produce the primary pulmonary infection; within the mammalian host tissues, inhaled mycelial fragments grow as uninuclear yeasts that reproduce by budding. This parasitic form may also be induced in the laboratory by culturing the mycelial phase at 37°C on a blood-enriched medium; growth reverts to the mycelial form when the temperature is below 37°C. H. var. duboisii causes a clinically distinct disease, African histoplasmosis, in which large yeast cells with thicker walls are found in tissues, in contrast to the small yeast cells of H. var. farciminosum, which causes epizootic lymphangitis. [histo- + G. plasma, something formed]

* * *

the etiologic agent of classic histoplasmosis, occurring as small, oval, yeastlike cells which in tissue seem to be encapsulated but are not. It grows as a mycelial fungus in the soil and as a yeast at 37° C on agar or in tissue. Its perfect (sexual) stage is Ajellomyces capsulatus.