Histoplasmosis

Abstract

Abstract Histoplasmosis, the most common endemic mycosis in the United States, is caused by Histoplasma capsulatum var. capsulatum. Histoplasma capsulatum var. duboisii causes African histoplasmosis, which has different clinical manifestations. Histoplasma capsulatum is a thermally dimorphic fungus; in the environment and at temperatures below 35°C, it exists as a mould, and in tissues and at 35°–37°C, as a yeast. In the endemic area, along the Mississippi and Ohio River valleys in the United States, most persons are infected in childhood. The primary site of infection is the lungs following inhalation of the conidia from the environment. The severity of disease is related to the number of conidia inhaled and the immune response of the host; the primary host defense mechanism against H. capsulatum is cell-mediated immunity. Pulmonary infection is asymptomatic or only mildly symptomatic in most persons who have been infected; acute severe pneumonia and chronic progressive pulmonary infection also can occur. Asymptomatic dissemination of H. capsulatum to the organs of the reticuloendothelial system occurs in most infected individuals; however, symptomatic acute or chronic disseminated histoplasmosis, which is a life-threatening infection, occurs almost entirely in persons who have deficient cell-mediated immunity. Antifungal therapy is highly effective. For most patients with histoplasmosis, itraconazole, a triazole drug, is the treatment of choice. Histoplasmosis was first described and the organism given its name in 1904 by Samuel Darling, a physician working at the Canal Zone Hospital in Panama. He erroneously thought the organism, which in tissues resembled Leishmania, was a parasite. Within a few years it became clear that this organism was indeed a fungus. Several decades later it was shown that H. capsulatum was a thermally dimorphic fungus, and by 1949, an environmental reservoir for H. capsulatum had been proved by Emmons.