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The present study investigated the effects of essential oils extracted from lemongrass (Cymbopogon citratus), Kaffir lime (Citrus hystrix) and peppermint (Mentha cordifolia) as feed supplement on growth performance, blood parameters, non-specific immunity and disease resistance against Streptococcus agalactiae of Nile tilapia (Oreochromis niloticus). The fish were fed with diets supplemented with lemongrass, Kaffir lime peel and peppermint essential oil at the same ratio of 0.5 ml/kg feed and a control group was fed with basal diet. After 75 days of feeding, growth performance, blood parameters and non-specific immunity of fish were investigated. After feeding trial, fish were also challenged with S. agalactiae and mortalities were recorded. Results of this study showed that peppermint essential oil at 0.5 ml/kg feed significantly improved final body weight, weight gain and specific growth rate (p < 0.05). All supplemented diets showed significantly increased total erythrocyte count, hematocrit levels and hemoglobin (p < 0.05). In addition, fish fed essential oil supplemented diets showed no significant increases of both lysozyme activity and survival rate following S. agalactiae infection, compared with those of the control group. The results indicated that peppermint essential oil supplementation potentially improved growth performance, some blood parameters and tendency toward decreased mortality after bacterial infection of cultured tilapia.
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