UTTAR PRADESH JOURNAL OF ZOOLOGY

Aquaculture production is increasingly constrained by bacterial diseases, among which Streptococcus agalactiae is an important pathogen causing streptococcosis in tilapia. Assessment of respiratory burst and lysozyme activity provides useful insight into the innate immune response of red tilapia (Oreochromis spp.) following bacterial challenge. This study evaluated respiratory burst activity, measured using the nitroblue tetrazolium (NBT) assay, and serum lysozyme activity in red tilapia (Oreochromis spp.) experimentally challenged with S. agalactiae. Fish were assigned to a saline-injected control (T1) or intraperitoneally challenged with 10⁶ (T2), 10⁷ (T3), or 10⁸ (T4) CFU/mL. Clinical signs were monitored for 10 days, after which NBT absorbance at 655 nm and lysozyme activity were determined. No visible clinical signs, morbidity, or mortality were observed in the challenged groups during the observation period. Mean NBT absorbance was 0.045 ± 0.001 in T1, 0.048 ± 0.001 in T2, 0.050 ± 0.001 in T3, and 0.053 ± 0.001 in T4. Although values increased with bacterial concentration, differences among treatments were not statistically significant. Mean lysozyme activity was 9.765 ± 0.402 units/min in T1, 10.333 ± 0.654 in T2, 13.235 ± 0.742 in T3, and 13.259 ± 0.986 in T4. Lysozyme activity was significantly higher in T3 and T4 than in T1. Under the conditions of this study, red tilapia displayed measurable innate immune activation following S. agalactiae challenge despite the absence of overt clinical disease. The findings indicate that NBT reduction and lysozyme activity can provide complementary information on the early cellular and humoral immune responses of red tilapia to experimental streptococcal exposure.