Dietary vitamin A can improve immune function in heat-stressed broilers

This experiment was undertaken to evaluate the effect of dietary vitamin A on the performance and immune competence of broilers under heat stress (HS). A total of 180 birds, at 22 days of age, were randomly assigned to be reared either at 248C (thermoneutral, TN, 248C, constant) or 248C to 388C (heat stress, HS, cycling) until the age of 42 days. Birds were then supplemented with vitamin A at 750, 1500, 15 000 IU/kg. Each of the 233 factorially arranged treatments were replicated in six cages, each containing five birds. Humoral immunity was assessed by intravenous injection of 7% sheep red blood cells (SRBC) followed by evaluation of serum for antibody titers in primary and secondary responses. Cell-mediated immunity was assessed by using a Sephadax stimulation method to recruit abdominal exudate cells (AEC) to evaluate macrophage phagocytic ability. Body weight (BW) and feed conversion were significantly affected by dietary vitamin A ( P,0.05). HS significantly reduced BW, feed intake and feed conversion ( P,0.05). Numbers of AEC, percentage of macrophages in AEC, phagocytic macrophages, internalized opsonized and unopsonized SRBC were increased by dietary vitamin A ( P,0.05). Both primary and secondary antibody responses were characterized by increasing titers of antibody to SRBC by dietary vitamin A when birds were exposed to HS ( P,0.05). Lymphoid organ weights, antibody responses, incidence of macrophages in AEC and phagocytic ability of macrophages were all significantly reduced under HS. These results indicated that HS severely reduced performance and immunocompetence of broilers, whereas the immune response of broilers improved by dietary vitamin A supplementation under HS.