Diagnosis and pathogenesis of dermatophyte infections.

Dissemination of dermatophyte fungi depends on direct or indirect contact between infected and uninfected hosts. The first step in the infection process is colonisation of the cornified surface of the stratum corneum. The fungal form which is the contagious and infectious particle is most likely to be the arthrospore or a hyphal fragment adhering to or contained within a corneocyte. It is the distinctive morphological forms of hyphae and arthrospores, and the relatively large size of dermatophytes in keratinous tissue, that provide the basis of a rapid diagnostic method using microscopy. Microscopy may also have to provide the definitive diagnosis of a dermatophyte infection, eg, in onychomycosis, where a significant proportion of the samples which are positive on microscopy fail to grow in culture. The sensitivity of microscopy can be enhanced using KOH that incorporates a variety of dyes, or a fluorescent brightener (eg, Calcofluor white or Blankophor BA). This type of agent can also be used to recognise fungal elements in biopsy sections. However, culture has certain benefits over microscopy; it is a more sensitive technique, since much more material can be examined and a definitive identity of the organism can be obtained.