The Emerging Terbinafine-Resistant Trichophyton Epidemic: What Is the Role of Antifungal Susceptibility Testing?

Dermatophytosis is commonly encountered in the dermatological clinics. The main aetiological agents in dermatophytosis of skin and nails in humans are Trichophyton (T.) rubrum, T. mentagrophytes and T. interdigitale (former T. mentagrophytes-complex). Terbinafine therapy is usually effective in eradicating infections due to these species by inhibiting their squalene epoxidase (SQLE) enzyme, but increasing numbers of clinically resistant cases and mutations in the SQLE gene have been documented recently. Resistance to antimycotics is phenotypically determined by antifungal susceptibility testing (AFST). However, AFST is not routinely performed for dermatophytes and no breakpoints classifying isolates as susceptible or resistant are available, making it difficult to interpret the clinical impact of a minimal inhibitory concentration (MIC).

PubMed was systematically searched for terbinafine susceptibility testing of dermatophytes on October 20, 2020, by two individual researchers. The inclusion criteria were in vitro terbinafine susceptibility testing of Trichophyton (T.) rubrum, T. mentagrophytes and T. interdigitale with the broth microdilution technique. The exclusion criteria were non-English written papers. Outcomes were reported as MIC range, geometric mean, modal MIC and MIC50 and MIC90 in which 50 or 90% of isolates were inhibited, respectively. The reported MICs ranged from <0.001 to >64 mg/L. The huge variation in MIC is partly explained by the heterogeneity of the Trichophyton isolates, where some originated from routine specimens (wild types) whereas others came from non-responding patients with a known SQLE gene mutation. Another reason for the great variation in MIC is the use of different AFST methods where MIC values are not directly comparable. High MICs were reported particularly in isolates with SQLE gene mutation. The following SQLE alterations were reported: F397L, L393F, L393S, H440Y, F393I, F393V, F415I, F415S, F415V, S443P, A448T, L335F/A448T, S395P/A448T, L393S/A448T, Q408L/A448T, F397L/A448T, I121M/V237I and H440Y/F484Y in terbinafine-resistant isolates.