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Pathophysiology
Clinical meaning
Blastomycosis is a systemic fungal infection caused by the dimorphic fungus Blastomyces dermatitidis (and the recently reclassified Blastomyces gilchristii), endemic to the Ohio and Mississippi River valleys, the Great Lakes region, and the St. Lawrence River valley in North America. Understanding the unique biology of this dimorphic fungus and the host immune response is essential for the nurse to recognize the diverse clinical presentations and implement appropriate care.
Blastomyces dermatitidis exists in two distinct morphological forms depending on temperature, a property called thermal dimorphism shared with other endemic mycoses (histoplasmosis, coccidioidomycosis). In the environment (soil, decaying organic matter near waterways at temperatures of 25 to 30 degrees C), the organism grows as a mold (mycelial form), producing hyphae with terminal conidia (asexual spores 2 to 10 micrometers in diameter). When soil is disturbed by activities such as excavation, hunting, camping near waterways, or agricultural work, these conidia become aerosolized and are inhaled into the lungs. Upon reaching the warm environment of the host pulmonary alveoli (37 degrees C), the conidia undergo a critical morphological transformation over 48 to 72 hours, converting to the yeast form: large (8 to 15 micrometers, sometimes up to 30 micrometers), thick-walled yeast cells that reproduce by broad-based budding (the bud has a wide attachment to the parent cell, which is the pathognomonic microscopic finding distinguishing Blastomyces from other yeasts).
The yeast form is the pathogenic form. Its large cell size prevents effective phagocytosis by alveolar macrophages, and its thick cell wall (composed of alpha-1,3-glucan, chitin, and the immunomodulatory surface glycoprotein BAD1, formerly WI-1) resists intracellular killing mechanisms. BAD1 (Blastomyces adhesin 1) is the principal virulence factor: it mediates adhesion to host tissues, suppresses TNF-alpha production by macrophages (undermining the inflammatory response needed for fungal containment), and inhibits complement-mediated opsonization.
The initial host defense depends on the innate immune system. When conidia are inhaled, alveolar macrophages and neutrophils are recruited to the site of infection. Neutrophils can kill conidia through oxidative burst mechanisms (myeloperoxidase, reactive oxygen species), but the yeast form is more resistant to neutrophil killing. Macrophages ingest the conidia but are unable to kill the yeast form effectively without T-cell-mediated activation. Dendritic cells process fungal antigens and migrate to regional lymph nodes, presenting antigens to naive T cells and initiating the adaptive immune response.
The adaptive immune response is critical for disease containment. CD4+ Th1 cells produce interferon-gamma (IFN-gamma) and TNF-alpha, which activate macrophages to enhance their fungicidal activity through increased production of nitric oxide, reactive oxygen intermediates, and phagolysosomal fusion. Activated macrophages can then kill intracellular yeast cells. The formation of well-organized granulomas (composed of epithelioid macrophages, multinucleated giant cells, and a surrounding rim of T lymphocytes) is the hallmark of effective immune containment. These granulomas wall off the organisms and prevent dissemination. When T-cell immunity is deficient (HIV/AIDS with CD4 count less than 200, organ transplant recipients on immunosuppression, TNF-alpha inhibitor therapy), the granulomatous response fails, and Blastomyces disseminates widely, causing fulminant and often fatal disease.
Clinically, blastomycosis has four major presentations. Pulmonary blastomycosis (60 to 90% of cases) presents as an acute pneumonia (fever, productive cough, chest pain, myalgias mimicking community-acquired bacterial pneumonia) or chronic pneumonia (cough, low-grade fever, weight loss, night sweats mimicking tuberculosis or lung cancer). Chest imaging shows lobar or multilobar consolidation, mass-like lesions (often mistaken for lung cancer), miliary patterns, or cavitary disease. Cutaneous blastomycosis (40 to 80% of disseminated cases) presents with verrucous (wart-like) lesions with raised, irregular borders and central ulceration, or ulcerative lesions with sharp undermined borders draining purulent material. These skin lesions are often painless and may be mistaken for skin cancer, tuberculosis, or pyoderma gangrenosum. Osteoarticular blastomycosis (10 to 25% of disseminated cases) affects long bones, vertebrae, and ribs, presenting as osteomyelitis with pain, swelling, and draining sinuses. Genitourinary blastomycosis (10 to 30% of male disseminated cases) presents as prostatitis or epididymoorchitis.
