Potency testing of a live, genetically attenuated vaccine for salmonids

Studies have been performed on the use of a live vaccine for immunization of salmonids against the bacterial disease furunculosis. The protection elicited by a kanamycin-resistant aroA mutant of A. salmonicida (Brivax I) and an unmarked aroA deletion mutant (Brivax II) has been examined, and data compared with protection seen using a freeze-dried Brivax II preparation and a commercial, oil-adjuvanted killed vaccine for furunculosis. Whilst high relative percent survival (RPS) values were seen in fish vaccinated with broth-grown Brivax I after a natural exposure to furunculosis (70-100%), much lower RPS values (30-40%) were seen with Brivax II vaccinated fish after an experimental challenge. Nevertheless, the freeze-dried Brivax II formulation performed as well as the broth-grown Brivax II formulation and a commercial vaccine in these studies. In addition, the environmental impact in terms of bacterial shedding into the tank water has been estimated, and shown to approximately 0.03% of the total inoculum used. Lastly, the freeze-dried formulation has been tested for its ability to infect fish and prime for lymphocyte proliferation and antibody production, relative to broth-grown preparations. In all three experiments no significant differences were seen between fish given the broth-grown and freeze-dried formulations. Such data, together with observations that the freeze-dried live preparation had an extended shelf life with the same potency as freshly grown bacteria, show that the potential exists for a commercially viable live vaccine to be produced for use in aquaculture.