Histoplasma capsulatum

Shmuel Shoham, M.D., John G. Bartlett, M.D.
Histoplasma capsulatum is a topic covered in the Johns Hopkins ABX Guide.

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  • Two varieties of Histoplasma capsulatum 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.
    • Fungus can also be found in parts of South America, Asia, 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).
  • Dimorphic growth: H. capsulatum may grow as a mold or as a yeast depending 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: February 26, 2017