Coccidioides immitis

Shmuel Shoham, M.D.


  • Causative organisms:
    • Coccidioides immitis
    • Coccidioides posadasii
    • Note: previously, all cases were attributed to C. immitis.
      • Now known that infections acquired in California are generally due to C. immitis while those acquired elsewhere (e.g. SW U.S., Mexico, Central and South America) are C. posadasii.
  • Dimorphic fungal characteristics:
    • Environmental form: filamentous with septated hyphae that grow in soil. When a dry spell follows rain, the hyphae become brittle, and small (~3-5 μm in diameter) fungal particles termed arthroconidia break off, become airborne, and potentially inhaled.
    • Invasive form at 37°C: following inhalation or occasionally via percutaneous entry, arthroconidia transform into large (~60-75 µm in diameter) structures termed spherules that contain hundreds of small (~2-5 µm in diameter) structures termed endospores. When the mature spherules rupture, endospores are released, which can in turn form additional spherules [Fig].
  • Preferentially grows in regions of California, Southwest U.S. and parts of Mexico that have hot summers, moderate winters and little rainfall.
    • Isolated pockets of C. posadasii are also found in parts of South and Central America.
    • Growth of the fungus as far north as central Washington state has been described.
  • Micro: grows readily on standard fungal media.
    • Coccidioides grown in culture can be a significant laboratory hazard and must be handled cautiously in an appropriate safety cabinet. Alert micro lab if suspected when submitting specimens.

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Last updated: August 29, 2021