The causative pathogens of onychomycosis include dermatophytes, Candida, and non-dermatophytic molds.
Dermatophytes are the fungi most commonly responsible for onychomycosis in the temperate western countries; meanwhile, Candida and non-dermatophytic moulds are more frequently involved in the tropics and subtropics with a hot and humid climate.
''Trichophyton rubrum'' is the most common dermatophyte involved in onychomycosis. Other dermatophytes that may be involved are ''Trichophyton interdigitale'', ''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A common outdated name that may still be reported by medical laboratories is ''Trichophyton mentagrophytes'' for ''T. interdigitale''.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 causative pathogens include Candida and non-dermatophytic moulds, in particular members of the mould generation Scytalidium (name recently changed to Neoscytalidium), Scopulariopsis, and Aspergillus.
Candida mainly cause fingernail onychomycosis in people whose hands are often submerged in water. Scytalidiummainly affects people in the tropics, though it persists if they later move to areas of temperate climate.
Other molds more commonly affect people older than 60 years, and their presence in the nail reflects a slight weakening in the nail's ability to defend itself against fungal invasion.
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