Evaluation of protective effects of heat-inactivated cyprinid herpesvirus-2 (CyHV-2) vaccine against herpesviral hematopoietic necrosis disease (HVHND) in goldfish (Carassius auratus)

Immune responses and protective efficacy of American eel (Anguilla rostrata) immunized with a formalin-inactivated vaccine against Anguillid herpesvirus

Anguillid herpesvirus 1 (AngHV), the causative agent of "mucus sloughing and hemorrhagic septicemia disease", causes serious infectious diseases in farmed eel. Among the effective prevention and control strategies, vaccination is one of the most effective approaches. However, no vaccine for AngHV is available. Our study developed a formalin-inactivated AngHV vaccine and evaluated its performance in American eels. Initially, AngHV-FJ, a strain of AngHV, was inactivated completely by 0.1 % formaldehyde, mixed with adjuvant Montanide ISA 763 A VG (763A). Then, vaccines containing different amount of antigen (3 × 106 PFU, 3 × 105 PFU, 3 × 104 PFU, 3 × 103 PFU) were immunized in each American eels. The results showed that the 3 × 105 PFU/fish was the proper dose. The inactivated AngHV vaccine was proven safe for American eels by back intramuscular injection. The results of twice immunization showed that antibody production peaked in the 8th week after the first immunization, and the antibody titer was 1:64,000. Furthermore, the immunized fishes challenged with AngHV (105 PFU/ml immersion) showed a significantly lower incidence rate (33.33 %) than the control group (95.65 %). The survival of the fish in the vaccine group (94.44 %) was significantly higher than the control group (60.87 %). The relative survival rate of the vaccinated group was 85.80 %. Also, vaccine group tissue collected at 7th d post-challenge showed reduced tissue damage and a lower virus load than the control group. The expression of cytokines of IL-1β, IFN-α, IFN-γ, Mx1, RIG-1, and IRF-3, were significantly lower in the vaccine group than the control group at the 7th and 14th d post-challenge. Overall, the formalin-inactivated AngHV vaccine was safe and had immune protective effects against AngHV infection.

Immersion prime and oral boost vaccination with an inactivated Vibrio harveyi vaccine confers a specific immune response and protection in Asian seabass (Lates calcarifer)

Asian seabass (Lates calcarifer) holds significant economic value in fish farming in the Asia-Pacific region. Vibriosis caused by Vibrio harveyi (Vh) is a severe infectious disease affecting intensive farming of this species, for which prevention strategies by vaccination have been developed. This study investigated an alternative approach to injectable vaccination to prevent vibriosis in Asian seabass juveniles. The strategy begins with an immersion prime vaccination with a heat-inactivated Vh vaccine, followed by two oral booster doses administered at 14- and 28-days post-vaccination (dpv). Expression of five immune genes TNFα, IL1β, CD4, CD8, and IgM in the head kidney and spleen, along with investigation of anti-Vh antibody response (IgM) in both systemic and mucosal systems, was conducted on a weekly basis. The efficacy of the vaccines was assessed by a laboratory challenge test at 43 dpv. The results showed that the immunized fish displayed higher levels of mRNA transcripts of the immune genes after the immersion prime and the first oral booster dose compared to the control group. The expression levels peaked at 14 and 28 dpv and then declined to baseline at 35 and 42 dpv. Serum specific IgM antibodies were detected as early as 7 dpv (the first time point investigated) and exhibited a steady increase, reaching the first peak at 21 dpv, and a second peak at 35 dpv. Although the antibody levels gradually declined over subsequent weeks, they remained significantly higher than the control group throughout the experiment. A similar antibody response pattern was also observed in the mucosal compartment. The laboratory challenge test demonstrated high protection by injection with 1.65 × 104 CFU/fish, with a relative percent of survival (RPS) of 72.22 ± 7.86 %. In conclusion, our findings highlight the potential of an immersion prime-oral booster vaccination strategy as a promising approach for preventing vibriosis in Asian seabass.

In Vivo Imaging Sheds Light on the Susceptibility and Permissivity of Carassius auratus to Cyprinid Herpesvirus 2 According to Developmental Stage

Cyprinid herpesvirus 2 (CyHV-2) is a virus that causes mass mortality in economically important Carassius spp. However, there have been no comprehensive studies into host susceptibility or permissivity with respect to developmental stage, and the major portal of viral entry into the host is still unclear. To help bridge these knowledge gaps, we developed the first ever recombinant strain of CyHV-2 expressing bioluminescent and fluorescent reporter genes. Infection of Carassius auratus hosts with this recombinant by immersion facilitated the exploitation of various in vivo imaging techniques to establish the spatiotemporal aspects of CyHV-2 replication at larval, juvenile, and adult developmental stages. While less susceptible than later developmental stages, larvae were most permissive to CyHV-2 replication, leading to rapid systemic infection and high mortality. Permissivity to CyHV-2 decreased with advancing development, with adults being the least permissive and, thus, also exhibiting the least mortality. Across all developmental stages, the skin was the most susceptible and permissive organ to infection at the earliest sampling points post-infection, indicating that it represents the major portal of entry into these hosts. Collectively these findings provide important fundamental insights into CyHV-2 pathogenesis and epidemiology in Carassius auratus with high relevance to other related economically important virus-host models.