This work aimed to investigate the role of some immunostimulants and probiotics in improving the response of overwintered tilapia to Aeromonas hydrophila vaccine. In this study, 15000 Nile tilapia fry (Orechromis niloticus) were collected and divided into five groups. Group 1 was the control, groups 2 to 5 were fed diet supplemented with garlic, Echinacea, Organic Green TM and Vet-Yeast TM respectively for 5 months. Vaccination with A. hydrophila bacteria was done by the end of the feeding experiment. The antibody titer of the vaccinated overwintered tilapia of all groups showed no significant changes during the same sampling time. A significant high value in the antibody titer was recognized in vaccinated overwintered tilapia at the end of 6 th -8 th week post-vaccination (PV) in the control group and between the 4 th -8 th week PV in the immunostimulant supplemented groups (Groups 2-3), and between the 2 nd -10 th week PV in probiotic supplemented groups (Groups 4 and 5). The challenge infection of the vaccinated tilapia showed the highest mortality in Group 1 while the lowest mortality was seen in Group 5. However, maximum protection after challenge was seen at 6 th week PV in other treated groups. The immunostimulants and probiotics under test proved efficient in improving the immune response to vaccination which will improve the resistance of tilapia fry against infection during the winter. The overall results are promising to implement overwintering fry culture program to economically maximize and efficiently use the available aquaculture facilities throughout the year.

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