histoplasmosis

Histoplasma capsulatum exists as a mold in the environment (25°C, associated with soil enriched by bird/bat droppings—classic exposure includes caves, chicken coops, and construction/demolition sites) and converts to a yeast form at body temperature (37°C) inside host macrophages, exhibiting thermal dimorphism. It is endemic to the Ohio and Mississippi River valleys in the US, as well as parts of Central and South America, Africa, and Asia.

Pathophysiology: Microconidia are inhaled into the lungs, phagocytosed by alveolar macrophages, and survive intracellularly by inhibiting phagolysosome fusion and altering phagosomal pH. Cell-mediated immunity (Th1 response, IFN-gamma activation of macrophages) is critical for containment; granuloma formation with central caseous necrosis occurs, often with later calcification (can mimic TB or malignancy on imaging).

Clinical presentations: - Acute pulmonary histoplasmosis: Often asymptomatic (90%) or presents with flu-like illness, cough, fever, in patients with heavy exposure. - Chronic pulmonary histoplasmosis: Resembles TB, occurs in patients with underlying lung disease (e.g., COPD), presents with cavitary lesions, chronic cough, weight loss. - Disseminated histoplasmosis: Seen in immunocompromised hosts (HIV/AIDS with CD4 <150, transplant patients, infants). Presents with hepatosplenomegaly, pancytopenia, fever, mucocutaneous lesions, and can be rapidly fatal if untreated. - Mediastinal complications: Mediastinal granuloma, fibrosing mediastinitis (rare, can cause SVC syndrome).

Diagnosis: Urine/serum Histoplasma antigen (most sensitive for disseminated disease), fungal culture (gold standard but slow), histopathology showing small intracellular budding yeasts within macrophages (2-4 micrometers), serology (complement fixation, immunodiffusion).

Key distinguishing feature from other dimorphic fungi: Small yeast forms found INSIDE macrophages (compare to Blastomyces - broad-based budding, Coccidioides - spherules with endospores, Paracoccidioides - captain's wheel appearance).

Treatment: Mild/moderate disease often self-resolves or treated with itraconazole. Severe/disseminated disease requires amphotericin B followed by itraconazole. Immunocompromised patients may need long-term suppressive therapy.

Board relevance: Classic association with spelunking (cave exploration), bird/bat droppings, Ohio/Mississippi River valleys, and AIDS-defining illness when disseminated. Often tested in differentiating from TB (both can cause cavitary lung disease and granulomas) and other endemic mycoses.