Abstract
A total of 250 whiteleg shrimp juveniles, Litopenaeus vannamei, clinically infected with black gill melanization were subjected to mycological examination to isolate the main causative agents. The results showed the dominance of Fusarium solani at a 66% incidence rate compared to 34% of F. oxysporum in the examined shrimp. The in vitro antifungal activity of Origanum vulgare and orange peel essential oils (EOs) against F. solani was determined in comparison with Biotronic (R) Top3 (commercial antimicrobial). In addition, four groups in triplicates of L. vannamei (20.01 +/- 3.02 g) were used to investigate the in vivo immunostimulant, antioxidant, gill histopathological changes, and antifungal effects of dietary O. vulgare and orange peel EOs for 28 day against the F. solani infection. The first group was a negative control that fed a basal diet, and the second group was a positive control that fed a basal diet and was artificially infected with 0.1 mL of 5.0 x 10(6) conidia/mL of F. solani. The third and fourth groups were fed diets supplemented with O. vulgare and orange peel EOs (1 mL/kg diet), respectively, and infected with F. solani (0.1 mL of 5.0 x 10(6) conidia/mL). The in vitro antifungal activity revealed a very strong inhibition effect of O. vulgare and orange peel EOs on F. solani (>50%) compared to 31-50% inhibition induced by Biotronic (R) Top3. In addition, the in vivo feeding experiment showed that the positive control had lower cellular (hemocyte count, phagocytosis activity, and index) and humoral)phenoloxidase and lysozyme activities) immune responses than other treatments. Also, superoxide dismutase activity was significantly decreased in F. solani-infected shrimp. The histopathological investigation of infected shrimp showed dense aggregation of black melanin pigment in gill filaments and variable lesions of necrosis, edema, and congestion. Dietary both EOs significantly alleviated the immune suppression and histopathological effects of F. solani, especially O. vulgare EOs. The mortality rate was decreased to 16.66 and 30.00% with O. vulgare and orange peel EOs, respectively, compared to 70% in the positive control. In conclusion, O. vulgare and orange peel EOs can be used as antifungal and immunostimulant supplementation in L. vannamei diets against F. solani infection.