Rainbow trout (Oncorhynchus mykiss) farming in India faces a significant threat from the bacterium Lactococcus garvieae strain RTCLI10 (KG+ phenotype). Thus, this study aims to develop an inactivated immersion vaccine (Lg IIV) against this pathogen. The biosafety of LgIIV was assessed in vitro using epithelioma papulosum cyprini (EPC) cell line, and in vivo in rainbow trout (body weight: 0.5 ± 0.01 g) challenge model by immersion (2.4 × 109 cells mL-1). For efficacy assessment of Lg IIV, rainbow trout (body weight: 1.26 ± 0.43 g) received primary (0 days) and booster vaccination (7 days post booster vaccination) (1.2 × 109cells mL-1), followed by challenge with homologous strain, L. garvieae RTCLI10 (KG+ phenotype; LD50: 2.6 × 105 CFU mL-1). EPC cell survivability was above 90 % and cytotoxicity were not observed in these cells, when exposed to LgIIV for 120 h at different dilutions. Lg IIV did not show any adverse effects on survival, behavior or health of exposed rainbow trout post booster vaccination, and no histological changes were observed in gill, liver, kidney, spleen, eye and muscle. The relative percentage survival (RPS) of rainbow trout was 83.78 %, with 96.15 % survival in vaccinated and challenge (VC) group, compared to 76.28 % in non-vaccinated and challenge (NVC) group. Bactericidal activity, lysozyme activity and serum antibody titre were significantly elevated in VC group in between 3 and 5 days post challenge (dpc). Quantitative PCR (qPCR) analysis of 12 immune-related genes in the spleen and anterior kidney showed significantly elevated inflammatory gene expression in VC rainbow trout, in comparison to NVC. Therefore, our study confirms the biosafety and protective efficacy of Lg IIV against L. garvieae infection for sustainable aquaculture in India.
Keywords: Antimicrobial resistance; India; Rainbow trout; Vaccine; Warm water lactococcosis.
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