Blastomyces dermatitidis
Summary
Blastomyces dermatitidis is a thermally dimorphic fungus endemic to the Mississippi and Ohio River valleys and Great Lakes region, causing blastomycosis. It exists as a mold in soil (25°C) and yeast in tissue (37°C), and infection typically begins with inhalation of conidia leading to pulmonary disease that can disseminate to skin and bone.
Detail
Blastomyces dermatitidis is a dimorphic fungus found in soil and decaying organic matter, particularly in the central and eastern United States (Mississippi/Ohio River valleys, Great Lakes, and St. Lawrence River areas), as well as parts of Canada. Infection occurs via inhalation of microconidia from disturbed soil, especially near waterways or after activities like construction or excavation.
Pathophysiology: At 25°C in the environment, it exists as a mold with septate hyphae producing conidia. Upon inhalation and exposure to body temperature (37°C), it converts to a yeast form characterized by broad-based budding, thick refractile walls, and large size (8-15 μm)—a key diagnostic feature distinguishing it from other fungi like Cryptococcus (narrow-based budding) or Histoplasma (smaller yeast).
Clinical presentation: Primary pulmonary blastomycosis can range from asymptomatic to a flu-like illness with cough, fever, and chest pain, often mimicking bacterial pneumonia or tuberculosis. Chronic pulmonary disease may resemble TB with cavitary lesions. Dissemination occurs in a subset of patients, particularly immunocompromised individuals, affecting skin (verrucous or ulcerative lesions), bone (osteomyelitis), and genitourinary tract (prostatitis). Skin lesions often show pseudoepitheliomatous hyperplasia on biopsy, which can mimic squamous cell carcinoma.
Diagnosis: Definitive diagnosis relies on visualization of the characteristic broad-based budding yeast on KOH prep, cytology, or histopathology (using special stains like GMS or PAS). Culture confirms diagnosis but takes weeks. Urine/serum antigen testing is available but may cross-react with Histoplasma.
Treatment: Mild-to-moderate disease is treated with itraconazole; severe or CNS disease requires amphotericin B, often followed by itraconazole maintenance therapy.
Key associations for boards: Geographic distribution (Ohio/Mississippi River valleys), broad-based budding yeast, dimorphic fungus, granulomatous inflammation, and pseudoepitheliomatous hyperplasia of skin lesions that can be mistaken for carcinoma.
Sources
- First Aid for the USMLE Step 1
- Sabiston Textbook of Surgery
- Harrison's Principles of Internal Medicine
- Murray's Medical Microbiology
- CDC Fungal Diseases Reference
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