Diagnosis is confirmed by visualizing the characteristic broad-based budding yeast on microscopy of clinical specimens (sputum, BAL fluid, wound drainage, tissue biopsy) using KOH preparation, calcofluor white staining, or histopathology with GMS (Gomori methenamine silver) or PAS (periodic acid-Schiff) stains. Culture on Sabouraud dextrose agar grows mold colonies in 1 to 4 weeks; definitive identification requires DNA probe or MALDI-TOF mass spectrometry. The Blastomyces urine antigen test (enzyme immunoassay for galactomannan) has 90% sensitivity in disseminated disease and 76% in pulmonary disease, but cross-reacts with Histoplasma (both produce galactomannan).
Treatment depends on disease severity. Mild to moderate pulmonary disease: itraconazole 200 mg three times daily for 3 days (loading dose), then 200 mg twice daily for 6 to 12 months. Moderate to severe pulmonary disease or any disseminated disease: IV amphotericin B lipid formulation (3 to 5 mg/kg/day) for 1 to 2 weeks or until clinical improvement, followed by step-down to itraconazole 200 mg twice daily to complete 12 months total. CNS blastomycosis: high-dose IV amphotericin B (5 mg/kg/day) for 4 to 6 weeks followed by oral fluconazole or voriconazole (which penetrate the CNS better than itraconazole) for at least 12 months.
Exam Focus
Exam relevance
Risk factors:
- Residence in or travel to endemic areas: Ohio and Mississippi River valleys, Great Lakes region, St. Lawrence River valley
- Outdoor occupational or recreational activities: hunting, fishing, camping, forestry work, farming, construction near waterways where soil contains the organism
- Soil disruption activities: excavation, demolition, digging near water, handling decaying organic matter in endemic areas
- Immunosuppression: HIV/AIDS (CD4 less than 200), organ transplant recipients, TNF-alpha inhibitor therapy, chronic corticosteroid use (risk of disseminated and fatal disease)
- Male sex (2 to 10 fold higher incidence in males, possibly due to greater outdoor occupational exposure)
- Dog exposure (dogs develop clinical blastomycosis from shared environmental exposure, serving as a sentinel for human risk; dogs do NOT transmit to humans)
- Middle-aged adults (peak incidence ages 30 to 60 years)
Diagnostics:
- Microscopy of clinical specimens (sputum, BAL fluid, wound drainage, tissue biopsy): KOH wet mount or calcofluor white staining revealing large (8 to 15 mcm) thick-walled yeast with broad-based budding (bud attachment is as wide as the parent cell - pathognomonic)
- Fungal culture: growth on Sabouraud dextrose agar at 25 C produces white mold colonies in 1 to 4 weeks; conversion to yeast form at 37 C confirms dimorphism; DNA probe for definitive species identification
- Histopathology (tissue biopsy with GMS or PAS stains): mixed pyogranulomatous inflammation with neutrophils and epithelioid granulomas containing large broad-based budding yeast forms
- Blastomyces urine antigen (enzyme immunoassay): sensitivity 90% in disseminated disease, 76% in isolated pulmonary disease; cross-reacts with Histoplasma antigen (both organisms produce galactomannan); useful for diagnosis and monitoring treatment response
- Chest imaging (X-ray and CT): lobar consolidation, mass lesions mimicking lung cancer, miliary pattern, cavitary disease, or mediastinal lymphadenopathy
- Itraconazole trough level monitoring: therapeutic range 1 to 5 mcg/mL; check after 2 weeks of therapy; subtherapeutic levels are a common cause of treatment failure
Core concept
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Clinical scenario
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