Correlation of immune assays with protection in rainbow trout, Salmo gairdneri, immersed in Vibrio bacterins

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Abstract
One hundred kg of Rainbow trout, Salmo gairdneri, weighing 80–100g each were immersed in a formalized, bivalent vaccine of Vibrio anguiliárum (VA) and Vibrio ordalii (VO), at a1/10 dilution for 30s at 10°C. The protection levels achieved at 8 and 10 weeks post vaccination were assessed by intraperitoneal challenge with 105 live V. anguillarnm or V. ordalii. No decline in protection due to antigen depletion was found between batches throughout the procedure. The protection conferred by one component of the vaccine was slightly better than the other (80% against VO and 42 % against VA). A number of measures of immunity were monitored before and after challenge. These included serum and mucus antibody litres, bactericidal effects of serum and mucus and phagocytosis by peritoneal exudate cells. Serum antibody was present (peak litre of-log2 3.25 for VO, and 4.4 for VA) but had declined to background levels by the time of challenge, whereas bactericidal effects and phagocytosis rate of 40% were not increased by vaccination. The most likely immune mechanism responsible for the observed protection was discussed and suggested to be a cellular mechanism where serum antibody acts as an opsonin to increase phagocytosis.

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