Histoplasma, Blastomyces, Coccidioides, and other dimorphic fungi causing systemic mycoses.

This chapter covers the dimorphic members of the families Onygenaceae and Ajellomycetaceae, which include Blastomyces dermatitidis, Histoplasma capsulatum, Paracoccidioides brasiliensis, and Coccidioides immitis and C. posadasii as well as Emmonsia species. B. dermatitidis is thermally dimorphic, converting from the mold phase to the yeast phase under appropriate conditions of temperature and nutrition. In the environment and in culture at room temperature, the Coccidioides fungus exists as a mold producing septate hyphae and arthroconidia that usually develop in alternate hyphal cells. Inhalation of conidia is the usual mode of infection leading to blastomycosis. The incubation period has been estimated to be 4 to 6 weeks. Inhalation of arthroconidia is the usual mode of infection leading to coccidioidomycosis in humans. The incubation period is 1 to 3 weeks. Paracoccidioidomycosis predominates in adults, who display 85% to 95% of cases, and in persons in agriculture-related occupations. The disease is more often diagnosed in males than in females. There is a wide spectrum of clinical manifestations of histoplasmosis, ranging from a transient pulmonary infection that subsides without treatment to chronic pulmonary infection or to more widespread disseminated disease. Tissue sections should be stained with periodic acid-Schiff, Grocott-Gomori methenamine silver, or hematoxylin and eosin to permit the detection of the characteristic large thick-walled spherules of Coccidioides species.