Histoplasma

Histoplasma is a genus of dimorphic fungi best known for causing histoplasmosis in humans. In the environment, Histoplasma exists as mold with mycelia that produce tuberculate macroconidia. In the human body, typically within the lungs, it shifts to a yeast phase that can survive and replicate inside macrophages. Histoplasma capsulatum is the most common species associated with human disease, though related taxa have been described.

Ecology and transmission: Histoplasma is found worldwide but is highly endemic in certain regions, particularly river valleys in North America. It thrives in soil enriched with bird or bat droppings. Transmission occurs primarily by inhalation of airborne microconidia or mold fragments; it is not transmitted from person to person. Most infections are asymptomatic or mild, but the organism can cause a range of disease manifestations.

Clinical spectrum: The disease ranges from asymptomatic infection to acute pulmonary histoplasmosis, which can resemble pneumonia or influenza. In some individuals, especially those with risk factors such as advanced age or chronic lung disease, chronic cavitary histoplasmosis may develop. Disseminated histoplasmosis, involving multiple organ systems, occurs mainly in immunocompromised patients (notably those with AIDS or on immunosuppressive therapy) and can be life-threatening if not treated.

Diagnosis and treatment: Diagnosis relies on fungal culture or histopathology, antigen detection (notably in urine and serum), serology, and imaging studies. Treatment depends on severity and form: mild cases may resolve without therapy; moderate to severe disease typically uses itraconazole for months to a year or longer, with amphotericin B used for severe disseminated cases, followed by itraconazole. Prevention focuses on minimizing exposure in endemic areas and using protective measures in high-risk environments such as caves or bird roosts.