Histoplasma capsulatum [hĭs′tə-plăz′mə kăp′sə-lā′təm]

Figure

Figure. Numerous, capsulated yeast cells (shown in pink) of Histoplasma capsulatum in a bone marrow aspirate (Giemsa-stained, original magnification ×400). Source: Shivaprakash Rudramurthy, PGIMER, Chandigarh, India.

In 1905, Samuel Taylor Darling serendipitously identified a protozoan-like microorganism in an autopsy specimen while trying to understand malaria, which was prevalent during the construction of the Panama Canal. He named this microorganism Histoplasma capsulatum because it invaded the cytoplasm (plasma) of histiocyte-like cells (Histo) and had a refractive halo mimicking a capsule (capsulatum), a misnomer (Figure).

Histoplasma capsulatum, a dimorphic fungus, now belongs to Kingdom Fungi and causes histoplasmosis (Darling’s disease) through inhalation of spores found in soil and bird droppings. The fungus thrives in the central and eastern parts of United States, especially around the Ohio and Mississippi River valleys, and in South America, Africa, Asia, and Australia. Three varieties exist globally: H. capsulatum var. capsulatum, H. capsulatum var. duboisii, and H. capsulatum var. farciminosum.