Bactericidal activity by sub-agglutinating levels of rainbow trout ( Oncorhynchus mykiss ) antiserum to Vibrio anguillarum serogroup O1

Immunisation against Vibrio anguillarum by oral or immersion delivery has often not been associated with a detectable increase in a specific serum antibody (Ab) response in sockeye salmon (Oncorhynchus nerka), ayu (Plecoglossus altivelis), or rainbow trout (Oncorhynchus mykiss), even though the fish were protected against experimental challenge by this bacterium. In other studies this Ab was demonstrated to contribute to protection against V. anguillarum in fish passively immunised with V. anguillarum antiserum. It is possible, therefore, that oral or immersion immunisation stimulate mucosal immunity which accounts for the protection, but an additional explanation may be that even low or undetectable levels of specific Ab are sufficient for an effective protection against V. anguillarum. Harrell et al. found that very diluted immune serum in the presence of normal serum was able to kill or inhibit the growth of V. anguillarum as demonstrated by a plate plaque assay. The purpose of this study was to test if sub-agglutinating levels of immune serum in the presence of normal serum as a complement source had the capacity to kill V. anguillarum serogroup O1.