Emergent insights on the spread of antifungal-resistant Trichophyton indotineae dermatophyte: Clonal expansion, adaptability dynamics and human-animal host adaptation.

Trichophyton indotineae is an emergent, clonally expanding, multidrug-resistant dermatophyte that emerged from the Indian subcontinent and has disseminated internationally, causing extensive, treatment-refractory tinea. Its phenotypic resemblance to Trichophyton mentagrophytes makes classical identification unreliable, so molecular and proteomic tools such as Internal Transcribed Spacer (ITS) with High Mobility Group (HMG) discrimination, updated MALDI-TOF and Squalene Epoxidase gene (SQLE) sequencing are essential. Resistance is driven by SQLE point mutations (notably Leu393Phe, Phe397Leu and Ala448Thr) that confer high terbinafine Minimum Inhibitory Concentrations (MICs) and have been detected in both human and animal isolates, indicating transmissible, stable resistant clones. The global expansion of Trichophyton indotineae is driven by clonal transmission of terbinafine-resistant strains harboring stable SQLE mutations, compounded by diagnostic ambiguity within the T. mentagrophytes complex and sustained antifungal selection pressure. Its demonstrated ability to persist in the environment and infect both human and animal hosts highlights the pathogen's adaptive capacity and underscores the need for accurate molecular identification and resistance-guided clinical management.