Evaluation of immersion immunisation of Atlantic salmon (Salmo salar) against yersiniosis

Yersinia ruckeri, a Gram-negative bacteria, is a pathogen which causes yersiniosis and significant losses in farmed Atlantic salmon (Salmo salar) in the Southern Hemisphere. Currently, Yersinivac-B, prepared from formalin killed whole-cells, is a commercial bacterin-based vaccine manufactured by MSD Animal Health. It is delivered by bath immersion vaccination against Y. ruckeri for most Tasmanian Atlantic salmon. Signifi-cant mortality events may still occur despite vaccination. This thesis focused on the improvement of yersiniosis vaccine efficacy for Atlantic salmon by using different administrated methods of immersion immunisation. The following aims were addressed in this thesis: • Evaluate the effects of different inactivation methods of Y. ruckeri on the effica-cy of single dip vaccines including formalin inactivation, ammonium sulphate inactivation, and pH-lysed then formalin treated inactivation. • Compare hyperosmotic pretreatment with direct immersion in Atlantic salmon (Salmo salar) by using an ammonium sulphate inactivated Y. ruckeri whole-cell vaccine. • Investigate the vaccine performance of immersion vaccination with single dip and double dip vaccination of small fish at hatchery stage. In this thesis, three different experiments were carried out to obtain the results. The first experiment where the vaccine efficacy of Atlantic salmon was determined using the relative percent survival (RPS), antibody production levels, and carrier status of Y. ruckeri and gene expression. The results showed that ammonium sulphate was success-fully used for Y. ruckeri inactivation and demonstrated further potential for bacteria inactivation using this method for commercial fish vaccines. Additionally, the serum humoral antibody levels did not correlate with the RPS and were not a good marker of protection of the fish against Y. ruckeri infection. The prevalence of asymptomatic carriers was low. The gene expression was consistent with previous successful use as a biomarker of vaccine success in Atlantic salmon against Y. ruckeri. In the second experiment, the vaccine efficacy was assessed when ammonium sulphate inactivated bacterin was combined with hyperosmotic infiltration in vaccination. The results showed that ammonium sulphate vaccinated fish with hyperosmotic infiltration had significantly better survival than the unvaccinated fish with 25.6% higher in the RPS. It has demonstrated that hyperosmotic infiltration could improve protection of a vaccine against Y. ruckeri for Atlantic salmon and has the potential to be used with other bacterin-based immersion vaccines. The vaccine efficacy using single dip and double dip immersion vaccination at first feeding was tested to determine potential of protection against the early outbreaks of yersiniosis in the hatchery stage. There was no significant difference in the survival of salmon regardless of vaccination. In addition, expression levels of IgM gene were not different between the unvaccinated fish (control) and other two groups of vaccinated fish. This suggested that the vaccine did not work with the small fry that were vaccinated twice at 0.13 g and 1 g (2 dips) and single vaccinated at 1 g size. This thesis provided further understanding into different methods of immersion immunisation to yersiniosis in Atlantic salmon. Ammonium sulphate was successfully used for Y. ruckeri inactivation in vaccine production. The minimum fish size for the earliest vaccination by immersion needs to be investigated further