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Asian seabass (Lates calcarifer) is an economically important fish species that is widely cultivated in Thailand. However, aquaculture of Asian seabass is limited by infectious diseases. One of the most serious diseases is photobacteriosis, caused by Photobacterium damselae. Vaccination is recognized as an efficient disease prevention and pathogen control method for strengthening the aquaculture industry. To promote vaccine development, the characterization of pathogenic bacteria and their pathogenesis is required. In this study, isolates of P. damselae were obtained from commercial aquaculture farms in Thailand during 2019–2021. Analyses of 16S rRNA and the urease subunit alpha genes identified the isolates as P. damselae subsp. damselae (Phdd). Antibiotic susceptibility analyses showed that all Phdd isolates were resistant to amoxicillin (10 μg). Haemolysis and phospholipase activities were used to categorize P. damselae into three groups based on their biological activities. The pathogenicity of four candidates (SK136, PD001, PD002 and T11L) was tested in Asian seabass. Isolate SK136 showed the highest virulence, with a lethal dose (LD50) of 1.47 × 105 CFU/fish, whereas isolate PD001 did not show any virulence. Genotypic characterization, based on multi‐locus sequence typing analysis, demonstrated that all candidates were novel strains with new sequence types (64, 65, 66 and 67). Preliminary vaccination using formalin‐killed cells (FKCs) protected Asian seabass from artificial challenges. Taken together, these results provide fundamental knowledge for vaccine development against Phdd infection in Asian seabass. [ABSTRACT FROM AUTHOR]

The live attenuated vaccine P7‐P8 strain against herpesviral haematopoietic necrosis, which is caused by cyprinid herpesvirus 2 (CyHV‐2), exhibits high protective efficacy in goldfish at 25°C, the predominant temperature for this disease; however, the effect of water temperature during the vaccination period on efficacy has not been determined. In this study, an in vitro experiment revealed that the vaccine strain grew between 15 and 30°C in the goldfish cell line RyuF‐2. Subsequent in vivo efficacy tests were conducted with vaccination temperatures ranging from 15 to 30°C. During the vaccination period, organs were sampled to determine the vaccine growth dynamics. Blood plasma was collected to assess anti‐CyHV‐2 antibody titres. The protective efficacy of the vaccine at 15, 20, 25, and 30°C after subsequent virulent CyHV‐2 challenge resulted in a relative percentage survival of 73.3%, 77.8%, 100%, and 77.8%, respectively, which indicated that the vaccine is effective over this temperature range. The vaccine virus load in the spleen was lowest at 15°C (103.7DNA copies/mg) and highest at 25°C (106.5DNA copies/mg). This indicates that the vaccine virus load over 104DNA copies/mg may elicit sufficient acquired immunity. No significant differences in antibody titre were observed between groups, which suggests that cell‐mediated immunity can be fundamentally involved in protection. [ABSTRACT FROM AUTHOR]