LAUSR

Streptococcus parauberis is an etiological agent that causes streptococcosis in fish, exerting a significant impact on aquaculture production. The available injectable vaccine against S. parauberis is commonly used in olive flounder species (Paralichthys olivaceus) in Korea. However, an oral alternative that overcomes the disadvantages of the injectable vaccine has long been desired in the aquaculture industry. Alginate-based microparticles have been widely used as delivery systems for oral administration in aquaculture. In the present study, we produced alginate microparticles encapsulating formalin-inactivated S. parauberis via ionic gelation, which remained stable under acidic conditions (pH 2 and 4) and at low temperatures (4 °C and 25 °C). Mixing the alginate–Gelzan complex with a commercial feed for ease of application induced a specific antibody response and exerted significant protective effects (relative survival rate: 50.0%) in small olive flounder fingerlings. Less pronounced protective effects were also observed in large fingerlings (relative survival rate: 37.5%), suggesting that protection against S. parauberis may be dependent on the size of the fish. Thus, our results indicate that alginate microparticles encapsulating an S. parauberis antigen can be used for initial vaccination in olive flounder at the fingerling stage.