Passive Protective Effect Of Specific Egg Yolk Antibodies Against Edwardsiella Tarda In Turbot(Scophthalmus Maximus)

With the development of intensive farming of turbot (Scophthalmus maximus), outbreaks of bacterial diseases have led to serious economic losses. Edwardsiella tarda has been regarded as the leading pathogen causing diseases in turbot farming these years. Due to the over use of antibiotics, food safety and human health were threatened by drug resistance inevitably. Egg yolk antibodies are recognized as an effective method to inhibit aquatic bacterias with all these advantages including high specificity, environment-friendliness and safety, no drug resistance and no need to slaughter animals. Oral delivery of chicken egg yolk immunoglobulins (IgY) serve as an interesting opportunity for controlling bacterial diseases in aquaculture and providing passive immunization of aquatic animals.Passive protective effect of IgY against Edwardsiella tarda infection in turbot was evaluated. IgY was isolated from egg yolks laid by hens immunized with inactivated Edwardsiella tarda. After water dilution, salting out andultrafiltration concentration, freeze drying purified IgY was obtained with a purity of 83.9% and titer of 1:25000 estimated by ELISA. Specific IgY showed high specific binding activity to other E. tarda strains isolated from infected turbots.Purified IgY inhibited the growth of Edwardsiella tarda in vitro with a dose-dependent manner at concentrations ranging from 1 to 10 mg/mL. Immunofluorescence and immunoelectron microscopy techniques confirmed the binding ability of specific IgY to E. tarda. With transmission scanning electron microscopy, it is noted that E. tarda treated with specific IgY showed much more rough surface than negative control, which indicated that the characteristic of bacteria surface has been changed by the treatment of specific IgY. Besides, he presence of specific IgY induced a attenuation of biofilm formation withthe hydrophobicity on bacteria surface decreased significantly（P<0.05).High survival rate (87.5%) of turbot was observed in groups treated with anti-E.tarda IgY after bacterial infection, compared with those treated with with non-specific IgY (31.25%) or no administration (18.75%). As well, the bacterial burden in liver, spleen, kidney and intestine was significantly (P<0.05) lower in turbot treated with specific IgY than those treated with non-specific IgY. Moreover, preventive protection was observed in groups fed with 2% IgY powder and 10% egg yolk powder, which provided 75% and 62.5% survival rate, respectively, had better performance compared to 2% egg yolk powder IgY group and 0.2% specific IgY powder group.10% IgY egg yolk powder group also showed dramatically difference compared to the negative control group (P＜0.05). More importantly, the macrophages of head kidney showed high phagocytic activity for E.tarda in the presence of specific IgY. Higher and earlier response of nonspecific immune enzymes was estimated in specific IgY group compared with nonspecific control.These results suggest that passive immunization with specific IgY by oral intubation may provide a potential treatment for Edwardsiella tarda infection in turbot.