Based on the status of oral vaccines in humans and veterinary species, there is no doubt that the generation of safe and efficacious oral vaccines is among one of the most difficult tasks of immunologists. This is illustrated by the very limited number or oral vaccines approved for use in humans and the slightly larger number approved for use in poultry, pigs and cattle. In this respect fish are not lagging much behind, with 5 oral vaccines available on the market. These vaccines however, are against only a very limited number of pathogens and are available for an even smaller number of fish species. When considering the vast diversity of cultured fish species and their pathogens, the current oral vaccines are by far insufficient to fulfil the market requirements. Such species diversity is in fact larger than the diversity in species and pathogens faced by for example the poultry or cattle vaccine industry.
Nonetheless, fish oral vaccine development can greatly profit from the progress made on human and veterinary oral vaccines. For example, the use of live vectors, e.g adenoviruses, or a more rational attenuation of enteric pathogens, e.g E. tarda or V. anguillarum, as well as the combination of weak oral antigens with strong mucosal adjuvants, e.g enterotoxins, leaves a vast number of combinations that have not been fully exploited in fish vaccine development. Fish mucosal immunology, despite the large body of work performed in the last 30 years, is still in its infancy, mostly due to the great heterogeneity in teleost species. Nevertheless, the discovery of new players in fish mucosal immunity within the last 5–10 years, including IgT or M-like sampling cells, keeps the field of mucosal immunology and vaccinology a dynamic and developing area. The possibility to specifically target M-like cells or putative APCs in the fish gut is becoming a viable option in fish vaccine delivery as well. This is being realized through the great advances in gene discovery and the several genome sequencing initiatives for several fish species and their pathogens. The bottleneck will of course be the functional characterization of most of the novel genes and the translation of this fundamental knowledge into practical applications linked to vaccine development. Molecular traceable and genetically modifiable models such as transgenic or mutant zebrafish can support and accelerate fish vaccine development as much as other animal models have helped the human and veterinary field. Information on host mucosal responses, together with insights in how fish gut microbiota might influence the response to oral vaccination is increasing at a rapid paste. This information will be essential to design strategies aimed at breaking mucosal tolerance while preventing inflammation for a greater variety of fish species. Finally, collaborations between academia, industrial partners and farmers will be instrumental to produce safe and efficacious vaccines for most commercially relevant fish species.