Antibody levels in salmon after furunculosis vaccine administration

Aeromonas salmonicida is a species of bacteria which causes furunculosis. It has been of major concern to salmon farmers and it is thought that the disease may also be contributing to the decline in salmon populations as wild salmon is becoming infected by farmed fish in sea cages. In recent years farmers have been getting the disease under control partially due to the introduction of vaccines.

            This study has investigated the best way of testing for potency of furunculosis vaccines in Salmo salar L (Atlantic salmon). The immune status of test fish was tested before the experiment had begun and many pre-existing antibodies were documented. The fish were divided into three groups, each kept at a different temperature (3, 12 and 18^oC). Two experimental vaccine formulations were produced for the study and two control groups were included. One of the vaccines contained the same quality and quantity of A. salmonicida, Moritella viscosa and Infectious Pancreatic Necrosis Virus (IPNV) antigen as a commercially available formation, the full dose (FD) formulation. The other vaccine contained only 5% of the full dose for bacterial antigens and 10% of the IPNV, the reduced dose (RD) formulation. The two controls were injected with either just an adjuvant (AD) or physiological saline (PS). The antibody response of a random selection of fish was then tested at 3, 6, 9 and 12 weeks.

            It was found that the level of protection did not significantly increase between 6 and 12 weeks after vaccination in fish which had been kept at 12 and 18^oC which shows that the fish have already developed protective immunity against A. salmonicida. Fish that were kept at 18^oC had peak antibody resistance at 3 weeks after vacciniation, which is much earlier than those kept at 12^oC (9 weeks), but the optimum temperature for Salmonid fish is 8-14^oC so other problems would likely occur if the fish were farmed at such a high temperature. The findings have also suggested that the best time to test the antibody levels in S.salar is between 500 and 700^o days (immunisation temperature (^oC) multiplied by the number of immunisation days at that temperature).

            At 6 and 12 weeks after vaccination some fish kept at 3 and 18^oC, of each vaccine group, were acclimatised to 12^oC for two days. After this period the fish were then kept with the same number of fish, taken from a 12^oC tank, and all kept together in a tank kept at 12^oC. A proportion of these fish were then injected with A. salmonicida. It was found that most of the fish were protected, however fish previously from a 3^oC tank that were put in the new tank after just 6 weeks were found to have low protection and antibody levels regardless of which vaccine group they were in. If the fish were reared for 12 weeks in the 3^oC tank and then transferred however, the FD vaccine group did have a protective effect. Fish from the control groups all had low survival rates as is expected.

            This study has shown that the vaccine is not very effective in fish kept at 3^oC and that it is best to test for antibody levels in fish that have been kept at 12^oC between 6-9 weeks after vaccination, as after this there is a sharp decrease in antibody levels.

Reference: Romstad, A. B. et al. (2011). Development of an antibody ELISA for potency testing of furunculosis (Aeromonas salmonicida subsp salmonicida) vaccines in Atlantic salmon (Salmo salar L). Biologicals, 40(1), 67-71.