Histoplasma capsulatum, a thermal dimorphic fungus, is the etiologic agent of histoplasmosis. In most endemic areas, H. capsulatum var. capsulatum is the causative agent. In Central and South America, histoplasmosis is common and is caused by genetically different clades of H. capsulatum var. capsulatum. In Africa, H. capsulatum var. duboisii is also found. Yeasts of var. duboisii are larger than those of var. capsulatum.
Mycelia—the naturally infectious form of Histoplasma—have a characteristic appearance, with microconidial and macroconidial forms (Fig. 207-1). Microconidia are oval and are small enough (2–4 μm) to reach the terminal bronchioles and alveoli. Shortly after infecting the host, mycelia transform into the yeasts that are found inside macrophages and other phagocytes. The yeast forms are characteristically small (2–5 μm), with occasional narrow budding (Fig. 207-2). In the laboratory, mycelia are best grown at room temperature, whereas yeasts are grown at 37°C on enriched media.
Spiked spherical conidia of H. capsulatum (lacto-phenol cotton blue stain).
A. Small (2–5 μm) narrow budding yeasts of H. capsulatum from bronchoalveolar lavage fluid (Grocott’s methenamine silver stain). B. Intracellular yeasts of H. capsulatum within an alveolar macrophage from a patient with AIDS and disseminated histoplasmosis (Giemsa stain).
Histoplasmosis is the most prevalent endemic mycosis in North America. Although this fungal disease has been reported throughout the world, its endemicity is particularly notable in the Ohio and Mississippi river valleys of North America and in certain parts of Central and South America, Africa, and Asia. In Europe, histoplasmosis is diagnosed fairly often, mostly in emigrants from or travelers to endemic areas on other continents. The geographic distribution of histoplasmosis is related to the humid and acidic nature of the soil in the endemic areas. Soil enriched with bird or bat droppings promotes the growth and sporulation of Histoplasma. Disruption of soil containing the organism leads to aerosolization of the microconidia and exposure of humans nearby. Activities associated with high-level exposure include spelunking, excavation, cleaning of chicken coops, demolition and remodeling of old buildings, and cutting of dead trees. Most cases seen outside of highly endemic areas represent imported disease—e.g., cases reported in Europe after travel to the Americas, Africa, or Asia. The epidemiology of histoplasmosis is changing with the continued expansion of at-risk populations and the acceleration of intercontinental and international travel that brings this infection to areas of the world that are not known to be endemic. The population at risk for histoplasmosis continues to grow as a result of increasing numbers of patients receiving immunosuppressive therapies for autoimmune disorders, cancers, and organ transplants.
PATHOGENESIS AND PATHOLOGY
Infection follows inhalation of microconidia (...