Abstract
Olive flounder (Paralichthys olivaceus) is the most valuable aquaculture species in Korea, corresponding to ~60% of its total production. However, infectious diseases often break out among farmed flounders, causing high mortality and substantial economic losses. Although some deleterious pathogens, such as Vibrio spp. and Streptococcus iniae, have been eradicated or contained over the years through vaccination and proper health management, the current disease status of Korean flounder shows that the viral hemorrhagic septicemia virus (VHSV), Streptococcus parauberis, and Miamiensis avidus are causing serious disease problem in recent years. Furthermore, these three pathogens have differing optimal temperature and can attack young fingerlings and mature fish throughout the year-round culture cycle. In this context, we developed a chitosan-poly(lactide-co-glycolide) (PLGA)-encapsulated trivalent vaccine containing formalin-killed VHSV, S. parauberis serotype-I, and M. avidus and administered it to olive flounder fingerlings by immersion route using a prime-boost strategy. At 35 days post-initial vaccination, three separate challenge experiments were conducted via intraperitoneal injection with the three targeted pathogens at their respective optimal temperature. The relative percentages of survival were 66.63%, 53.3%, and 66.75% in the group immunized against VHSV, S. parauberis serotype-I, and M. avidus, respectively, compared to the non-vaccinated challenge (NVC) control group. The immunized fish also demonstrated significantly (p < 0.05) higher specific antibody titers in serum and higher transcript levels of Ig genes in the mucosal and systemic tissues than those of NVC control fish. Furthermore, the study showed significant (p < 0.05) upregulation of various immune genes in the vaccinated fish, suggesting induction of strong protective immune response, ultimately leading to improved survival against the three pathogens. Thus, the formulated mucosal vaccine can be an effective prophylactic measure against VHS, streptococcosis, and scuticociliatosis diseases in olive flounder.
Highlights
Disease prevention is the key to improving production and profitability in aquaculture worldwide
In the viral hemorrhagic septicemia virus (VHSV) challenge experiment, majority of the infected fish in the NVC control group showed visible ascites at 3–5 days post-challenge with the first mortality occurring at 6 dpc, whereas in the immunized groups, 4–5 fish out of 30 fish showed apparent bulging of the abdominal cavity relatively later at 7–8 dpc, and the remaining fish showed no remarkable macroscopic signs
The immunized fish demonstrated a steady and significant (p < 0.05) increase in antibody titers postimmunization and post-challenge, in which the anti-VHSV (Figure 2A) and anti-M. avidus (Figure 2C) titers increased to ~60% percentage inhibition (PI) after primary immunization and reached up to 80% PI at 48 hpc, whereas the anti-S. parauberis type I (Figure 2B)
Summary
Disease prevention is the key to improving production and profitability in aquaculture worldwide. With the increase in variation of pathogens affecting a single fish species during its year-round culture cycle, it is imperative to develop multivalent vaccines to protect the cultured stock in a cost-effective manner using a single-vaccination program. In Korea, olive flounder (Paralichthys olivaceus) is one such economically important aquaculture species, which contributes approximately 58.1% of Korea’s total aquaculture production [2], but it is affected by various pathogenic diseases throughout the culture cycle, causing huge economic losses every year. This study initially aimed to develop a multivalent vaccine against VHSV, S. parauberis serotype I, E. tarda, and M. avidus, but due to the low efficacy (i.e., no protection) of the vaccine against E. tarda, we excluded the E. tarda antigen and re-formulated the vaccine against three pathogens
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