Emergence of Trichophyton indotineae: in vitro susceptibility data from a mycology reference laboratory in Australia

Management of Trichophyton indotineae dermatophytosis is challenged by high terbinafine resistance rates within this species. Antifungal susceptibility data for Australian T. indotineae isolates are lacking. Here we determined the in vitro activity of five antifungal agents, including olorofim, against T. indotineae to guide therapeutic choices.

Forty-four clinical T. indotineae isolates (cultured from skin, 2005-2025) across several Australian jurisdictions were tested for susceptibility to itraconazole, voriconazole, posaconazole, terbinafine and olorofim using CLSI broth microdilution methodology. MIC endpoints were determined at 80% growth inhibition for azoles and terbinafine, and 100% inhibition for olorofim. Isolates with terbinafine MICs ≥0.5 mg/L were considered as non-WT or resistant.

Twenty-eight of 44 (63.6%) T. indotineae isolates had terbinafine MICs ≥0.5 mg/L [geometric mean (GM) MIC 0.570 mg/L; MIC₉₀ 4 mg/L] with 21 (47.7%) displaying MICs ≥4 mg/L. The modal terbinafine MIC was 4 mg/L. Lower MICs were observed for azoles, with posaconazole demonstrating the greatest activity, i.e. posaconazole (GM 0.058 mg/L; MIC₉₀ 0.25 mg/L), itraconazole (GM 0.087 mg/L; MIC₉₀ 0.5 mg/L) and voriconazole (GM 0.201 mg/L; MIC₉₀ 1 mg/L). Olorofim demonstrated good in vitro activity with all 44 isolates exhibiting MICs  ≤ 0.06 mg/L (GM 0.023 mg/L; MIC₉₀ 0.06 mg/L).

Terbinafine resistance in T. indotineae is common in Australia but not universal. Susceptibility testing can inform treatment choice. The azoles and olorofim demonstrate good in vitro activity.