Increasing Terbinafine Resistance in Danish Trichophyton Isolates 2019-2020

Terbinafine resistance in species has emerged and appears to be increasing. A new EUCAST susceptibility testing method and tentative ECOFFs were recently proposed for . Terbinafine resistance and target gene mutations were detected in 16 Danish isolates in 2013-2018. In this study, samples/isolates submitted for dermatophyte susceptibility testing 2019-2020 were examined. Species identification (ITS sequencing for species complex (SC) isolates), EUCAST MICs and squalene epoxidase (SQLE) profiles were obtained. Sixty-three isolates from 59 patients were included. accounted for 81% and SC for 19%. Approximately 60% of and SC isolates were terbinafine non-wildtype and/or had known/novel SQLE mutations with possible implications for terbinafine MICs. All infections with terbinafine-resistant SC isolates were caused by . Compared to 2013-2018, the number of patients with terbinafine-resistant isolates increased. For , this is partly explained by an increase in number of requests for susceptibility testing. Terbinafine-resistant was first detected in 2018, but accounted for 19% of resistance (4 of 21 patients) in 2020. In conclusion, terbinafine resistance is an emerging problem in Denmark. Population based studies are warranted and susceptibility testing is highly relevant in non-responding cases.