THE SERUM ANTIBODY-LEVELS IN ATLANTIC SALMON (SALMO-SALAR L) AFTER VACCINATION WITH VIBRIO-SALMONICIDA AT DIFFERENT TIMES DURING THE SMOLTING AND EARLY POST-SMOLT PERIOD

The time course of the specific antibody response to vaccination with formalin-killed Vibrio salmonicida was studied in Atlantic salmon vaccinated in the smelting and sea water transfer period. Twelve groups received primary vaccination in Feb-May, and eight received secondary vaccination 4 or 8 weeks later, and one 20 weeks later. Each group had its own control receiving the medium of the vaccine only. Fish were transferred to sea water in June. Primary vaccination only resulted in low antibody levels and in September there were no significant differences between vaccinated and unvaccinated fish. With secondary vaccination 4 or 8 weeks after the primary vaccination, all vaccinated groups had significantly higher antibody values in September compared with unvaccinated fish. Groups vaccinated during the first months of the year had the highest antibody levels, while vaccination in May to July i.e. during smelting, resulted in lower antibody levels. Groups primary vaccinated in Feb-Mar had no increase in specific antibody values 1 month after secondary vaccination in contrast to groups primary vaccinated in Apr-May. In September, however, the early vaccinated groups showed higher antibody values compared with those vaccinated during smelting or after sea water transfer. Challenge experiments performed in Jan-Mar the following year showed no reproducible difference between the different vaccinated groups with respect to relative percent survival values, but all vaccinated groups had a good protection compared with the unvaccinated group. The results show that early vaccination i.e. before smelting and sea water transfer gives rise to high antibody levels at least 6 months later, even when there was low water temperatures (3.4-7.5 degrees C) at the time of primary and secondary vaccination.