COMPARISON OF DIFFERENT CHALLENGE METHODS TO EVALUATE THE EFFICACY OFFURUNCULOSIS VACCINES IN ATLANTIC SALMON, SALMO-SALAR L

Four challenge methods, intraperitoneal (i.p.) and intramuscular inocu lation, bath and cohabitation exposure, were evaluated as methods for testing the efficacy of furunculosis vaccines in Atlantic salmon, Salm o salar L. Groups of fish vaccinated with one of Mio different vaccine s containing aluminium phosphate or animal/vegetable oil as an adjuvan t were challenged with Aeromonas salmonicida 6 and 12 weeks after vacc ination. Relative per cent survival (RPS) was calculated daily during a 3-week observation period post-challenge. A large variation in prote ction measured by RPS, both between methods and between different time points for each method, was found. Towards the end of the observation period, RPS tended to be similar in the i.p. and cohabitation challen ge groups. A high degree of protection was demonstrated for the oil ad juvanted vaccine. The i.m. challenge produced very low RPS numbers for both vaccines, but this was most marked for the aluminium-phosphate-a djuvanted vaccine. The bath exposure resulted in RPS values intermedia te to the cohabitation and injection methods. The study also demonstra ted that, after i.p. and i.m. challenges, the initial peak mortality c aused by the inoculation was followed by a secondary increase in morta lity, probably because of shedding of bacteria into the water during t he first mortality phase and hence contributing to a superinfection st ate.