Etiology

Histoplasmosis is caused by the spores of the fungus Histoplasma capsulatum.

- H. capsulatum var capsulatum is now recognized to be multiple species: H. capsulatum sensu stricto, which is the originally described H. capsulatum species (in Panama), H. mississippiense (associated with infections in the Mississippi River region), H. ohiense (associated with infections in the Ohio River region), and H. suramericanum (associated with infections in the South American continent). ¹
- H. duboisii (previously H. capsulatum var duboisii) is restricted chiefly to western Africa.
Geographic distribution: H. capsulatum is found in multiple locations across the globe.
- Important endemic areas in North and Central America include (but are not limited to) the Ohio and Mississippi River valleys, the Caribbean basin, and Mexico. However, histoplasmosis can occur anywhere in the U.S. While it is most common in central and southern U.S. states, it is also endemic in the upper Midwest states, and the geographic range may change with a warming climate.
- The fungus can also be found in parts of South America, Asia, Australia, Europe, and Africa.
- Important to note that microfoci of endemicity exist both within and outside areas traditionally associated with the fungus.
Fungus grows in soil, particularly if it is enriched by bird or bat guano (e.g., bird roosts, chicken coops, caves with bats). However, most people with histoplasmosis do not recall encounters with such material.
Dimorphic growth: Depending on the situation, H. capsulatum may grow as a mold or yeast.
- Mold form predominates in the environment (or in vitro when incubated at < 35°C).
  - Morphology as mold
    - Aerial hyphae: long tubular structures.
    - Macroconidia: thick-walled spherical structures 8-15 μm diameter with surface projections. This distinctive structure is diagnostic in Fig 1.
    - Microcondia: smooth spherical structures 2-4 μm in diameter. When contaminated soil is disturbed, microconidia can become airborne and settle in patients’ alveoli to cause infection.
- Yeast form: with infection, the fungus transitions to yeast, seen in tissues (or in vitro when incubated at temperatures of ≥37°C).
  - Appearance: oval, narrow-based, budding yeast, 2-4 μm in diameter Fig 2.
  - It may be seen within macrophages or tissues and facilitates dissemination from the lung to multiple other sites.

References

1. https://www.hopkinsguides.com/hopkins/view/Johns_Hopkins_ABX_Guide/540273/all/Histoplasma_capsulatum#:~:text=Yeast%20form:%20with%20infection%2C%20the,lung%20to%20multiple%20other%20sites.