Preventive effect of starch hydrogel-based oral vaccine against Aeromonas salmonicida infection in rainbow trout (Oncorhynchus mykiss)

In rainbow trout aquaculture, furunculosis infection, caused by Aeromonas salmonicida, results in significant losses. Although vaccination is considered one of the most effective procedures against furunculosis, in Korea, there is no vaccine available for rainbow trout. Thus, the present study investigated the effect of a starch hydrogel-based oral (SHO) vaccine on antibody reactivity, immune-related genes expression, and protection against furunculosis infection in rainbow trout. In addition, the effect of booster vaccination was evaluated by comparing the immunostimulatory effect of a single vaccination with that of a booster vaccination. To determine the optimal SHO vaccination strategy, fish were individually orally administered the vaccine as follows: vaccinated only once (VT1), booster-vaccinated once 3 weeks after the initial vaccination (VT2), booster-vaccinated twice at 3-week intervals (VT3), or booster-vaccinated three times at 3-week intervals (VT4). Serum agglutination test showed that agglutination titers in booster-vaccinated groups were higher than those in the single-vaccinated group. The survival rate of rainbow trout was assessed after introduction of A. salmonicida at 11 weeks post vaccination. The relative survival rate was investigated as follows: 100% when challenged with 10² colony-forming units (CFU)/fish; 69.84 ± 2.75% when challenged with 10³ CFU/fish; and 65.28 ± 2.41% when challenged with 10⁴ CFU/fish. The results revealed that the expression of immune-related genes such as tumor necrosis factor α (TNFα), interleukins (IL-1β, IL-6, IL-8, and IL-10), IgM, IgDs, heat shock protein (HSP) 70, complement factor C3, serum amyloid protein A (SAA), MHC II, and transforming growth factor β (TGF-β) increased in SHO-vaccinated fish, whereas IgDm decreased. These results indicate that the SHO vaccine may be a promising oral vaccine for rainbow trout aquaculture.