Bwala , D.G., F.O. Fasina, A. Van Wyk, and N.M. Duncan. 2011. “Effects of Vaccination With Lentogenic Vaccine and Challenge With Virulent Newcastle Disease Virus (NDV) on Egg Production in Commercial and SPF Chickens”. International Journal of Poultry Science 10 (2): 98–105. https://doi.org/10.3923/ijps.2011.98.105.