High multi-azole-resistant Malassezia pachydermatis clinical isolates from canine Malassezia dermatitis

Malassezia pachydermatis, a lipophilic and aerobic yeast, is a causative agent of Malassezia dermatitis, a common skin mycosis in dogs and cats. This fungus is also responsible for zoonotic fungal infections in human neonates. Ravuconazole (RVZ) is an antifungal azole compound and the active metabolite of fosravuconazole, which was approved for use in humans in Japan in 2018. In the present study, in vitro RVZ susceptibility and multi-azole resistance of 13 clinical M. pachydermatis strains was investigated using the modified Clinical and Laboratory Standards Institute M27-A3 test. The minimum inhibitory concentrations (MICs) for the 13 isolates ranged from 0.094 to >32 mg/L for itraconazole (ITZ) and from 0.5 to >32 mg/l for RVZ. Similarly, MICs for ITZ- or RVZ-resistant strains (MICs >32 mg/l) were also >32 mg/l for clotrimazole (CTZ), >32 mg/l for miconazole (MCZ), and 0.25 to >32 mg/L for voriconazole (VRZ). BLAST analysis using the NCBI database showed that ERG11 cDNA of the RVZ-resistant strain encoded Gly at codon 461 and Asp in cytochrome p450 encoded by M. pachydermatis ERG11 mRNA. This work is the first report to describe that an RVZ-resistant M. pachydermatis strain contains ERG11 mutations. The affinity of the protein encoded by ERG11 for RVZ may differ from that of ITZ. Therefore, RVZ has considerable therapeutic potential for treating ITZ-resistant canine Malassezia dermatitis. However, RVZ-resistant strains already exist in canine Malassezia dermatitis in Japan.