Hyphal forms of Histoplasma capsulatum. A common manifestation of intravascular infections.

The usual tissue form of Histoplasma capsulatum is a small intracellular yeast, though extracellular organisms of unusually large size have also been described. The production of hyphae in tissue has been regarded as rare and we believe not generally appreciated. We report a case of probable Histoplasma endocarditis from which an embolus demonstrated abundant mycelial forms on histologic examination. Of previously reported cases of H capsulatum endocarditis with detailed pathologic description, nine of 27 cases are found to demonstrate hyphal forms. Hyphal structures should be considered a common morphologic variant of H capsulatum in intravascular lesions.