ABSTRACT
The endemic dimorphic pathogenic fungi have two distinct morphological forms. These fungi include Histoplasma capsulatum, Blastomyces dermatitidis, Paracoccidioides brasiliensis, Coccidioides immitis, C. posadasii, Sporothrix schenckii, agents of chromoblastomycosis (mainly Fonsecaea pedrosoi, Phialophora verrucosa, Cladophialophora carrionii) and Penicillium marneffei
Saint James School of Medicine, Kralendjik, Bonaire (Dutch Caribbean), West Indies. Email: harish.gugnani@gmail.com, harishgugnani@yahoo.com
(Kwon-Chung and Bennett, 1992; de Hoog et al., 2000; Brandt and Warnock, 2007; DiSalvo, 2009). In culture and in the environment, they grow as fi laments with characteristic spores; while in the infected tissue they grow mostly as yeasts, and sometimes as other forms, viz. spherules containing endospores or septate bodies. The yeast form can also be produced in vitro by cultivating the fungi at 37ºC on a rich medium, e.g., brain-heart infusion agar or blood agar supplemented with 0.1% cysteine. Except for S. schenckii and agents of chromoblastomycosis, human infections due to these fungi occur initially in the lungs by inhalation of infectious spores or hyphal elements from an environmental source; later the infection disseminates to other internal organs. Infections due to S. schenckii and agents of chromoblastomycosis are initiated by traumatic implantation of the causative fungus from the environment into the skin/subcutaneous tissue (Kwon-Chung and Bennett, 1992), the fungi invading the dermis, subcutaneous tissue, muscle and facia. Dimorphic fungi are identifi ed in the laboratory by several morphological and sometimes also by biochemical characteristics. The morphological characters include the colonial characters, appearance and arrangement of their asexual spores, which may be large (macroconidia) or small (microconidia, arthroconidia) and the tissue form, which may be yeast, sometimes spherules or septate bodies. In vitro conversion to yeast from and exo-antigen tests are required where applicable, for confi rming the identity of the isolate. Salient clinical symptoms of human diseases caused by dimorphic fungi, the diagnostic characters of the mycelial and tissue forms of the causative agents are given in Table 15.1. The microscopic features of the culture and tissue forms of the dimorphic fungi are illustrated in Figures 15.1-15.15. Ecology of these fungi and the mechanism of spread of their infectious spores to humans are of great importance in understanding the epidemiology of their infections, and that has implications in public health (Ajello and Hay, 1998; Brandt and Warnock, 2007). This chapter attempts to present a concise update of our knowledge of the ecology of different species of dimorphic pathogenic fungi.
