Question
Piedra
Answer
Piedra is a superficial fungal infection of the hair shaft, characterized by the formation of firm nodules along the hair, occurring in two distinct clinical forms caused by different, unrelated fungi.
White piedra — caused by Trichosporon species (e.g., Trichosporon ovoides, formerly T. beigelii), producing soft, white-to-tan, less firmly adherent nodules on the hair shaft, most commonly affecting facial/genital/axillary hair in temperate/subtropical climates; the causative organism can occasionally cause more serious invasive disease (disseminated trichosporonosis) in severely immunocompromised patients.
Black piedra — caused by Piedraia hortae, an ascomycete fungus, producing hard, dark brown-to-black, firmly adherent nodules on scalp hair, endemic to tropical regions (e.g., parts of South America, Southeast Asia).
Pathogenesis: both organisms colonize and grow along the external surface of the hair shaft cuticle, forming compact fungal nodules composed of tightly packed hyphae/spores cemented together — a purely superficial infection without invasion of the hair follicle or surrounding skin.
Laboratory diagnosis: direct microscopy (KOH mount) of affected hair, demonstrating the characteristic nodule structure — white piedra nodules show a loose meshwork of hyphae with arthroconidia/blastoconidia; black piedra nodules show a dense, compact mass of pigmented hyphae with asci containing ascospores. Culture on Sabouraud dextrose agar confirms the specific organism.
Treatment: shaving/cutting off affected hair is the simplest and most effective measure; topical antifungal agents can be used as an adjunct/alternative to prevent recurrence.