Evaluation of immunity and protection induced in pullets by the V4 oral vaccine against a pneumotropic velogenic Newcastle disease virus (NDV) strain

Newcastle disease (ND), caused by Newcastle disease virus, is an acute, contagious and pathogenic infection of pet, free living and domestic birds. ND is an epidemic disease and it is responsible for high economic losses due to up to 100 % mortality. The control of ND in the intensive commercial poultry farms is largely dependent on prophylactic immunisation using conventional vaccines. The ND V4 vaccine and its derivative ND V4-HR vaccine were selected originally for use in village chickens, due to their immunogenicity, thermostability, transmissibility and ease of administration. The efficacy of V4 and V4HR vaccines have been established in many Asian and African countries in their ability to challenge a wide range of recognised and local velogenic NDV. Therefore, ND V4 was tested for efficacy against B1172 challenge NDV isolated in south Africa in 1993. Twenty-eight one day-old replacement pullets were vaccinated by eye-drop route at 21 and 49 days old. Chickens vaccinated by eye-drop route were left to mingle with the unvaccinated in-contact chickens. At 63 days all chickens including the unvaccinated control group were individually challenged with B1172 NDV. Serological monitoring of NDV antibody response was done using HI and ELISA tests. The ND V4 vaccine induced full protection against B1172 NDV in chickens vaccinated by eye-drop vaccination and in 55 % of chickens vaccinated by the in-contact method. No association was seen between NDV antibody titer at pre-challenge and the ability to withstand B1172 challenge NDV. A fair to good agreement was seen between the HI and ELISA test in monitoring NDV antibody response during the experiment. Although, the ELISA showed a higher sensitivity and specificity than the HI test, further studies are required using this method of comparison.