Effects of β-Thujaplicin on Anti-Malassezia pachydermatis Remedy for Canine Otitis Externa

Abstract

The antifungal activity of β-thujaplicin was evaluated against 51 Malassezia pachydermatis strains isolated from canine ear canals with or without otitis externa. For comparison, sensitivity tests were performed on M. pachydermatis isolates for nystatin, ketoconazole, and terbinafine HCl, all clinically available antifungal agents. The minimal inhibition concentrations over 50% of the tested isolates (MIC₅₀) were 3.13 μg/ml for β-thujaplicin and nystatin, 0.016 μg/ml for ketoconazole, and 1.56 μg/ml for terbinafine HCl. The antifungal effect for M. pachydermatis of β-thujaplicin compared favorably with commercial antifungal agents. None of the 51 M. pachydermatis isolates showed resistance against any of the tested antibiotics investigated in this study. Ten representative isolates of M. pachydermatis were subcultured for 30 generations at concentrations close to the MIC levels of β-thujaplicin, nystatin, ketoconazole, and terbinafine HCl, and examined to determine whether they had acquired resistance to each drug. As a result, M. pachydermatis was found to achieve resistance more easily for ketoconazole and terbinafine HCl than for β-thujaplicin or nystatin. The MIC₅₀ of β-thujaplicin did not change during the course of subculture, and it is thought that the potential development of a resistant strain is low, even with continuous infusion for otitis externa therapy. β-Thujaplicin is an inexpensive and safe treatment with anti-inflammatory and deodorant effects that can be recommended as an effective remedy for canine otitis externa.