One of the most significant bacterial diseases affecting the Nile tilapia is
streptococcus, which has a significant negative impact on the global economy. As
a result, vaccination is seen to be the best strategy for controlling bacterial disease
in farmed fish. The objective of the current study was to assess the effects of
formalin and autoclaved polyvalent vaccinations on immunological responses,
oxidative stress biomarkers, antioxidant defense, and streptococcal protection in
the Nile tilapia. Healthy fish (30±5g) were intraperitoneally vaccinated with
0.1ml of polyvalent vaccine, containing either formalin or autoclaved
inactivated streptococcus agalactiae, streptococcus iniae and lactococcus
garvieae, while the control group was injected with sterile saline. Serum samples
were withdrawn 14 days post-immunization to assess immunoglobulin M (IgM),
lysozyme activity (LYZO), alkaline phosphatase (AKP) and acid phosphatase
(ACP). In addition, oxidative stress biomarkers [malondialdehyde (MDA), nitric
oxide (NO)], hepatic antioxidant activities [superoxide dismutase (SOD), catalase
(CAT), glutathione reductase (GR), glutathione peroxidase (GPx)] were recorded.
All groups were separately challenged by virulent strains of S. agalactiae, S.
iniae and L. garvieae at the end of the immunization period (14 days). The results
showed that IgM level and LYZO, AKP and SOD activities were significantly
increased in both vaccinated groups, compared to the control group. The relative
percent survival of the immunized Nile tilapia varied from 88.8- 94.0%. These
results confirmed that both polyvalent formalin and autoclaved inactivated
vaccine boost immune response, antioxidant activity and confer an excellent
protective effect against streptococcal infection in the Nile tilapia.