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Vaccination promises to be the most sustainable means of preventing and managing disease outbreaks in aquaculture. Fish vaccines should be efficient, potent and safe, and should not have adverse effects on humans or the environment. Although vaccination for fish includes immersion and oral delivery routes, injection delivery is currently the most widely applied method industry wide. This is effective for highly valued fish species; however, injection vaccination is not practical for species of lower value or for diseases affecting fish at small sizes. Under these circumstances, oral and immersion delivery holds the greatest promise, but requires antigens to be efficiently taken up through mucosal surfaces. Oral delivery of vaccines in the feed is difficult due to challenges in delivering a consistent and adequate dosage, potential for degradation of antigens in the gastrointestinal tract, and risks of developing oral tolerance. Immersion delivery of killed antigens (bacterins) or live attenuated vaccines is more common and much less stressful than injection delivery. This method involves dipping or bathing fish in a vaccine solution for a period of time to allow antigen uptake across mucosal surfaces resulting in stimulation of both a mucosal and systemic immune response. This review discusses multiple delivery methods for vaccination of fish with primary emphasis on immersion delivery and the factors affecting efficacy such as dose, duration of protection, delivery time, size at first vaccination, booster regimes, and storage requirements. All of which must be optimized before a vaccine is commercialized. Such criteria have been evaluated for a recently developed live attenuated immersion vaccine, Flavobacterium psychrophilum 259-93-B.17 grown in iron limiting medium (called B.17-ILM) that protects trout and salmon against coldwater disease (CWD). This vaccine requires a booster immunization but protects rainbow trout for at least 24 weeks with a relative percent survival of 70%. A vaccine dose as low as ~ 105 cfu/mL has been shown to provide significant protection following pathogen challenge, and fish as small as 0.5 g can be administered the vaccine by immersion and protected from CWD. This review highlights aquaculture vaccines and emphasizes the potential to utilize live attenuated vaccines and mucosal vaccination (immersion) for aquaculture, but it is clear that a better understanding of antigen uptake mechanisms could aid in designing and optimizing future vaccines for fish. As newer developments in vaccine production and processing technologies, storage technologies and novel delivery strategies become available for aquaculture, it is likely that immersion vaccination will be the method of choice for most fish farmers.
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