Immunomodulation by B-glucan in tilapia (Oreochromis niloticus) vaccinated and challenged with Streptococcus agalactiae: immunological and hematological variables

Abstract

The growth of Brazilian fish farming has led to an increase in the occurrence of diseases and consequent losses in production, and several studies have evaluated the use of immunostimulants for disease prevention and control. Among them, ²-glucan is the most studied compound and used in fish feeding. There is evidence that ²-glucan administration can effectively activate various defense mechanisms. Thus, the objective of the present work will be to evaluate the effects of this immunostimulant on innate immune responses of tilapia through immunological and hematological variables. A total of 300 tilapia will be distributed in 20 polyethylene boxes and divided into five groups: G 1. Control diets (free of ²-glucan) for 30 days; G 2. Experimental diets (10g kg-1 of ²-glucan) for 15 days, and then control diet (²-glucan free) for 15 days; G 3. Experimental diets (10g kg-1 of ²-glucan) for 15 days, fish will be challenged, and then control diet for 15 days; G 4. Experimental diets (10g kg-1 of ²-glucan) for 15 days, and will be challenged, and then experimental diet for 15 days; and G 5. Experimental diets (10g kg-1 of ²-glucan) for 15 days, will be challenged, and then a diet with antioxidant vitamins and minerals (²-glucan free) for 15 days. At 15 days of the experiment, fish from all treatments will be sampled, blood will be collected to evaluate immunological variables (lysozyme activity, complement hemolytic activity, leukocyte respiratory activity); hematological variables (hemogram, leukogram, and hematimetric indexes); and biochemical variables (total protein, albumin, serum globulin and A: G index) and evaluation of the variables of productive performance (final weight, weight gain, specific growth rate, feed conversion, protein efficiency rate, and condition factor). Then groups 3, 4, and 5 will be challenged with stressful management (stress and infection with 1.0 x 108 CFU of S. agalactiae / fish). At 30 days of the experiment, 4 fish from each box (n = 12) will be sampled and will be collected for the evaluation of the same analyzes. This experiment will be conducted in a completely randomized design with 10 treatments (five groups x 2 sampling times: 15 and 30 days of the experiment) with 4 replicates each.(AU)