Aquaculture is the fastest growing food producing sector worldwide and is deemed vital to future food security. The importance of aquaculture is further highlighted by the declining capture fishing industry and diminishing wild stocks. The success and sustainable rapid growth of intensively farmed fish has largely been possible due to vaccination of fish against potentially devastating bacterial and viral pathogens. Since the first licensed fish vaccine in 1976 the use of fish vaccines is now an integral part of fish health management of intensively farmed species worldwide. High value species such as salmon and trout in Norway, Chile, the UK and Canada were the first to adopt the use of vaccines and currently dominate the market for fish vaccines.
Despite development of sub-unit, live-attenuated and DNA vaccines to a variety of pathogens the majority of licensed fish vaccines in use consist of relatively simple inactivated microorganisms. In fact, little has changed in the last 10 years concerning the formulation and delivery of fish vaccines. Intraperitoneal (i.p.)injection of multivalent vaccines to fish of injectable size and immersion delivery to juvenile fish too small to be handled and injected effectively are industry standards. Whilst effective, i.p.-injection and the necessary oil-based adjuvants used often result in adhesions to the peritoneal cavity and are of growing concern. Similarly, immersion vaccination also has its limitations such as limited duration of protection and ineffective protection in fish less than 2g.
Since the earliest development of fish vaccines in the 1950s, oral vaccination was believed to be optimal for fish vaccination. However, oral in-feed immunogen delivery has produced inconsistent outcomes in fish. This is primarily attributed to antigen degradation, solutions to which are typically complex and expensive. Here, we discuss the development and validation of an oral vaccine preparation using alginate microcapsules to deliver inactivated and live-attenuated experimental bacterial vaccines to Atlantic salmon. Furthermore, we demonstrate the improved efficacy of our microencapsulated chemically-inactivated bacterin compared to delivery by immersion in first-feeding fish of approximately 0.2g. These results suggest that continued development of oral immunoprophylaxis may provide a viable alternative to traditional i.p. and immersion vaccination of fish.