Intestinal histostructure and immune protection activity of age -1+ carp after consumption of a prebiotic feed additive

Currently, it is important in aquaculture to study and use products and additives, which have positive effects on the digestibility of nutrients by helping to normalize the intestinal microflora, thereby modulating fish immune response. The aim of the study was to assess the effect of a prebiotic based on mannan oligosaccharides produced from the outer walls of Saccharomyces cerevisiae cells on the histological structure, morphometric state of the intestine and activity of cellular and humoral components of nonspecific resistance of age -2+ carp. The experiment was conducted in four analogous ponds, which were stocked with age-1 scaly carp with an average weight of 55–58 g with a stocking density of 1000 fish/ha. Carp had been fed balanced compound feed for 60 days. Experimental groups of fish were fed with a prebiotic as a feed additive in the amount of: 0.025 % – Experiment 1, 0.05 % – Experiment 2 and 0.075 % – Experiment 3. Use of the studied prebiotic did not damage the intestinal histological structure of carp of the experimental groups. The villi height and intestinal crypt depth of carp in Experiment 1 (P < 0.05) and Experiment 2 (P < 0.01) were larger compared to the control group. Experiment 3 showed destructive changes in intestinal mucosa. The results of histological examination indicated the effectiveness of the use of the studied prebiotic in the amount of 0.025 and 0.05 % in the diet of age -1+ carp due to the absence of pathological changes and active morphofunctional state of the intestine, which, in turn, had a positive effect on feed absorption and metabolic processes in fish body. Analysis of the indicators of cellular and humoral links of natural resistance showed that in carp of the second and third experimental groups lysozyme activity of serum was 5 and 11 % (P < 0.001) higher than in the control group. Similar changes, though found in a greater extent, were recorded in the study of bactericidal activity of blood serum, especially in individuals of the second and third experimental groups (by 6.8 (P < 0.01) and 15.2 %). While with carps of the third experimental group, which used a prebiotic drug in the amount of 0.075 %, the content of CEC was 20.5 % higher than in the control group, which indicates an additional antigenic load on the body and is an unfavorable diagnostic factor. At the same time, with the fish of the second experimental group, which received, respectively, 0.05 % of the supplement of the drug “Actigen” to the main diet, probably higher phagocytic activity of blood neutrophils was recorded. A direct dependence of the growth of the phagocytic index and the phagocytic number of neutrophils on the dose of the studied prebiotic was detected. Immunological studies indicated a dose-dependent effect of the prebiotic on the activity of cellular and humoral components of non-specific resistance of carp. Given this, for the effective functioning of innate protective mechanisms, for the increase of the body's immune potential and productivity, it is most appropriate to apply to the diet of carp prebiotic supplement “Actigen” in the amount of 0.025 and 0.05 %.