MICROBIOLOGY

• Evolving nomenclature of Histoplasma capsulatum:
  
  • H. capsulatum var capsulatum is recognized to be multiple species: H. capsulatum sensu stricto, which is the species described initially, 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). This module will discuss these organisms together under the general term H. capsulatum.
  • H. duboisii (previously H. capsulatum var. duboisii) is restricted chiefly to western Africa and will not be covered in this module.
• Geographic distribution: H. capsulatum is found in multiple locations worldwide.
  • Important endemic areas include (but are not limited to) the Ohio and Mississippi River valleys, Central and South America, the Caribbean basin, and Mexico. However, histoplasmosis can occur anywhere in the U.S. (Histoplasma in the United States).
    
  • Histoplasma can also be found in parts of Asia, Australia, Europe, and Africa.
    • Areas with the presence of Histoplasma worldwide
  • 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 a specific exposure.
• Dimorphic growth: Depending on the temperature, H. capsulatum can grow as either a mold or a 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 in 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, causing infection.
  • Yeast form: at body temperature, 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 can be observed within macrophages or tissues and facilitates dissemination from the lung to multiple other sites (Fig. 3).