Changes in lymphatic organs of layer chickens following vaccination against Marek’s disease: Histological and stereological analysis

Abstract

The aim of this study was to investigate histomorphometrical characteristics of the thymus, bursa of Fabricius and spleen in the chickens vaccinated with a vaccine against Marek’s disease. For this purpose, we used newly hatched chickens of the light hybrid line, obtained from a local hatchery. The chickens were vaccinated on the 5th day after hatching with a bivalent cell-associated Marek's disease vaccine (PFU-2000 per dose). On day 13 both vaccinated chickens and unvaccinated controls were sacrificed, and thymus, bursa and spleen were removed and processed for light microscopy. The serial tissue sections, hematoxylin-eosine stained, were used for histomorphometric analysis. Vaccination against Marek’s disease decreased the relative mass of the lymphoid organs, and caused significant damage of the thymus and spleen in experimental chicken. In addition, vaccination, similar to Marek's disease virus, induced morphometric changes in the lymphoid organs. Namely, it significantly decreased the diameter and volume of lymphoid follicles, volume of follicular medulla and number of cells in the follicular cortex in the bursa of Fabricius. In the thymus, vaccination reduced the thymus volume and the absolute number of thymocytes. However, vaccination against Marek’s disease caused an increase in the diameter, number and volume of lymphoid follicles in the spleen. The present data suggest that vaccination against Marek’s disease was able to induce the immune response in processed organs, although it reduced the mass and number of lymphocytes in the major lymphoid organs.