Bit · Micro
Dimorphic Fungi — Histoplasma, Blastomyces, Coccidioides, Paracoccidioides
Four endemic systemic fungi. All dimorphic. The pivot is geography + signature tissue form.
Mechanism
Dimorphic fungi exist as mold in the cold (environment) and yeast in the heat (37 °C, in tissue). All cause primary pulmonary infection that can disseminate in immunocompromised hosts. The pivot is geography + tissue morphology:
Differentiator Table
| Fungus | Geography | Tissue form | Distinctive features | Treatment |
|---|---|---|---|---|
| Histoplasma capsulatum | Mississippi & Ohio River valleys; bird/bat droppings | Small (2–4 μm) intracellular yeasts within macrophages | Pulmonary; can disseminate to RES (liver, spleen, BM); HIV patients with pancytopenia + hepatosplenomegaly | Itraconazole; amphotericin if severe |
| Blastomyces dermatitidis | Mississippi & Ohio River valleys + Great Lakes | Broad-based budding yeasts | Lung + skin (verrucous lesions) + bone | Itraconazole; amphotericin if severe |
| Coccidioides immitis | Southwest US (San Joaquin Valley, Arizona) | Spherules filled with endospores | Valley fever (flu-like + erythema nodosum + arthralgia); meningitis in disseminated | Fluconazole (meningitis); itraconazole; amphotericin |
| Paracoccidioides brasiliensis | Latin America (esp. Brazil, Argentina, Venezuela) | 'Captain's wheel' or 'mariner's wheel' (mother yeast with multiple peripheral buds) | Lung + mucocutaneous lesions; predominantly affects rural adult men | Itraconazole; sulfadiazine |
The Pivot
Two questions:
- Where is the patient from / where did they travel? Geography is half the diagnosis.
- What is the yeast form on biopsy? Small intracellular → Histo. Broad-based budding → Blasto. Spherules with endospores → Cocci. Captain's wheel → Paracocci.
NBME-Style Stem
A 34-year-old construction worker from Arizona develops fever, fatigue, cough, and arthralgias 3 weeks after starting a job clearing desert brush. Chest x-ray shows multiple nodular infiltrates. He also has tender erythematous nodules on his shins. Biopsy of a lymph node shows large spherules filled with endospores. Which of the following is the most likely organism?
Concept Anchor
Each endemic mycosis is mold in soil, yeast in tissue — geography narrows the differential to one fungus, and the tissue form (intracellular vs broad-based bud vs spherule vs captain's wheel) closes the diagnosis.