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Dimorphic fungi

Blastomyces


Deputy Editor-in-Chief: Patricia Tsang, M.D., M.B.A.
Smitha Mruthyunjayappa, M.D.
Sixto M. Leal, Jr., M.D., Ph.D.

Last author update: 20 April 2022
Last staff update: 20 April 2022

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PubMed Search: Blastomyces

Smitha Mruthyunjayappa, M.D.
Sixto M. Leal, Jr., M.D., Ph.D.
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Table of Contents
Definition / general | Essential features | Epidemiology | Sites | Pathophysiology | Clinical features | Laboratory | Case reports | Treatment | Microscopic (histologic) description | Microscopic (histologic) images | Positive stains | Molecular / cytogenetics description | Differential diagnosis | Additional references | Board review style question #1 | Board review style answer #1 | Board review style question #2 | Board review style answer #2
Cite this page: Mruthyunjayappa S, Leal SM. Blastomyces. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/microbiologyblastomyces.html. Accessed April 25th, 2024.
Definition / general
  • Taxonomy:
    • Class: Eurotiomycetes; order: Onygenales; family: Ajellomycetaceae
  • Common species:
    • Blastomyces dermatitidis: most common cause of blastomycosis
    • Blastomyces gilchristi: northern U.S. and Canada
    • Blastomyces helicus: western U.S
    • Blastomyces percursus: Africa
Essential features
  • Dimorphic mold
  • Body temperature, uniformly sized, extracellular round yeasts with a thick, refractile, double contoured cell wall and broad based budding
  • Immunocompetent: a chronic granulomatous and suppurative disease of the lung or skin
  • Immunocompromised: disseminated disease with poor prognosis
Epidemiology
  • Cause of blastomycosis, a chronic pyogranulomatous inflammatory disease
  • Soil organisms, associated with moist areas near riverbeds
  • Midwestern, south central and southeastern U.S., particularly in areas surrounding the Ohio and Mississippi River valleys, the Great Lakes and the Saint Lawrence River
  • Present in Canada, with a small number of documented cases in Africa and India
  • References: J Clin Aesthet Dermatol 2009;2:22, Semin Respir Crit Care Med 2020;41:31
Sites
  • Pulmonary blastomycosis: can be asymptomatic or range from self limited pulmonary infection to life threatening (eMedicine: Acute Respiratory Distress Syndrome (ARDS) [Accessed 26 January 2022])
  • Extrapulmonary blastomycosis:
    • Cutaneous: development of verrucous (wart-like) or ulcerative skin lesions
    • Osseous: osteomyelitis along with contagious tissue abscesses and draining sinuses
    • Genitourinary (usually male): involvement of the prostate and epididymis
    • Central nervous system: brain abscess, cranial or epidural abscess and meningitis
Pathophysiology
  • Inhaled spores transform into yeasts resulting in acute and chronic lung inflammation
  • Cutaneous lesions can demonstrate pseudoepitheliomatous hyperplasia of the epidermis
  • Dissemination to extrapulmonary sites, especially the skin, is presumed to be hematogenous
  • Reference: J Clin Aesthet Dermatol 2009;2:22
Clinical features
  • Involves the lung in over 90% of cases; infection can range from asymptomatic self limited infection (about 50% of cases) to severe diffuse pneumonia causing respiratory failure
  • Extrapulmonary dissemination occurs in approximately 25 - 50% of cases of blastomycosis (Mycopathologia 2009;167:115)
Laboratory
  • Direct exam shows large, thick walled, yeast forms with single broad based budding cells (8 - 10 μm)
  • Slow growing yeasts that transition to a fluffy white mycelium or glabrous, tan, nonsporulating colonies within 1 - 4 weeks
  • Microconidia resemble Histoplasma capsulatum but macroconidia are not formed
  • Microscopic examination of growth demonstrates thick walled yeast cells with broad based budding
  • Conversion to the yeast phase can occur on routine media incubated at 37 °C
  • Antibody detection tests
    • Immunodiffusion utilizes purified B. dermatitidis A antigen (relatively low sensitivity and specificity)
    • Enzyme immunoassays use BAD1 antigen: most sensitive approach (cross reactivity seen with histoplasmosis and other fungal infections)
  • Reference: Semin Respir Crit Care Med 2020;41:31
Case reports
Treatment
  • Acute pulmonary blastomycosis can be mild and self limited, not requiring treatment in immunocompetent hosts
  • More severe disease, particularly in immunocompromised hosts, requires itraconazole with or without liposomal amphotericin B
    • Treatment duration can range from 6 months to 1 year
  • References: Clin Infect Dis 2000;30:679, S D Med 2006;59:255
Microscopic (histologic) description
  • Pyogranulomatous inflammation: neutrophil infiltration admixed with epithelioid histiocytes and granulomatous inflammation
  • Cutaneous lesions demonstrate pseudoepitheliomatous hyperplasia of the epidermis
  • Uniformly sized, refractile round yeast cells may be observed at low power in H&E stained tissue sections
  • High power may show cell contents within the refractile cell wall but often this material washes away during processing
  • Periodic acid-Schiff (PAS) and Gomori methenamine silver (GMS) stains highlight organisms
  • Mucicarmine may be weakly positive; contrast with a strongly positive Cryptococcus
  • Fontana-Masson stain for melanin: negative
  • Reference: Semin Respir Crit Care Med 2020;41:31
Microscopic (histologic) images

