Investigation of the growth performance, liver and intestinal histopathology, and immune response of juvenile gilthead seabream (Sparus aurata, L.1758) fed diets containing the leaf essential oil of niaouli (Melaleuca viridiflora)

This study was carried out to investigate the growth performance, liver and intestinal histopathology, and immune response of juvenile gilthead seabream (Sparus aurata, l.1758) fed diets containing the leaf essential oil of Niaouli (Melaleuca viridiflora). It involves two stages. The first stage of the study involves the investigation of the effect of Niaouli essential oil-added diet on the growth of fish through a feeding experiment, while the second stage of the study involves the observation of the effects of vibrio anguillarum vaccine on the blood parameters and liver and intestinal histopathology of the groups of gilt-head seabream fed diet containing Niaouli essential oil and the control groups fed diet consisting of unmixed feed pellets. The leaf essential oil of Niaouli was added to diets at four different levels (0%, 1%, 1,5% and 3%). The study consisted of 3 repetitions. A total of 600 seabream (mean initial body weight of each: 11,770±0,006g) were stocked randomly in 12 tanks (each containing 50 seabream) and fed for 105 days. At the end of the study, the feed conversion ratio (FCR), weight gain (WG) and specific growth rate (SGR) values of the treatment group fed diet containing 1% Niaouli essential oil were found statistically different and significant compared to the other groups. In terms of intestinal histopathology, no pathological difference was observed in any group. Similarly, pathological difference was not detected in the livers of control groups among both the feeding groups and vaccine groups. However, hepatocyte degeneration at varying levels, ranging from mild to severe, was observed in all the groups treated with diet containing the essential oil additive of Niaouli, and dose-dependent hepatocyte degeneration was revealed. It was observed that the majority of epithelial cells in the treatment group fed diet containing 1% Niaouli essential oil approached the normal pattern, while the cytoplasm of some hepatocytes had a granular pattern. The group fed diet containing 1% Niaouli essential oil revealed less severe degeneration when compared with the other groups. In the other stage of the study, the comparison of vaccinated and unvaccinated groups and the evaluation of their feeding parameters manifested the positive effects of the vaccine, reducing feed evaluation ratio, while increasing live weight gain and specific growth rate. Hematological values, on the other hand, revealed that the vaccine caused an increase in the number of erythrocytes, stimulated erythrocyte production in bone marrow, and generated erythrocytosis. In conclusion, an update of the literature is suggested through experiments concerning the effects of the essential oil of Niaouli and the applied vaccine in different time periods, oil rates, and fish species.