Effects of dietary acidifiers on growth, hematology, immune response and disease resistance of Nile tilapia, Oreochromis niloticus

Organic acids or their salts can be used as feed additives in aquaculture. This study was conducted to evaluate the use of a mixture of formic acid, propionic acid and calcium propionate compared with oxytetracycline (OTC). A total of 720 apparently healthy Oreochromis niloticus fingerlings with an average body weight of 28.8 +/- 0.14 g (mean +/- SE) were randomly divided into four equal groups (G(1), G(2), G(3) and G(4)). G(1) was fed a basal diet with no additive as the control group, while G(2) and G(3) were fed a formic and propionic acid/salt mixture in 1 and 2 g/kg, respectively. G(4) was fed OTC (0.5 g/kg). Each group was subdivided into tow subgroups A and B (30 fish/subgroup) in triplicate. The first subgroup was used to evaluate growth performance, hematology and body composition for 60 d. The second subgroup was used to examine immunity, gut microbiota and resistance to infection for 30 d. At the end of the feeding period (60 d), G(3) had significant improvements in final body weight (FBW), weight gain (WG), specific growth rate (SGR) and food conversion ratio (FCR) compared with other groups. The total erythrocyte count, hemoglobin content, platelet count, hematocrit, mean corpuscular hemoglobin and total leukocyte count were significantly increased in G(3) and G(2) compared with G(1) and G(4). Mean corpuscular volum, lymphocyte and neutrophil percentages had the highest significant improvement in G(3). There were no significant differences among the groups in mean corpuscular hemoglobin concentration and monocyte percentage. The protein and fat contents of the whole body were the highest in G(3). The widest inhibition zones against Aeromonas sobria were at the 30, 40 and 50% concentrations of acidifiers, which were equivalent to OTC (30 mu g). G(3) showed the lowest total gastrointestinal bacterial counts, followed by G(2). After 15 and 30 d, G(3) had the highest serum killing, lysozyme and nitric oxide activities. Serum lysozyme activity and nitric oxide assay had no significant difference between G(1) and G(2) after 30 d. The lowest immune parameters were recorded in G(4). After 30 d, the highest expression of interleukin-1 beta and tumor necrosis factor-alpha in the liver and kidney were found in G(3). The best protection against challenged Aeromonas sobria was in G(3), followed by G(2) and G(4). Dietary supplementation with a combination of formic acid, propionic acid and calcium propionate improves the performance of Nile tilapia. (C) 2016 Elsevier Ltd. All rights reserved.