Histoplasma, Blastomyces, Coccidioides, and Other Dimorphic Fungi Causing Systemic Mycoses*

Superficial fungal infections constitute some of the most common infectious conditions and include dermatophytosis (tinea corporis, tinea capitis, tinea pedis, and tinea unguium) and pityriasis versicolor, as well as rarer disorders like tinea nigra and black and white piedra. The etiologic agents of dermatophytosis are classified, along with some nonpathogenic relatives, in three genera: Trichophyton, Microsporum, and Epidermophyton. Dermatophytes are keratinophilic fungi that are capable of invading the keratinous tissues of living animals. They are grouped into three categories based on host preference and natural habitat: (i) anthropophilic species almost exclusively infect humans, with animals being rarely infected; (ii) geophilic species are soil-associated organisms that can occasionally cause infections in humans and other animals; and (iii) zoophilic species are essentially pathogens of nonhuman mammals, although animal-to-human transmission is not uncommon. This chapter describes the taxonomy, epidemiology, transmission, and clinical significance of dermatophytes. The methods of diagnosis of dermatophyte infections, including specimen collection, transport, and processing; the microscopic examination and culture of clinical specimens from suspected cases of dermatophytosis; and the conventional (phenotypic) and molecular methods of identification of the causative agents, are discussed in detail. Attention is also given to the clinical presentation, diagnosis, and identification of the causative agents for those superficial infections in which the causative fungi colonize the cornified layers of the epidermis or the suprafollicular portion of the hair (tinea versicolor, tinea nigra, black piedra, and white piedra).