Effect of Terminalia catappa methanol leaf extract on nonspecific innate immune responses and disease resistance of red hybrid tilapia against Streptococcus agalactiae

The current study focused on the immunostimulatory effects of intraperitoneal administration of the medicinal plant, Terminalia catappa leaf extract in red hybrid tilapia (Oreochromis mossambicus x O. niloticus). Fish were divided into four groups of 30 fish each. Group 1 served as the methanol control, administered 0.1 mL of methanol. Group 2 served as the negative control without any extract induced. Group 3 and 4 were administered intraperitoneally with 31.25 mg kg(-1) and 62.5 mg kg(-1) of Terminalia catappa methanol extract, respectively. The effect of the extract on some innate immune responses such as phagocytosis, respiratory burst activity, complement activity, lysozyme activity and resistance against Streptococcus agalactiae was determined after weeks 1, 2 and 4 post-injection respectively. Results indicated that the extract of 31.25 mg kg(-1) and 62.5 mg kg(-1) significantly enhanced the phagocytic, respiratory burst and lysozyme activities after two weeks. No significant difference was observed in the complement activity assay throughout the experimental period in the groups administered Terminalia catappa methanol leaf extract. However, a sequential decline in the innate immune response was observed in the methanol control group as the week progressed from week 1-4. Following the challenge with S. agalactiae, there was significantly lower percentage mortality in the group of fish administered 31.25 mg kg(-1) and 62.5 mg kg(-1) Terminalia catappa methanol leaf extract, thereby increasing the relative percentage survival of the fish. The highest dose of 62.5 mg kg(-1) gave better protection than the 31.25 mg kg(-1). The results indicated that the intraperitoneal injection of Terminalia catappa leaf extract enhanced the nonspecific immune responses and disease resistance of red hybrid tilapia against S. agalactiae.