Histoplasma capsulatum

Shmuel Shoham, M.D.


  • Two varieties of Histoplasma capsulatum are relevant to human infections: this module will focus on var. capsulatum, which will be referred to as H. capsulatum.
    • H. capsulatum var. capsulatum: nearly worldwide distribution
    • H. capsulatum var. duboisii: mostly restricted 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, Europe, and Africa.
    • Important to note that microfoci of endemicity exist both within and outside of areas traditionally associated with the fungus.
  • Fungus grows in soil, particularly if 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: H. capsulatum may grow as a mold or as a yeast depending on the situation.
    • 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 in diameter with surface projections. This distinctive structure is diagnostic 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, which is the form 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.
      • May be seen within macrophages or in tissues and serve to facilitate dissemination from the lung to multiple other sites.

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Last updated: May 8, 2022