Case 159 - Discussion

Uploaded: 2007-12-19, Updated: 2007-12-21



GMS, 20

GMS, 40

PAS, 20

PAS, 40



AFB stain is negative for tuberculosis bacteria.


Histoplasma Granuloma (Histoplasma capsulatum)

Description and Natural Habitats

  • Thermally dimorphic fungus found in nature;

  • Common natural habitat: soil contaminated with bird droppings or excrements of bats;

  • Endemic in the Tennessee-Ohio-Mississippi river basins.

Pathogenicity and Clinical Significance

  • Systemic (endemic) mycosis/histoplasmosis: varying from an acute benign pulmonary infection to a chronic pulmonary or fatal disseminated disease.  In cases of dissemination of Histoplasma capsulatum var. capsulatum infection, reticuloendothelial system (RES) is most frequently involved. The fungus resides intracellularly in RES cells;

  • Dissemination and fatal course are more common in the immunocompromised and elderly. Chronic cavitary histoplasmosis is most commonly observed in individuals with underlying pulmonary disease.

Macroscopic Features

  • Being a thermally dimorphic fungus, Histoplasma capsulatum grows in mould form at 25C, and in yeast form at 37C. Below are the macroscopic characteristics at varying temperatures and for both varieties.

  • At 25C: Colonies are slow growing and granular to cottony in appearance. The color is white initially and usually becomes buff brown with age. The colonies are not sensitive to cycloheximide in the culture media. From the reverse, a yellow or yellowish orange color may be observed. While these features are best observed on Sabouraud dextrose agar (SDA), brain heart infusion agar (BHIA) enhances growth more efficiently.

  • At 37C:  Creamy, slowly growing, moist and yeast-like colonies are formed. This phase is observed in infected tissues and in vitro on enriched media, such as BHIA containing 5-10% blood.

Microscopic Features

  • At 25C: hyphae-like conidiophores which arise at right angles to the parent hyphae. It has both macro- and microconidia. Macroconidia are tuberculate, thick-walled, round, unicellular, hyaline, large and often have fingerlike projections on the surface. These macroconidia are also referred to as tuberculochlamydospores or macroaleurioconidia. Microconidia (microaleurioconidia) are unicellular, hyaline and round, with a smooth or rough wall.

  • At 37C: narrow-based, ovoid, budding yeast cells are formed. Yeasts of var. capsulatum are smaller than (2-4 m) those of var. duboisii (12-15 m).