A Case of Kerion Celsi Caused by Trichophyton tonsurans, a Plate Culture of Which Showed Yellow-Green Fluorescence Under UVA Light

We herein report a case of kerion celsi of the scalp and tinea corporis due to Trichophyton tonsurans. A 17-year-old Japanese male high school student who practiced judo had alopecic patches with severe inflammation on the scalp. We performed a fungal culture and identified the causative fungus as T. tonsurans. A plate culture of T. tonsurans showed lemon-yellow colonies with yellow-green fluorescence under UVA light, which are typical findings for Microsporum canis. However, genetic analysis of the ribosomal RNA gene of the isolate facilitated differential diagnosis of T. tonsurans. In contrast to dermatophytosis due to other dermatophytes, the clinical features of infection caused by T. tonsurans, an anthropophilic dermatophyte, are initially not very apparent and, thus, are frequently overlooked. We herein present a case of a severe type of kerion celsi caused by T. tonsurans with a fluorescence pattern mimicking M. canis colonies under UVA light. We suspect that yellow pigment metabolites, such as riboflavin, which are fluorescent under UV when secreted into the culture medium, are the virulence factors for not only M. canis, but also T. tonsurans, as shown in the present case.