Administration of either 0.001 g/kg ampicillin (A), 0.05 g/kg oxytetracycline (O) or 0.05 g/kg sulphadimidine (S) in feed to broiler chicks for 50 days caused an increased serum concentration of the drug compared to the control birds that were given no drugs. O and S but not A resulted in a significant decrease of the total number of leukocytes, lymphocytes and the size of bursa of Fabricius and thymus but not spleen or body weight. The antibacterials significantly reduced the macrophage phagocytic activity compared to controls. It is suggested that the prolonged administration of O and S to chickens may induce an immunosuppressant effect.
Al-Ankari, A. S., & Homeida, A. M. (1996). Effect of antibacterial growth promoters on the immune system of broiler chicks. Veterinary Immunology and Immunopathology, 53(3–4), 277–283. https://doi.org/10.1016/S0165-2427(96)05609-7