Infectious Disease · Fungal Infections
The facts most likely to be tested
Blastomycosis is endemic to the Ohio and Mississippi River valleys and the Great Lakes region.
The classic microscopic finding is broad-based budding yeast with a thick, double-refractive cell wall.
Patients frequently present with pulmonary symptoms that mimic tuberculosis or lung cancer, including cough, hemoptysis, and night sweats.
Cutaneous involvement is highly characteristic, manifesting as verrucous or ulcerative skin lesions with well-demarcated borders.
Bone involvement is a common site of disseminated disease, often presenting as lytic bone lesions.
Diagnosis is confirmed via sputum culture or tissue biopsy showing the pathognomonic yeast, or by urine antigen testing.
Treatment for severe or disseminated disease requires intravenous amphotericin B, while mild to moderate disease is treated with oral itraconazole.
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A 45-year-old male presents to the clinic with a 3-month history of productive cough, low-grade fevers, and weight loss. He lives in rural Wisconsin and works as a logger. Physical examination reveals a non-healing, verrucous ulcer on his left forearm with an indurated border. A chest radiograph demonstrates a dense consolidation in the right upper lobe. A biopsy of the skin lesion is performed.
What is the most likely microscopic finding on the biopsy specimen?
Broad-based budding yeast
The patient's geographic location, pulmonary symptoms, and characteristic verrucous skin lesion are classic for Blastomycosis, which is identified by its distinct broad-based budding yeast.
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Etiology / Epidemiology
Caused by Blastomyces dermatitidis; endemic to Ohio and Mississippi River valleys and Great Lakes region.
Clinical Manifestations
Presents as pneumonia or verrucous skin lesions; broad-based budding yeast is pathognomonic.
Diagnosis
Fungal culture is the gold standard; antigen testing (urine/serum) is highly sensitive.
Treatment
Itraconazole for mild-moderate disease; Amphotericin B for severe or CNS involvement.
Prognosis
High cure rate with treatment; respiratory failure is the primary cause of mortality.
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Epidemiology & Etiology
Infection follows inhalation of conidia from moist soil, decaying wood, or leaf litter. It is most common in middle-aged men with occupational or recreational exposure to endemic areas. Unlike other dimorphic fungi, it is not strictly associated with bird or bat guano.
Pertinent Anatomy
Primary infection occurs in the lungs via inhalation. Hematogenous dissemination frequently targets the skin, bones, and genitourinary tract.
Pathophysiology
The organism exists as a mold in the environment and converts to a yeast at body temperature. It evades host immunity via the BAD-1 surface protein, which inhibits phagocytosis. This leads to a pyogranulomatous inflammatory response in affected tissues.
Clinical Manifestations
Patients often present with chronic cough, fever, and night sweats mimicking tuberculosis. Skin involvement manifests as verrucous, ulcerated, or crusted lesions with irregular borders. Acute respiratory distress syndrome (ARDS) is a rare but life-threatening presentation.
Diagnosis
The fungal culture of sputum or tissue is the gold standard. Microscopic visualization of broad-based budding yeast in tissue biopsy is diagnostic. Urine antigen testing is useful for rapid screening but may cross-react with Histoplasma.
Treatment
Itraconazole is the first-line therapy for mild to moderate pulmonary or disseminated disease. Amphotericin B is reserved for severe, life-threatening infections or CNS involvement. Treatment duration is typically 6 to 12 months.
Prognosis
Most immunocompetent patients recover fully with appropriate antifungal therapy. Bone and joint involvement may require longer treatment courses. Relapse can occur, necessitating long-term monitoring.
Differential Diagnosis
Histoplasmosis: associated with bird/bat guano and narrow-based budding yeast
Coccidioidomycosis: associated with desert southwest and spherules
Tuberculosis: chronic cough/night sweats without fungal morphology
Squamous cell carcinoma: mimics verrucous skin lesions
Lung cancer: presents as a pulmonary mass/nodule