Onychomycosis causes
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
Causes
Dermatophytes are the fungi most commonly responsible for onychomycosis.
Two dermatophyte species, Trichophyton rubrum and Trichophyton interdigitale, cause the vast majority of onychomycosis cases worldwide.
Other related dermatophyte fungi that may be involved are Epidermophyton floccosum, Trichophyton violaceum, Microsporum gypseum, Trichophyton tonsurans, Trichophyton soudanense (considered by some to be an African variant of T. rubrum rather than a full-fledged separate species) and the cattle ringworm fungus Trichophyton verrucosum. A common outdated name that may still be reported by medical laboratories is Trichophyton mentagrophytes for T. interdigitale.
The name T. mentagrophytes is now restricted to the agent of favus skin infection of the mouse; though this fungus may be transmitted from mice and their danders to humans, it generally infects skin and not nails.
Other causal fungi include yeasts (5-17%), e.g., Candida, and non-dermatophytic moulds, in particular members of the mould genera Scytalidium (name recently changed to Neoscytalidium), Scopulariopsis, and Aspergillus.
Yeasts mainly cause fingernail onychomycosis in people whose hands are often submerged in water. Scytalidium mainly affects people in the tropics, though it persists if they later move to areas of temperate climate.
Other moulds mainly affect people over the age of 60, and their presence in the nail reflects a slight weakening in the nail's ability to defend itself against fungal invasion.