Prolonged immersion improves the effectiveness of dilute vibrio vaccine for rainbow trout

In order to clarify the effect of a prolonged exposure period on vaccine effectiveness, juvenile rainbow trout were immersed in 100, 1,000 and 10,000 times dilutions of a commercial vibrio bacterin for 3 min (brief immersion, BI) and for 24 h (prolonged immersion, PI). Two weeks and 2, 4 and 6 mo after the vaccination, fish were challenged by waterborne exposure to Vibrio ordalii Prolonged exposure to dilute bacterin resulted in better protection than the standard BI method. Two weeks after vaccination, the cumulative mortality was 0% in the group vaccinated by PI with the 100, 1,000 and 10,000 times diluted vaccine. In contrast, mortality was 58 and 77% in the groups vaccinated by BI with the 100 and 10,000 times diluted vaccine respectively, and was 100% in the unimmunized control group. Furthermore, 6 mo after vaccination, the mortality was 20% (100 times dilution) to 77% (10,000 times dilution) in the PI group, while it was 64% (100 times dilution) to 93% (10,000 times dilution) in the BI group. Mortality in the corresponding control group was 97%. These results suggest that prolonged exposure period improves vaccine effectiveness.