New case study: WraAs OptiVacc - advancing vaccination strategies for Ballan wrasse

Assessing the efficacy of 'dip' vaccination in juvenile Ballan wrasse

Ballan wrasse are vital to sustainable salmon farming in the UK, acting as cleaner fish that naturally control sea lice. However, their effectiveness is threatened by atypical Aeromonas salmonicida, a bacterial pathogen that can cause high mortality, especially in juveniles. With no licensed vaccines available, Scottish producers rely on autogenous vaccines, which provide over 90% survival in larger wrasse when injected, but challenges remain for smaller fish.

The WraAs OptiVacc project aimed to refine vaccination strategies and improve understanding of wrasse immunity. Valued at £352,807, the partners were Mowi Scotland, University of Stirling, Otter Ferry Seafish, and Ridgeway Biologicals. The partners aimed to improve vaccination strategies and understand wrasse immunity better. Researchers tested different approaches, including dip vaccination (where fish are immersed in vaccine solution) and injection, using both single-target (monovalent) and multi-target (multivalent) vaccines.

Key findings

• Single-target vaccines produced stronger antibody responses than multi-target ones, suggesting that combining too many antigens can reduce effectiveness.
• Dip vaccination followed by an injection boost improved immune responses, especially for certain bacteria.
• Genetic analysis showed that dip vaccination activated key immune markers, indicating the fish were mounting a defence. However, some responses, like antibody production, were stronger with single-target vaccines.
• The study also highlighted the complexity of the bacteria, which varies genetically, making vaccine development harder.

Although the immersion vaccine did not provide full protection in field trials, the project delivered valuable insights. It confirmed that mucosal vaccination - targeting the fish’s skin, gills, and gut - could be a promising method for young fish, as it reduces handling stress and works at early life stages.

Future research will focus on improving vaccine design using advanced techniques and better delivery systems. These efforts aim to protect cleaner fish, reduce antibiotic use, and make salmon farming more sustainable.

The full title of this project is ‘Optimising vaccination formulations for Ballan wrasse’.

Read the case study