Antifungal Resistance Patterns of Microsporum canis: A 27-Year MIC Study in Mainland China

Background: Microsporum canis, a dermatophyte commonly associated with pets, is a leading cause of severe tinea capitis. The increasing prevalence of antifungal resistance among dermatophytes poses a significant global health challenge.

Objectives: This study aims to define the updated antifungal susceptibility profile of M. canis to enhance treatment strategies for dermatophyte infections.

Methods: This study analysed 348 M. canis isolates from mainland China for their susceptibility to 11 antifungal agents, following the CLSI M38-A3 guidelines. Additionally, we investigated the susceptibility of M. canis to antifungal agents and analysed the correlation between in vitro drug susceptibility and clinical outcomes in 54 cases of tinea capitis.

Results: The majority of strains showed low MICs to all 11 drugs. We described the upper limits of wild-type (WT) minimal inhibitory concentrations (UL-WT) for 10 of these agents; however, no clear resistance patterns were identified through MIC distribution analysis. Notably, fluconazole had the highest MICs among the tested classes, while the novel agent olorofim showed superior activity. Resistance was detected in two strains to griseofulvin (MIC 64 μg/mL), one to fluconazole (MIC 64 μg/mL) and two to terbinafine (MIC 16 μg/mL). Although azoles and terbinafine remain effective against M. canis. Clinical outcomes indicate that terbinafine may be less effective in treating M. canis infections.

Conclusion: Establishing a clinical breakpoint for M. canis is urgently needed to improve treatment protocols. While azoles and griseofulvin are still recommended for M. canis infections, ongoing surveillance of dermatophyte species and their susceptibility to antifungal agents is crucial to guide treatment strategies.