The effectiveness of various forms of Zn as stimulators of the immune response in broiler chickens (Gallus gallus L.)

A significant problem of modern poultry farming is insufficient viability of broiler chickens due to various reasons, including immunosuppression. Various biologically active substances and trace elements are successfully used to increase the adaptive capacity and immunoreactivity of birds. The use of zinc (Zn) in feed additives which has immunotropic properties, stimulates immune and antioxidant systems, increases the productivity and safety of animals is of undoubted interest. Here, for the first time the influence of zinc from different sources on natural resistance and morphofunctional reorganization of immunocompetent organs of Smena 7 cross broiler chickens (Gallus gallus L.) was shown. These data also point to the advantage of organic and ultrafine dietary forms of zinc for chick feeding compared to the inorganic form. Our goal was to compare the effectiveness of using different forms of zinc in the diet as modulators of the immune system based on biochemical parameters and characterization of the microstructure of immunocompetent organs. Studies were performed on broiler chickens of Smena 7 cross (three groups, 24 animals in each group) in the vivarium of the Federal Research Center of Biological Systems and Agrotechnologies RAS (Orenburg). Sources of trace elements were asparaginate Zn (organic form, OF; LLC V-Min+, Sergiev Posad, Russia), mineral salts ZnSO4·7H2O (inorganic form, IF; Lenreaktiv, St. Petersburg, Russia) and powder of ultradispersed Zn particles (UDP Zn; LLC Advanced Powder Technologies, Tomsk, Russia). Chickens in the control group received a basic diet throughout the experiment, in which Zn was normalized by the introduction of ZnSO4·7H2O. In the test groups from day 14 to day 42 Zn sulphate was replaced with UDP Zn at a dose of 49 mg/kg feed (group I) or with asparaginate Zn at the same dose (group II). Samples for analysis were collected after poultry slaughtering at 3, 4, 5, and 6 weeks of age. Biochemical studies of blood serum were performed on an automatic analyzer CS-T240 (DIRUI Industrial Co., Ltd., China) using commercial kits for veterinary research DiaVetTest (JSC Diakon-DS, Russia). Morphological composition was determined using an automatic hematological analyzer URIT-2900 Vet Plus (URIT Medial Electronic Co., Ltd., China). Iindices of natural resistance, i.e., the bactericidal activity of blood serum (BABS), lysozyme activity (AL), β-lysine activity (Aβ-L), and immunological indices, the phagocyte number (PN) and phagocytic index (PI) were evaluated. Morphological characteristics of cloacal bursa (CB), thymus and spleen were determined on 5-6 microns thick histological sections stained with hematoxylin and eosin. General structural changes were assessed on paraffin sections stained with hematoxylin and eosin using a light-optical microscope with MT 5300L software (Meiji Techno Co., Ltd., Japan). The area of the follicle and medullary region, the width of the cortical zone were determined in the CB sections; the area of red and white pulp, cell density of red and white pulp in the spleen; the area of cortical and brain substance, their ratio (cortical index), cell density of red and white pulp in the thymus. The area of structures was determined at 125000 μm2, density at 1 mm2. At 3 weeks of age, chickens of group II showed a 37.5 % (p ≤ 0.05) increase in leukocytes compared to control. By 4 weeks of age, the index also increased by 40.7% (p ≤ 0.05) when UDP Zn was fed. The increase was due to lymphocytes and monocytes. In the test groups, by the end of the experiment, the number of leukocytes was lower than the control. All indicators of white blood cells were within the normal range. Introduction of UDP Zn contributed to an increase in BABS in the range from 5.8 % to 16.7 % and AL by 8.2 % in the later stages of the experiment compared to control. The tendency to an increase in Aβ-L was recorded at 5 weeks of age with a subsequent decrease by the end of the experiment. Application of OF Zn led to a statistically significant (p ≤ 0.01) increase in the activity of BABS by 13.4 % at 3-weeks of age and AL at the end of the experiment (by 8.8 %). The OF Zn provided a smooth progressive increase in Ab-L throughout the experiment. Histological evaluation revealed that dietary OF Zn increased CB activity with an increase in lymphoid follicle area by 64.5 % (p ≤ 0.01) due to expansion of medulla zones, as well as increased cellular density of the cortical zone with a large number of macrophages and mitoses. In the thymus, together with a 20.9 % expansion of the medullary zone, there was a greater number of Hassall’s corpuscles, thickening of cells in the cortical layer and proliferative activity of lymphocytes with activation of macrophages. In the spleen after the introduction of UDP Zn, enlargement of follicles by increasing the area of white pulp 2.2-fold (p ≤ 0.05) was noted.