Health screening and autogenous vaccination strategies for atypical Aeromonas salmonicida in farmed ballan wrasse (Labrus bergylta, Ascanius)

Abstract

Ballan wrasse (Labrus bergylta, A.) are deployed in Atlantic salmon (Salmo salar, L.) pens to control sea lice (Lepeophtheirus salmonis, K) infestations in the Northern hemisphere. Bacterial disease outbreaks related to atypical Aeromonas salmonicida (aAs), however, cause significant mortalities in ballan wrasse at hatcheries and cage sites. Although a multivalent autogenous vaccine, including aAs, was being used disease outbreaks still occurred. The aims of this project were initially to identify the most prevalent and predominant bacterial pathogens from ballan wrasse in hatcheries and cage sites in Scotland with the goal of optimising the formulation of the autogenous vaccine used in hatcheries (Chapter 3). Then, the effect of the enzyme Alcalase® was investigated on bacterial load which could provide essential information for disinfection protocols in commercial hatcheries (Chapter 4). Also, the egg microbiota of farmed ballan wrasse was characterised shortly after spawning and during incubation, and the possibility of vertical transmission of aAs from broodstock to offspring was investigated. The above would give an insight into egg bacteria dynamics and possibly identify when ballan wrasse encounter important bacterial pathogens during production (Chapter 4). Furthermore, the pathogenicity and virulence of bacterial isolates collected during disease outbreaks in Scotland were evaluated and experimental bacterial challenge models were developed both by bath and i.p injection. These had the objective to determine the efficacy of the immersion and injectable version of the autogenous polyvalent vaccine that has been used in commercial hatcheries (Chapter 5, 6 and 7). Finally, the ontogeny of elements of the adaptive immune system of farmed ballan wrasse was verified on juvenile ballan wrasse (Chapter 7). Atypical As was the most prevalent and predominant bacterial species isolated during the health survey (Chapter 3) with Vibrionaceae isolates also recovered from diseased fish. The former bacterium was detected and recovered from 162 days post hatch (dph) in diseased ballan wrasse but not from water samples. A large reduction in bacterial load was recorded on eggs treated with the enzyme Alcalace® (gum layer removed, Chapter 4). Atypical As was detected both by real time quantitative PCR (qPCR) and MiSeq but the very low positive results and reads, respectively, were not sufficient for a conclusion to be drawn (Chaper 4). Pre – deployment ballan wrasse (> 25 g) were immunised by i.p. injection vaccination with the vaccine and then experimentally challenged (i.p) against homologous and heterologous strains of aAs. The vaccine was highly protective for the above stains with RPS values of 91 % and 95% (homologous) and 79 % (heterologous). In addition, specific antibody titers (IgM) were measured by enzyme linked immunosorbent assay (ELISA). In contrast, no protection was achieved by immersion vaccination of juvenile ballan wrasse (0.5 g ± 0.2 g and 1.5 ± 0.4 g) challenged with aAs. Furthermore, no differences were observed in the expression of adaptive immune genes (MHC II – CD74 and IgM) in vaccinated fish. However, the ontogeny of MHC II – CD74 and IgM molecules were reported for the first time at 35 and 90 dph, respectively. Overall, this work has provided essential information on the pathogenicity, susceptibility and vertical transmission of routinely recovered bacteria from ballan wrasse during disease outbreaks in hatcheries and cage sites in Scotland. This research also contributed to a better understanding of acquired immunity upon different vaccination delivery methods and life stages and proved insights on the ontogeny of adaptive immune molecules. The findings of this PhD are important for disease mitigation in ballan wrasse production, particularly in the hatchery through vaccination and can facilitate future immersion vaccine optimisation.