Antifungal Effects of Citrus maxima Cultivar Tubtim-Siam Peel Extract Against Malassezia pachydermatis Isolated from Dogs

Otitis externa in dogs is primarily caused by Malassezia pachydermatis. Treatment involves antifungal and antiseptic agents; however, resistance among causative organisms has been noted. Pomelo (Citrus maxima) is a source of bioactive compounds with antimicrobial activity. Its extract mainly includes essential oils, which are mostly applied for alternative treatment for M. pachydermatis. The study aimed to investigate the anti-M. pachydermatis effects of pomelo peel extracts and their potential use in topical solutions for canine infections. M. pachydermatis was isolated from dogs and confirmed with Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry (MALDI-TOF/MS). Antifungal susceptibility of M. pachydermatis to itraconazole was evaluated. Phytochemicals of essential oil and crude extract from C. maxima peel were determined using Gas Chromatograph–Mass Spectrometry (GC-MS/MS). In addition, the antifungal activity of the extracts was assessed using an agar plate dilution assay. The essential oil was formulated into a prototypic topical solution, and its effects on M. pachydermatis were observed in vitro. The prevalence of M. pachydermatis was 42%, with 53% having yeast on both ear sides. The minimum inhibitory concentrations (MIC) of itraconazole, essential oil, and crude extract to M. pachydermatis were 0.03–0.25 µg/mL, 1.0% v/v, and >200 mg/mL, respectively. The prominent phytochemicals in peel extracts were meranzin hydrate and D-limonene, identified in the crude extract and essential oil, respectively. Moreover, a topical solution containing essential oils inhibited M. pachydermatis growth and showed destructive effects on the yeast cell wall at higher concentrations. The essential oil exhibits antifungal activity against M. pachydermatis, primarily due to the high concentration of D-limonene. The growth was inhibited completely at MIC, observed over a 5-day period. Furthermore, the prototypic topical solution demonstrated an anti-M. pachydermatis effect. These findings suggest potential veterinary applications for pomelo peel extract, though further studies are necessary to assess stability, mechanism of action, and industrial suitability.