Structural-functional characteristics of thymus in piglets with signs of latent and subclinic PCV2-infection
Authors: Evert V. V. 
Published: November 2018

Circovirus type II disease is an infectious disease with significant economic impacts on the pig industry, which has become global in the pig population (PCVD). The most common form of PCV2-infection has long been considered a PCV2-systemic disease, but recently latent and subclinical forms of PCV2-infection have become of particular importance. The main “target” for PCV-2 is the immune system, and the multiplication of PCV-2 in the cells of the immune system leads to their death and development of immunodeficiency state. The immune system, together with the nervous and endocrine systems, plays an important role in maintaining the body's homeostasis and ensures its adaptation to the effects of unfavorable environmental factors. The organs of the immune system of pigs include bone organs, thymus, spleen, lymph nodes, as well as diffuse and aggregated lymphatic formations of the walls of the digestive canal and respiratory tract. Thymus is the central organ of lymphocytopoiesis and imunopoiesis, in it occurs differentiation of all cellular forms of mature T-lymphocytes in the pre-T-lymphocyte stage, which migrate from the bone marrow to the bloodstream. The growth and development of the thymus, as well as the bone organs, is in direct interaction with the growth and development of the spleen, lymph nodes and is reflected in the structure of the interstitial connective tissue and lymphoid parenchyma, and the question of the structural and functional characteristics of the thymus piglets with signs of latent and subclinical PCV-2 infection in accessible the scientific literature is not sufficiently elucidated and requires a detailed study. Purpose: determination of the structural and functional characteristics of the thymus of piglets with signs of latent and subclinical PCV-2-infection. The work was carried out in pig farms of Ukraine, with intensive technology of pig rearing, at the department of normal and pathological anatomy of agricultural animals and the Scientific and Research Center for Biosafety and Environmental Control of the Agroindustrial Complex Resources of Dnipro State Agrarian and Economic University. To determine the structural and functional characteristics of the thymus of piglets at different forms of the of PCV-2-infection, monitoring studies of blood sera were carried out by quantitative PCR-analysis. A total of 275 samples of blood serum from piglets of 5–16 weeks of age were investigated. For pathomorphological studies with the method of acute bleeding, animals with signs of latent PCV2-infection (in 1 cm3 of blood serum of 103–104 copies of the genome of PCV-2 equivalents) were slaughtered and animals with signs of subclinical PCV2 infection (in 1 cm3 of blood serum 105–106 copies of the gene equivalents of PCV-2) 6 heads. By anatomical preparation, the thymus was removed, fixed in a 10% solution of neutral formalin, poured into paraffin, sections were stained with hematoxylin and eosin and azur II eosin according to generally accepted techniques. In the paraffin sections, the presence of PCV-2 antigen (immunohistochemical staining) was also determined. In the lymphoid parenchyma of the thymus, the percentage ratio between individual cells (large, medium, small lymphocytes, plasmocytes and reticular cells), macrophages and other cells was determined. Cell counting of parenchyma was performed on dasgs stained with azur II eosin at an increase of 10 x 100 under the immersion for every 100 cells on ten preparations in 20 fields of vision for each individual group of animals. Based on the data obtained, the average percent content of each cell type was determined. The author, on the basis of pathohistological and immunohistochemical studies, determined the structural and functional characteristics of the thymus of piglets with signs of latent and subclinical PCV2-infection. It has been established that in piglets with signs of subclinical and latent PCV2-infection, pathogistological changes of the thymus do not differ significantly from the corresponding thymus parameters of clinically healthy animals free from PCV-2. At the same time, there is a tendency to decrease the number of lymphoid cells, mainly immunoblasts and lymphocytes, and, accordingly, the area of the particle cortex, which is more pronounced in thymus of pigs with signs of subclinical PCV2-infection.