blastomycosis

Pathophysiology: Blastomyces dermatitidis is a dimorphic fungus—existing as a mold in the environment (25°C) and converting to a thick-walled, broad-based budding yeast form at body temperature (37°C). Infection begins with inhalation of conidia, which convert to yeast form in the lungs, triggering a granulomatous immune response (macrophages, neutrophils, giant cells). Most infections are subclinical or self-limited; however, in immunocompromised hosts or with high inoculum exposure, progressive pulmonary or disseminated disease can occur.

Clinical features: Pulmonary blastomycosis presents with fever, cough, weight loss, and night sweats, mimicking tuberculosis or lung cancer (can form mass-like lesions). Chest imaging may show nodules, infiltrates, or cavitary lesions. Extrapulmonary dissemination (10-40% of symptomatic cases) commonly involves skin (verrucous or ulcerated lesions), bone (osteomyelitis), and genitourinary tract (prostatitis). CNS involvement is rare but serious.

Diagnosis: Definitive diagnosis via culture or histopathology showing broad-based budding yeast (differentiates from Cryptococcus, which has narrow-based budding and a capsule, and from Paracoccidioides, which has multiple narrow buds). Urine/serum antigen testing is useful but cross-reacts with Histoplasma. Serology and PCR are adjunctive.

Treatment: Mild-to-moderate disease: itraconazole. Severe or CNS disease: amphotericin B, followed by itraconazole maintenance. Treatment duration is typically 6-12 months.

Key associations for boards: Geographic distribution overlapping with Histoplasma (Ohio/Mississippi River valleys) but also Great Lakes and Canadian border states; exposure to soil, decaying wood, or river banks; broad-based budding yeast on KOH/histology; can mimic malignancy or TB on imaging.