In vivo testing of any candidate vaccine is influenced by the choice of challenge method and the external environmental conditions. In the present study, a comparative challenge study was performed to evaluate the efficacy of different vaccines against the bacterial pathogen Aeromonas salmonicida causing furunculosis. Rainbow trout (Oncorhynchus mykiss) were vaccinated with two trivalent adjuvanted experimental vaccines containing formalin-killed A.
salmonicida, Vibrio anguillarum O1 and O2a and a commercial corresponding vaccine (Alpha Ject 3000). Fish were challenged by i.p. injection or cohabitation at both freshwater and saltwater conditions at different time points post vaccination. The cohabitation challenge method represents a more natural mode of infection and provided a better differentiation of vaccine groups compared to i.p. injection of live bacteria. However, cohabitation challenge in
saltwater condition produced less mortality compared to freshwater, probably due to the growth inhibition of A. salmonicida in saline condition which was also verified by in vitro assay. One of the experimental vaccines emulsified in water in oil adjuvant showed a protection comparable to that of the commercial vaccine with lower side effects as observed by the Speilberg scoring system. Gene expression analysis did not show a clear trend for Th1 or Th2 response in the vaccinated fish. Exposure of fish to saltwater increased the IgT production. Overall, the immune response in vaccinated fish, the side effects due to oil
adjuvants, and the importance of choice of challenge methods used will be discussed.