Blastomyces spp.

MICROBIOLOGY

• 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 done through a DNA probe but can also be done by converting mold to yeast form (but labor intensive).
      • The 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.
    • It is found in the midwestern, southeastern, and south-central 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[12].
    • Instead of a single broad-based bud, multiple buds are attached to the mother cell, and no conidia are produced.
    • Cutaneous findings have not yet been described with this species.
    • This species may be misdiagnosed as H. capsulatum or B. dermatitidis.
  • B. percursus[9]
    • Middle East and Africa
  • Blastomyces emzantsi[9]
    • Only described in S. Africa to date.
  • B. parvus
    • Rare, described only in severely immunosuppressed people.