Optimising the detection of Trichophyton indotineae and its prevalence in a large Australian laboratory

The aim of this study was to estimate the prevalence of Trichophyton indotineae, an emerging drug-resistant fungus, in specimens submitted for dermatophyte examination at a large laboratory in Melbourne, Australia, from January to June 2024, and to examine approaches for best laboratory practice for detection of this species. T. indotineae has not been previously isolated at our laboratory. We examined all skin and hair specimens (nail specimens were excluded) ​for dermatophyte presence by microscopy and culture; species identification was performed by routine phenotypic methods. Trichophyton interdigitale, Trichophyton mentagrophytes and Trichophyton isolates identified to genus level only, which were urease-negative or urease-indeterminate after 7 days, underwent internal transcribed spacer (ITS) region sequencing for definitive identification. A total of 202 Trichophyton isolates from 202 specimens (196 patients) were studied. The most common site of infection was the foot (tinea pedis, 39.3%) followed by, collectively, sites not specified (27.6%) ​and the groin (tinea cruris, 13.8%). Final identification revealed Trichophyton rubrum (n=128, 63.4%) as the most frequent species, followed by T. interdigitale (n=38, 18.8%) and T. indotineae (n=13, 6.4%). All T. indotineae isolates were initially phenotypically identified as T. interdigitale (n=7) or as Trichophyton species (n=6). T. indotineae caused tinea corporis (n=4, 30.8%), tinea cruris (n=3, 23.1%), tinea manuum (n=2, 15.4%), tinea pedis (n=1, 7.7%), tinea capitis (n=1, 7.7%) ​and tinea in an unspecified site (n=2, 15.4%). The estimated prevalence of T. indotineae was 0.6% [95% confidence interval (CI) 0.3-1.0]. T. indotineae cannot be identified using phenotypic methods, and identification requires ITS sequencing. Whilst apparently uncommon herein, further studies are warranted to more accurately determine the likelihood of encountering this important species in different populations.