National Belgian Study on Terbinafine Resistance in Trichophyton interdigitale/mentagrophytes/indotineae (2022–2023): Epidemiology and Molecular Features

<b>Objectives</b>: <i>Trichophyton indotineae</i>, a dermatophyte closely related to <i>T. interdigitale</i> and <i>T. mentagrophytes</i>, is of growing concern due to its high terbinafine resistance and widespread presence in India. Its emergence in Europe calls for enhanced surveillance. Resistance is linked to mutations in the squalene epoxidase (<i>SQLE</i>) gene. This multicentric national study aimed to evaluate the prevalence, terbinafine susceptibility, and phylogenomics of <i>T. interdigitale/mentagrophytes/indotineae</i> strains in Belgium, with a focus on SQLE substitutions. <b>Methods</b>: Between February 2022 and April 2023, 137 isolates from 16 Belgian labs were analyzed for antifungal susceptibility using the EUCAST E.Def.11.0 method. Whole-genome sequencing (WGS) was performed via Illumina sequencing method. <b>Results:</b> Phylogenomic analysis identified 8 <i>T. indotineae</i>, 91 <i>T. interdigitale</i>, and 38 <i>T. mentagrophytes</i> (including 7 genotype VII strains). Terbinafine resistance (5.1%) was mainly found in <i>T. indotineae</i> (87.5%), always linked to SQLE substitutions. <i>T. interdigitale</i> was fully susceptible. <i>T. mentagrophytes</i> showed mildly elevated MICs, often associated with K276N substitution. <b>Conclusions:</b> Terbinafine-resistant <i>T. indotineae</i> is emerging in Belgium, mostly via imported cases. Continued molecular surveillance and species-specific treatment strategies are essential.