In this study Artemia nauplii were used as an antigen delivery system for oral vaccination of young fish with a Vibrio anguillarum bacterin. Juvenile carp of 15-, 29- and 58-days old (stomachless fish) and gilthead seabream of 57- and 69-days old (stomach-containing fish) were fed with this bioencapsulated bacterin. Antigen transport in the endgut was studied using bioencapsulated bacterin. In carp of 29- and 58-days old antigenic determinants of V. anguillarum were transported from lumen to supranuclear vacuoles in carp and finally to intraepithelial macrophages. In carp of 15-days old a similar transport was observed, only macrophages could not be found. In gilthead seabream a different (more diffuse) transport was observed and no macrophages were found. Ten weeks after oral vaccination fish of both species received an intramuscular booster. Three weeks later, serum was collected and measured for specific antibodies by ELISA. Carp orally vaccinated at 15- or 39-days old showed a significantly lower response after the booster compared with non-vaccinated controls. However, carp orally vaccinated at 58-days old tended to have a higher antibody response compared with the control. Seabream orally vaccinated at 57 or 69 days showed significantly higher secondary responses compared with the control. It is concluded that oral vaccination with bioencapsulated vaccines is very promising when applied at the right age. Application at too young an age seems to induce immunosuppression, instead of immunisation.
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