Wood’s Light, Immunology and Serology With Fungal Culture
Hair infected with M. audounii, M.canis and T. schoenleinii green fluoresce under Wood’s light, a source of a long-wave ultraviolet light. Wood’s light can be used to assist clinical diagnosis and to select suitable scalp material for laboratory investigation.
It also enables selection of the best part of the hair for culture and direct examination. Care must be taken to differentiate between true fungal fluorescence (bright green) and the auto fluorescence of keratin (dull blue) or the fluorescence of creams and ointments that may have been applied to the lesion.
Immunology and Serology
The immunological aspects of “ringworm” are incompletely understood. It is clear that a primary infection produces partial local immunity to reinfection but this protection varies in duration and extent depending on the host, the site of infection and the species of Dermatophytes. Cutaneous hypersensitivity (immediate and/or delayed) may occur and circulating antibodies have been detected in infected individuals but neither phenomenon has been shown to be of any diagnostic value.
Fungal Culture
Dermatophytes develop well on culture media containing an organic source of nitrogen. Those commonly used for isolations are 4% malt extract and Saboraud’s dextrose agar. It is usual to add chloramphenicol to these media and reduce bacterial growth. Inoculation of an adequate number (>10) of small (<1 mm) fragments of the specimen should be made. Although many dermatophytes may develop recognizable colonies with in 5-7 days, cultures should be retained for at least 3 weeks at 25-300C and longer at lower temperatures before making a final diagnosis.
Either Petri dish or test tube culture is satisfactory and there is little risk of laboratory infection.
Dermatophyte isolates can usually be distinguished from contaminants by the occurrence of compact growth around the inocula and the color of the colony Dermatophytes are never green, blue or black.
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