Question
Histoplasma capsulatum.
Answer
Histoplasma capsulatum is a dimorphic fungus (mould in the environment/at room temperature, small intracellular yeast at body temperature), the cause of histoplasmosis, endemic in soil enriched with bird or bat droppings (classically associated with caves, chicken coops, and bird roosts — “spelunker’s disease” for cave-associated exposure), particularly in the Mississippi/Ohio river valleys of the USA, though found in other regions worldwide including parts of India.
Transmission: inhalation of airborne microconidia (spores) from disturbed contaminated soil/droppings.
Clinical spectrum: most infections are asymptomatic or self-limiting (a mild, flu-like respiratory illness) in immunocompetent hosts, given effective cell-mediated immune containment (forming granulomas, sometimes with subsequent calcification visible on chest X-ray, mimicking healed tuberculosis); disseminated, progressive disease occurs predominantly in the immunocompromised (advanced HIV/AIDS, an AIDS-defining opportunistic infection in endemic areas), presenting with fever, weight loss, hepatosplenomegaly, and pancytopenia.
Key pathogenic feature: after inhalation, the organism converts to its yeast form and is phagocytosed by alveolar macrophages, within which it survives and multiplies as a facultative intracellular pathogen (unusual among fungi) — this intracellular yeast form, seen classically packed within macrophages on tissue/blood smear, is a key diagnostic and pathogenic hallmark.
Laboratory diagnosis: microscopy of bone marrow/blood/tissue biopsy showing small, oval, intracellular yeast forms within macrophages (Giemsa/Wright stain); culture on Sabouraud dextrose agar (slow-growing mould at 25°C with characteristic tuberculate macroconidia, converting to yeast at 37°C — demonstrating dimorphism); urine/serum antigen detection (Histoplasma polysaccharide antigen) — a rapid, sensitive test particularly useful in disseminated disease; serology (complement fixation, immunodiffusion) for less severe/localized disease.
Treatment: itraconazole for mild-moderate disease; amphotericin B for severe/disseminated disease, particularly in immunocompromised patients.