A commercial autogenous injection vaccine protects ballan wrasse (Labrus bergylta, Ascanius) against Aeromonas salmonicida vapA type V

Atypical Aeromonas salmonicida (a As ) and Vibrionaceae related species are bacteria routinely recovered from diseased ballan wrasse used as cleaner fish in Atlantic salmon farming. Autogenous multivalent vaccines formulated from these microorganisms are widely used by the industry to protect farmed wrasse despite limited experimental proof that they are primary pathogens. In this study, the components of a commercial multivalent injection wrasse vaccine were tested for infectivity, pathogenicity and virulence via intra peritoneal injection at pre-deployment size (25-50g) and the efficacy of the vaccine for protection against a As assessed. Injection with 3.5×109, 8×109 1.8×109 and 5×109 cfu/fish of Vibrio splendidus , V. ichthyoenteri , Aliivibrio logeii and A. salmonicida , respectively, did not cause significant mortalities, lesions or clinical signs after a period of 14 days. IP injection with both a As and Photobacterium indicum successfully reproduced the clinical signs and internal lesions observed during natural outbreaks of the disease. Differences in virulence (LD50 at day 8-post infection of 3.6×106 cfu/fish and 1.6×107 cfu/fish) were observed for two a As vapA type V isolates. In addition, the LD50 for Photobacterium indicum was 2.2×107 cfu/fish. The autogenous vaccine was highly protective against the two a As vapA type V isolates after 700-degree days of immunisation. The RPSFINAL values for the first isolate were 95 and 91% at 1×106 cfu/fish and 1×107 cfu/fish, respectively, and 79% at 1×107 cfu/fish for the second isolate tested. In addition, significantly higher anti a As seral antibodies (IgM), were detected by ELISA in vaccinated fish in contrast with control (mock vaccinated) fish. These results suggest wrasse can be effectively immunised and protected against a As infection by injection with oil adjuvanted vaccines prepared with inactivated homologous isolates. Further work should assess the efficacy of vaccination against other isolates that have proven to be pathogenic such as a As type VI and Photobacterium indicum and explore the feasibility of immersion vaccination. In addition, a full characterisation of a As isolates within the same vapA types should be performed as differences in virulence between vapA type V isolates were observed and partial genome analysis indicated small but potentially important genomic differences in these isolates. ### Competing Interest Statement The authors have declared no competing interest.