Dietary effects on immunity, stress, and efficacy of two live attenuated Flavobacterium psychrophilum vaccine formulations

• Feeding a commercial functional diet enhanced the efficacy of Flavobacterium psychrophilum B.17-ILM vaccine in rainbow trout.
• The improved ILM vaccine formulation produced significantly lower mortality in rainbow trout than the standard B.17 vaccine.
• Fish vaccinated with ILM vaccine produced significantly higher specific antibody titres than unvaccinated control fish.
• Feeding functional diet can significantly reduce mortality in rainbow trout exposed to acute chlorine toxicity.

The effects of feeding a commercially available health enhancing aquaculture feed (Bio-Oregon's BioPro/Protec® diet) on the efficacy of two coldwater disease (CWD) vaccine formulations consisting of Flavobacterium psychrophilum 259-93B.17 (B.17) and B.17 grown in iron limited medium (ILM) were carried out in two separate trials. The primary aim of this study was to compare efficacy of the B.17 and ILM formulations in rainbow trout (Oncorhynchus mykiss). A secondary aim was to determine if dietary alterations could enhance the immune response and/or elicit greater levels of protection. Therefore, a basal diet (BioOlympic fry diet; control feed) and an immunostimulatory diet (BioPro; IS feed) were fed to rainbow trout at 2% body weight (bwt) per day for 2 weeks prior to immersion immunization with the B.17 vaccine, ILM vaccine, or a TYES media control. A booster immunization was administered two weeks post initial vaccination, and feeding of the respective feeds continued for one additional week at which time all groups received the control diet until the end of the experiment. In trial 1, an acute chlorine spike occurred just prior to pathogen challenge (week 7) resulting in substantial mortality among treatment groups. Interestingly, fish fed the IS feed had significantly lower mortality (36.7%) than fish fed the control feed (67%). Vaccinated fish had significantly higher antibody titers than the control fish at week 2 through 7 in trial 1. In trial 2, antibody titers for all vaccinated fish were significantly greater than controls at week 4, but at weeks 6 and 8 the titers of B.17 vaccinated fish were not significantly different from controls. However, average titers for ILM vaccinated were significantly greater than control fish at weeks 6 and 8. Cumulative percent mortality (CPM) in B.17 and ILM vaccinated groups was significantly lower than control groups at the point where CPM reached 60% (mid-way through outbreak), but by the end of the challenge (28 days) ILM vaccinated fish had significantly lower CPM than either control or B.17 vaccinated fish. When comparing dietary effects, CPM was not significantly different but RPS values for fish fed the BioPro IS feed were consistently elevated over groups fed the control diet regardless of the vaccine formulation administered. This study demonstrates increased efficacy of the ILM vaccine in rainbow trout, and provides evidence that a commercial “health enhancing” diet can provide benefits in response to an acute stress event and may contribute to increased vaccine performance.Statement of relevanceThis manuscript addresses an important and very practical problem in aquaculture. Coldwater disease is a major disease affecting salmonid aquaculture around the world. There is no approved vaccine available in the US. This manuscript reports the beneficial effects of feeding commercial immunostimulatory functional feeds on the efficacy of two formulations of a coldwater disease vaccine. The results presented here are highly relevant to the development of aquaculture and finding sustainable and non-antibiotic based aquatic animal health management strategies. This study will contribute towards coldwater disease management and identifies potential benefits of commercial functional feeds commonly used in the aquaculture industry.