The Efficacy of a Combination of Selected Azole Antifungals and Plant Essential Oil Components Against Malassezia pachydermatis

Infections caused by <i>Malassezia</i> (<i>M.</i>) <i>pachydermatis</i> in dogs are mostly treated with azole antifungals. Excessive use of these drugs is usually associated with an increased incidence of resistant isolates, which can be prevented by combining commonly used antifungals with natural bioactive compounds. The present study aimed at testing the effectiveness of a combination of selected azole derivatives showing low antifungal activity against <i>M. pachydermatis</i> isolates, with plant essential oil components displaying the highest efficacy. Among the four azole antifungals tested (itraconazole, posaconazole, clotrimazole, and miconazole), clotrimazole (a mean MIC of 7.62 μg/mL at 72 h and 7.24 μg/mL at 96 h) and miconazole (a mean MIC of 1.71 μg/mL at 72 h and 2.33 μg/mL at 96 h) exhibited the lowest antifungal efficacy. Out of the four plant essential oil components tested (eugenol, terpinene-4-ol, geraniol, and limonene), eugenol (an average MIC of 378.57 μg/mL at 72 h and 1180 μg/mL at 92 h) showed the highest antifungal activity. The checkerboard method was used to assess the interaction of these agents. The fractional inhibitory concentration index (FICI) values for the combination of clotrimazole with eugenol reached 1.43 at 72 h and 0.70 at 96 h and for the combination of miconazole with eugenol, 1.30 at 72 h and 0.45 at 96 h. A higher effect of the combinations was recorded at 96 h, when the combination of clotrimazole with eugenol showed an additive effect in 66.67% of the isolates, and the combination of miconazole and eugenol brought a synergistic effect in 57.14% of the isolates. The obtained results indicate that eugenol is a suitable agent for enhancing the efficacy of poor azoles against <i>M. pachydermatis</i>.