MICROBIOLOGY

• 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.