Contributed by Sixto M. Leal, Jr., M.D., Ph.D.
Lollipop-like aleurioconidia

Lollipop-like aleurioconidia

Multinucleated giant cell with yeasts

Multinucleated giant cell with yeasts

GMS stain of yeasts

GMS stain of yeasts

PAS stain of yeasts

PAS stain of yeasts

Positive stains
Molecular / cytogenetics description
  • Nucleic acid probe hybridization assays are commercially available (AccuProbe by Hologic, Inc.) for definitive identification on culture
  • Additional lab developed PCR tests, sequencing and matrix assisted laser desorption / ionization time of flight (MALDI-TOF) mass spectrometry (MS) may enable identification
Differential diagnosis
Board review style question #1
BRQ image


A 36 year old Caucasian woman from Tennessee develops flu-like symptoms with fever, headache, diffuse joint pain and cough. She currently takes ibuprofen, which does not alleviate symptoms. An Xray shows diffuse bilateral pneumonia. After 1 week at 30 °C, sputum cultures show unique colonies with central yeast-like creamy morphology and peripheral fuzzy mold-like extensions. At 2 weeks, the colonies are almost entirely mold-like and a tape prep shows thin hyaline septate hyphae with lollipop-like structures called aleurioconidia. No large tuberculate macroconidia are noted at 2 weeks but are spotted at 4 weeks. A molecular test is performed to confirm the diagnosis. What is the most likely cause of respiratory illness?

  1. Blastomyces dermatitidis
  2. Coccidioides immitis
  3. Histoplasma capsulatum
  4. Paracoccidiodes braziliensis
Board review style answer #1
C. Histoplasma capsulatum. Coccidiodes species exhibit alternating barrel shaped arthroconidia. Paracoccidiodes, Blastomyces and Histoplasma species cultured at 30 °C form lollipop-like aleurioconidia but only Histoplasma proceeds to make large tuberculate macroconidia in late cultures.

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Reference: Blastomyces
Board review style question #2
Which of the following is true for Blastomyces infection?

  1. Cutaneous lesions demonstrate pseudoepitheliomatous hyperplasia of the epidermis
  2. Direct exam shows large, thick walled, refractile yeast forms (8 - 10 μm) with narrow based budding
  3. Extrapulmonary dissemination does not occur in Blastomyces
  4. Late cultures show large tuberculate macroconidia
Board review style answer #2
A. Cutaneous lesions demonstrate pseudoepitheliomatous hyperplasia of the epidermis

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Reference: Blastomyces
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