Dermatophytes are a unique group of fungi that are capable of infecting nonviable keratinized cutaneous epithelium including stratum corneum, nails, and hair. It can be further specified according to the tissue mainly involved: epidermomycosis (epidermal dermatophytosis), trichomycosis (dermatophytosis of hair and hair follicles), or onychomycosis (dermatophytosis of the nail apparatus).
Dermatophytes synthesize keratinases that digest keratin and sustain existence of fungi in keratinized structures. Cell-mediated immunity and antimicrobial activity of polymorphonuclear leukocytes restrict dermatophyte pathogenicity.
The clinical presentation of dermatophytoses depends on several factors: site of infection, immunologic response of the host, species of fungus. Dermatophytes (e.g., T. rubrum) that initiate little inflammatory response are better able to establish chronic infection. Organisms such as M.canis cause an acute infection associated with a brisk inflammatory response and spontaneous resolution. In some individuals, infection can involve the dermis, as in kerion and Majocchi’s granuloma.