Adjuvants and immunostimulants for enhancing vaccine potency in fish

Adjuvants combined with immunogens are effective enhancers of the immune response in fish. As in other animals, substances such as Freund's complete and incomplete adjuvants, light oils and bacterial lipopolysaccharides have been shown to induce elevated antibody production when added to bacterins and administered to fish. Light oil adjuvants are now used successfully with injectable bacterins of Aeromonas salmonicida, and in multivalent fish vaccines where A. salmonicida is combined with Yersinia sp., and/or Vibrio spp. antigen preparations. The oils are thought to act as depots or reservoirs, holding the antigens in globules at the site of injection, thus allowing prolonged dosage. Additions to the oils such as muramyl dipeptide, bacterial lipopolysaccharides and other bacterial and animal-extracted products may activate specific immune cell populations such as T-cells, neutrophils and other phagocytic cells important in the cellular mediated response. Conjugation of antigens with alum is another traditional approach which has been used for A. salmonicida and Y. ruckeri bacterins with varying results. The process enables antigen to be held in a reservoir and also may detoxify harmful substances. Recent research on substances such as beta-1,3 glucans, chitosan, levamisole, and other inflammatory agents shows enhancing effects on the specific immune response when added to immunogens and administered by injection, bath or by feeding. These substances may also act to elevate non-specific defence mechanisms against disease agents as most of them are active even when given alone.