Effects of Streptococcus agalactiae Infection on Blood and Hepatopancreatic Tissue Biochemical Indices in Different Species of Tilapia

Abstract:

Nonspecific immunity plays a vital role in pathogen infection in fish. The bacterium Streptococcus agalactiae is an important pathogen in tilapia cultivation, but its effects on tilapia blood and hepatopancreatic biochemical indices are unclear. We injected three strains of Nile tilapia (Oreochromis niloticus)  a disease-resistant genetically improved farmed tilapia (GIFT) strain, a GIFT Baigui strain, and hybrid tilapia (O. aureus × O. niloticus)  with S. agalactiae, then monitored superoxide dismutase (SOD), catalase (CAT), acid phosphatase (ACP), phosphatase (AKP), lysozyme (LZM), and total antioxidant capacity (T-AOC) enzyme activities in blood and hepatopancreatic tissues. Mortality rates in the hybrid tilapia, GIFT disease-resistant strain, and GIFT Baigui strain were 55.4%, 60.5%, and 78.6%, respectively. Post-infection, enzyme activities of ACP, AKP, SOD, and T-AOC in hepatopancreatic tissues of all strains changed, first increasing then decreasing, and AKP and CAT activities after 24 h of infection were significantly higher in the hybrid tilapia than in the two GIFT strains. ACP, AKP, LZM, CAT and T-AOC enzyme activities in blood increased after infection, but SOD activity decreased; ACP, AKP, CAT and T-AOC activities in the hybrid strain were significantly higher than in the two GIFT strains. Using activity changes of the six enzymes and infection mortality rates, we identified AKP and CAT as potential indicators of streptococcal resistance in strains of tilapia.