Protective Effects of Olive Leaf Extract as a Natural Growth Promoter and Immune Modulator in Broilers Challenged with Velogenic Newcastle Disease Virus

ABSTRACT Newcastle disease (ND) is a serious menace to poultry production and requires stringent measures to enhance growth and improve resistance against viral challenges in infected flocks. In the present study, the immunostimulatory and growth-promoting effects of olive (Olea europaea) leaf extract (OLE) in velogenic Newcastle disease virus (NDV)-infected broilers were evaluated. A total of 150-day-old broilers (Cobb strain) were randomly allotted to five groups: Blank Control (BC), Negative Control (NC), OLE-1, OLE-2, and OLE-Only. OLE-1 and OLE-2 groups were given 10 mL and 20 mL/L of OLE-supplemented drinking water on day 3, and ND vaccinations on days 0, 3, and 14. The NC, OLE-1, and OLE-2 groups were given an intraperitoneal challenge of NDV on day 21. Weekly inspections comprised clinical signs, survival, antibody levels, weight gain, and hematological indices. Body weight gain and leukocyte number (p<0.05) were significantly enhanced with OLE supplementation, whereas mortality was lowered to 30%. Proventriculus hemorrhage and intestinal ulcers were more severe in the NC group but less severe in the OLE-treated groups. OLE also attenuated ND-caused tissue injury and increased erythrocyte number, hemoglobin content, and packed cell volume. In conclusion, OLE boosts broiler immunity and growth performance without causing any unfavorable physiological effects, hence being a possible natural additive to improve disease resistance and poultry production.