CHAPTER 150 Fungal Skin Diseases
Fungal diseases are commonly subdivided into groups of superficial, subcutaneous, and systemic conditions. This chapter focuses primarily on superficial and subcutaneous fungal infections.
SUPERFICIAL FUNGAL INFECTIONS OF THE SKIN
Dermatophytosis
Diagnosis
Dermatophytosis is diagnosed on the basis of clinical signs coupled with positive results of fungal culture. Woods lamp evaluation is not recommended as a diagnostic tool because only a few strains of M. canis elicit fluorescence. To complicate assessment even further, topical therapy may cause false-positive reactions. For fungal cultures, hair is collected at the periphery of the lesion and placed on a dermatophyte test medium (DTM) plate. This medium is essentially Sabouraud’s dextrose agar containing cycloheximide, gentamicin, and chlortetracycline, with phenol red added as a color indicator. To encourage growth of T. equinum var. equinum, it is important to add vitamin B complex to the medium. This can be done by adding a few drops of injectable vitamin B complex to the medium. To minimize growth of saprophytes, the area should be pretreated with alcohol. It is important to wait until the area is completely dry before plucking hairs for culture to avoid false-negative culture results. Growth of either saprophytic fungi or dermatophytes can change the color of DTM medium to red. Dermatophytes usually grow in 3 to 5 days and utilize protein first. The resulting alkaline metabolites turn the medium to a red color in several days, concomitantly with colony growth. By contrast, saprophytic fungi utilize carbohydrates first and protein later and thus the change in color of the medium develops only after prolonged incubation (10 to 14 days). Plates should be checked frequently for fungal growth and color change. Inappropriate and frequent diagnosis of dermatophytosis results if the plate is not assessed daily and diagnosis is made solely on the basis of color change of the medium.
