ABSTRACT
H. capsulatum is thermally dimorphic and can survive at two different temperatures as a saprophytic mycelial mold and as a parasitic yeast. Approximately 250#000 individuals are infected annually with H. capsulatum in the US and 500#000 worldwide. H. capsulatum is found in damp soil rich in bird droppings or excrement of bats.
Inhaled mycelial fragments and microconidia of H. capsulatum are ingested by lung phagocytes, where they reside and replicate. In response, macrophages enhance oxidative burst and the release of nitrogen intermediates. Recruited PMNs release fungistatic defensins. T cells contribute to protective immunity via production of cytokines.
In about 80–90% of infected immunocompetent individuals histoplasmosis is asymptomatic and subclinical. In a small group of the infected individuals (5–10%), mostly immunocompromised, young children and elderly, histoplasmosis presents as chronic progressive lung disease, chronic cutaneous or systemic disease or an acute fatal systemic disease. Presumed ocular histoplasmosis syndrome is only found in 1–10% of individuals who live in endemic areas.
A social and occupational history is important for the initial evaluation. The methods of diagnosis include radiography, microscopy, and ELISA. Differential diagnosis needs to be carried out with other dimorphic fungi, Aspergillus and Sporothrix species.
In most cases, infection with Histoplasma species of immunocompetent individuals is self-limiting and does not require therapy. Severe cases of acute histoplasmosis and all cases of chronic and disseminated disease require treatment with antifungal drugs itraconazole and amphotericin-B.
