<i>In vitro</i> susceptibility testing of essential oils and their main components against antifungal-resistant <i>Malassezia pachydermatis</i>

Canine Malassezia dermatitis (CMD) and otitis externa are generally treated by antifungal drugs. However, azole-resistant strains have been isolated from canine skin and ear canals worldwide. Phytochemicals isolated from essential oils are effective alternatives for inhibiting Malassezia pachydermatis. To evaluate the usefulness of phytochemicals against azole-resistant isolates, we performed in vitro susceptibility testing using the phytochemicals carvacrol, citral, and thymol. Eight antifungal-resistant isolates were obtained from 7 cases of dermatitis and 1 case of otitis externa during 2022 and 2023 from dogs in Tokyo and Kanagawa, Japan. Fungal susceptibility to carvacrol, citral, and thymol were assessed using the modified broth microdilution method. The minimum inhibitory concentrations (MICs) of the phytochemicals in all isolates were as follows: 0.03 to 0.125% for carvacrol; 0.03 to 0.125% for thymol; and 0.03% to 0.125% for citral. Based on these results, carvacrol, citral, and thymol appear to be effective against azole-resistant strains. The phytochemicals appear to be effective for treating antifungal-resistant cases of CMD and otitis externa.