Blastomyces spp.


  • Thermally dimorphic fungus: mycelial in nature (room temperature) and yeast in tissue (37°C).
  • The yeast form is 8 x 30 microns with broad-based budding [Fig 1], differentiating from Histoplasma narrow-based budding.
    • A fungal culture is often performed using Sabouraud’s dextrose agar, incubating at 25–27 °C.
      • Slow growth, up to 4 weeks to display an off-white mold.
      • Confirmation is now typically through DNA-probe but can also be done by converting mold to yeast form (but labor intensive).
        • Probe cannot differentiate between B. dermatiditiis, B. gilchristii and B. helicus.
  • Fungus is predominantly found in moist, acid soils in forests, decomposing matter.
  • Blastomycosis:
    • Blastomyces dermatiditis: usually strikes immunocompetent people.
      • Found in the midwestern, southeastern and southcentral U.S., bordering the Mississippi and Ohio River basins, the St. Lawrence Seaway and the Canadian provinces of Manitoba, Ontario and Saskatchewan.
    • B.gilchristii
      • Seen in northern Wisconsin and western Ontario.
    • B. helicus (previously Emmonsia helicus): this organism does not appear to produce conidia. Described to date as a rare cause of often disseminated human infection in the Western U.S., Rocky Mountains, Western Canada, and in immunocompromised hosts--differentiating from B. dermatitidis[8].
      • Instead of a single broad-based bud, multiple buds are attached to the mother cell, and no conidia are produced.
      • Cutaneous findings not yet described with this species.
      • This species may be misdiagnosed as H. capsulatum or B. dermatitidis.
    • B. percursus[5]
      • Middle East and Africa
    • Blastomyces emzantsi[5]
      • Only described in S. Africa to date.
    • B. parvus
      • Rare, described only in severely immunosuppressed people.

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Last updated: April 15, 2023