This study investigated the antifungal activity of 5 essential oils (EOs) towards yeasts recovered from diseased fishes; and focused on the efficacy of one EO (carvacrol) on growth performance, non-specific immunity, and disease resistance of Nile tilapia Oreochromis niloticus against Cryptococcus uniguttulatus challenge. Thymoquinone, thymol, carvacrol, eugenol, and cinnamon were first tested in vitro against 20 clinical yeast strains in comparison with antifungal drugs (fluconazole, ketoconazole, itraconazole, amphotericin B, nystatin, and clotrimazole) using disc diffusion and broth microdilution methods. For the in vivo challenge, fish (n = 150) were divided into 5 groups (carvacrol prophylaxis, carvacrol treatment, itraconazole treatment, unchallenged control, and positive control; 30 fish group-1) with 3 replicates. Phagocytic activity, reactive oxygen species production, reactive nitrogen species production, myeloperoxidase, lysozyme activity, and total immunoglobulins were tested before and after challenge. Relative percent survival (RPS) and mortality percent were determined as indicators for functional immunity. EOs displayed divergent degrees of antifungal activity, and carvacrol was the most effective against the tested yeasts. The dietary additive of carvacrol significantly enhanced growth performance, all immunological parameters, and the RPS values (90%) compared to other treatments. This unique experimental model indicates that carvacrol seems promising not only for enhancing immunity and promoting fish growth, but also for controlling emerging fungal infections. Future studies should investigate different concentrations of carvacrol as well as its antifungal activity in different fish species.
Keywords: Antifungal activity; Disease resistance; Essential oils; Immunostimulant; Myeloperoxidase; Oreochromis niloticus; Phagocytosis; Reactive nitrogen species; Respiratory burst; Serum lysozyme.