Abstract
Background: An increased number of dogs with unresolved yeast otitis externa (OE) after miconazole treatment was observed at a tertiary practice.
Hypothesis/objectives: To evaluate miconazole susceptibility in Malassezia pachydermatis isolates from canine OE.
Animals: Eleven client-owned dogs (16 ears) with clinical and video otoscopic signs of OE and cytological evidence of yeast.
Materials and methods: Yeast cultures were derived from ear swabs. PCR amplified portions of the internal transcribed spacer region (ITS), and partial sequences encoding the 18S rRNA small subunit (SSU), 26S rRNA large subunit (LSU), and actin (ACT1) loci for phylogenetic analysis, as well as the ERG11 gene that encodes lanosterol 14α-demethylase. Miconazole susceptibility was assessed by disk diffusion on Mueller-Hinton agar. Molecular modelling was used to evaluate the relationship between lanosterol 14α-demethylase amino acid substitutions and miconazole susceptibility.
Results: Isolates of M. pachydermatis from canine OE formed a three-clade phylogeny. Most ERG11 mutations predicted lanosterol 14α-demethylase substitutions that were clade-specific without an effect on miconazole susceptibility. Reduced susceptibility was associated with substitutions of amino acid A302, located in the lanosterol 14α-demethylase azole-binding site. Other altered amino acids were located elsewhere in the lanosterol 14α-demethylase protein without an apparent effect on miconazole binding.
Conclusions and clinical relevance: Clinical isolates of M. pachydermatis with reduced miconazole susceptibility were easily found, suggesting that miconazole resistance is present in routine patients. Lanosterol 14α-demethylase interactions with longer-tailed azoles involve additional contacts beyond A302 suggesting these drugs will be more effective against miconazole-resistant strains. However, effective antifungal stewardship is needed to slow development of pan-azole resistance that would limit treatment options for dogs with M. pachydermatis-associated disease.