Use of a genetically attenuated strain of Aeromonas salmonicida to vaccinate salmonid fish

A genetically attenuated strain of Aeromonas salmonicida has been developed that has a complete deletion of the aroA gene (Brivax II), making it suitable for development as a commercial vaccine. Brivax II was effectively cleared from Atlantic salmon, Salmo salar, a species highly susceptible to furunculosis, confirming that it is attenuated. Clearance rate was dependent on the vaccine dose administered, being longer with higher doses. Immunological studies using Brivax II injected in rainbow trout, Oncorhynchus mykiss, confirmed that live vaccines stimulate a greater response, in terms of generating leucocytes able to proliferate to a subsequent encounter with antigen, relative to killed vaccines. Development of strains of Brivax II as carriers of heterologous antigens was also investigated. Escherichia coli β-galactosidase was chosen as the model antigen, and three strains containing plasmids with the LacZ gene were constructed (Brivax 12, Brivax 61 and Brivax 107). All three strains were shown to express β-galactosidase in vivo in rainbow trout and to be cleared effectively. Interestingly, Brivax 107 was cleared faster than the other two Lac⁺ strains and had the highest level of β-galactosidase activity. The two strains expressing lower levels of activity also behaved differently in vivo, in that Brivax 12 accumulated derivatives expressing lower levels of β-galactosidase activity, suggesting that mutants are being selected in vivo. The potential advantages of live vaccines over killed vaccines are discussed.