author(s)
Salma T Salih and Sanaa A Mustafa

abstract
This study was carried out to assess the efficiency of synbiotic on hematological, histopathological changes and resistance against Saprolegnia spp. in Cyprinus carpio. A total of 100 C. carpio weighing 49.55–50.50 g, were randomly stocked into five treatment groups. Fishes were fed different synbiotic concentrations: 0.5% (T1), 1.0% (T2), 1.5% (T3) and 2% (T4), as well as the control group (C) were fed basal diet without any addition of synbiotic. All treatments were fed twice a day at rate of 2% body weight for 56 days. At the end of the feeding trail, all the treatment groups were challenged in a viable fungal suspension of Saprolegnia spp. (1×10⁵ live zoospores/ml). Considerable changes have been recorded in the differential leucocyte count, percentage of lymphocyte and monocyte showed significantly decrease (P<0.05) in T4 compared to the C+ group. Neutrophils (%) was significantly increased in T4 compared to C+ group. Histopathologically, gill sections exhibited epithelial lifting, epithelial hyperplasia and cellular necrosis. Quantitative analysis of gill lesions revealed that the lifting of the epithelium, epithelial hyperplasia were significantly increase (P<0.05) in groups treated with synbiotic compared to C+ group. Skin of infected fish (C+) showed loss of the epidermis, necrosis in dermis and penetration of hyphae in underlying musculature, these changes are lesser in extent in fish fed with synbiotic supplemented diet. Although, all levels of dietary synbiotic significantly increase resistance against Saprolegnia challenge, the highest survival rate was found in T4 (83%) followed by T3 and T2 (66%), T1 (50%) and C- (16%) respectively. The results indicated that supplementation of dietary synbiotic at 2.0 % in diet are more resistant against infection by Saprolegnia spp. Therefore, dietary treatment using probiotic and prebiotic combination had a positive influence on the survival rate and disease resistance in C. carpio.