capsulatum

Histoplasma capsulatum is a dimorphic fungus that causes histoplasmosis in humans. In the environment it exists as mold and produces microconidia, often in soil enriched with bird or bat droppings. When inhaled into the lungs, the organism converts to a yeast phase at body temperature and can multiply within host macrophages. The fungus is geographically widespread, with endemic regions in the Americas, Africa, and parts of Asia; the Ohio and Mississippi River valleys in the United States are classic areas of historical emphasis.

Infection commonly results in asymptomatic or mild pulmonary illness. Clinical presentations range from acute pulmonary histoplasmosis, which resembles community-acquired pneumonia, to chronic cavitary disease in older individuals with underlying lung disease. Disseminated histoplasmosis can occur, particularly in immunocompromised patients, and may involve the reticuloendothelial system, causing multiorgan symptoms and fever. Immunologic control of the infection is a key determinant of disease severity.

Diagnosis relies on a combination of methods. Antigen detection in urine or serum is useful for acute and disseminated forms. Serology can support diagnosis but may be limited by timing and immune status. Cultures from respiratory or tissue samples can confirm the organism, while histopathology may reveal small intracellular yeasts within macrophages.

Treatment is tailored to severity. Mild or asymptomatic cases may resolve without antifungals. Moderate to severe disease typically uses itraconazole for extended periods, often 6 to 12 months. Severe acute pulmonary or disseminated disease is treated initially with amphotericin B (liposomal formulation) followed by itraconazole. Fluconazole or other azoles may be options in some cases. Prevention focuses on reducing exposure to contaminated soil in endemic areas; there is no widely available vaccine.