Efficacy and safety of a non-mineral oil adjuvanted injectable vaccine for the protection of Atlantic salmon (Salmo salar L.) against Flavobacterium psychrophilum

Abstract

Flavobacterium psychrophilum is the causative agent of Rainbow Trout Fry Syndrome which has had a major impact on global salmonid aquaculture. Recent outbreaks in Atlantic salmon in Scotland and Chile have added to the need for a vaccine to protect both salmon and trout. At present no licensed vaccines are available in Europe, leaving antibiotics as the only course of action to contain disease outbreaks. Outbreaks generally occur in fry at temperatures between 10 and 15 °C. Recently outbreaks in larger fish have given added impetus to the development of a vaccine which can provide long term protection from this highly heterogeneous pathogen. Most fish injectable vaccines are formulated with oil emulsion adjuvants to induce strong and long lasting immunity, but which are known to cause side effects. Alternative adjuvants are currently sought to minimise these adverse effects. The current study was performed to assess the efficacy of a polyvalent, whole cell vaccine containing formalin-inactivated F. psychrophilum to induce protective immunity in Atlantic salmon. The vaccine was formulated with an adjuvant containing squalene and aluminium hydroxide, and was compared to a vaccine formulated with a traditional oil adjuvant, Montanide ISA 760VG, and a non-adjuvanted vaccine. Duplicate groups of salmon (23.5 ± 6.8 g) were vaccinated with each of the vaccine formulations or phosphate buffered saline by intraperitoneal injection. Fish were challenged by intramuscular injection with F. psychrophilum six weeks post-vaccination to test the efficacy of the vaccines. Cumulative mortality reached 70% in the control salmon, while the groups of salmon that received vaccine had significantly lower mortality than the controls (p = 0.0001), with no significant difference in survival between vaccinated groups. The squalene/alum adjuvant was safe, more readily metabolised by the fish and induced less histopathological changes than the traditional oil adjuvant.

Efficacy of a polyvalent injectable vaccine against Flavobacterium psychrophilum administered to rainbow trout (Oncorhynchus mykiss L.)

“…Previous studies using inactivated F. psychrophilum in conjunction with oil adjuvants have demonstrated protection, usually associated with increased levels of specific IgM in the serum of vaccinated fish (Fredriksen et al., ; Hoare, Jung, et al., ; Hoare, Ngo, et al., ; Högfors, Pullinen, Madetoja, & Wiklund, ; Madetoja et al., ). The protection in the current study appears to be at least partly mediated by specific serum antibodies (IgM) as shown by ELISA, agglutination and Western blotting.…”

“…Two isolates of F. psychrophilum recovered from trout and one recovered from Atlantic salmon in the UK in 2013 were used to make a formalin‐killed whole cell vaccine (FKC; AVU‐1T/13, serotype Fd; AVU‐2T/13, serotype Th; and AVU‐3S/13, serotype FpT) as described previously (Hoare, Jung, et al., ; Hoare, Ngo, et al., ). The three cultures were mixed in equal parts to form the whole cell vaccine at a final concentration of 1 × 10 9 colony‐forming units (CFU) per ml.…”

“…Enzyme‐linked immunosorbent assay (ELISA) was used to assess specific IgM titres to F. psychrophilum in serum according to Hoare, Jung, et al. () and Hoare, Ngo, et al. ().…”

“…Suspensions of the F. psychrophilum vaccine and challenge strains were aliquoted into 1.5‐ml micro‐centrifuge tubes (1 ml of 2 × 10 8 CFU/ml) and centrifuged for 15 min at 3,000 × g . Bacterial pellets were prepared and applied to a 12% polyacrylamide gel (Bio‐Rad) according to Hoare, Jung et al (). Gels were stained with Coomassie Brilliant Blue (Bio‐Rad) or subjected to Western blotting.…”

“…The work revealed high heterogeneity in the strains of F. psychrophilum and highlighted the need for such studies to enable selection of vaccine candidates with the potential for protection of both trout and salmon. The vaccine developed from these studies has been applied to salmon via injection and trout fry by immersion vaccination, resulting in excellent levels of protection (Hoare, Jung, et al., ; Hoare, Ngo, Bartie, & Adams, ). As immersion vaccines are currently lacking mucosal adjuvants, the protection afforded by this method would only cover the early life stages.…”