The humoral immune response of rainbow trout, Salmo gairdneri Richardson, and rabbits to Aeromonas salmonicida extracellular products

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Abstract
Controversy exists concerning the efficacy of vaccinating fish against furunculosis. Where success is claimed, there has been little attempt to characterize the protective antigens or confirm their immunogenicity. In this report, the immunogenicity of native extracellular products (ECP) of Aeromonas salmonicida and a formalin‐inactivated toxoid of ECP (f‐ECP) was studied in rainbow trout and rabbits, with particular attention to the putative bacterial virulence factors protease and haemolysin. Using crossed immunoelectrophoresis and Protein‐A absorption, antibodies to seven ECP components were detected in the rabbit following immunization with native ECP; antihaemolysin antibodies were found but antibodies to the protease could not be detected. Antibodies to at least 14 components of ECP, including haemotysin and protease, were detected in the rabbit following immunization with f‐ECP. In trout immunized either with native ECP or f‐ECP, antibodies to only four ECP antigens were detected and no antibodies to haemolysin or protease were found. The results may explain previous reports that passive immunization with rabbit antisera gave superior protection against furunculosis compared with antisera raised in fish, and indicate that many extracellular antigens of A. salmonicida may require modification in order to improve their immunogenicity in fish.