Evaluation of protection efficiency of different vaccination programs against velogenic Newcastle disease virus in broiler chickens: Comparative in field and in laboratory studies

This study was conducted to evaluate the protective efficiency of different vaccines and vaccination programs against Newcastle disease virus (NDV) in chickens, in the field and the laboratory. In the field, three commercial chicken farms (LA, LB, and QC) used different types of live vaccines and vaccination programs via drinking water (DW) were serologically estimated by hemagglutination inhibition (HI) assay to detect the level of circulating antibodies against NDV. The laboratory study was carried out to assess whether the types of vaccine and their route of administration influence the HI level and viral shedding. Chicken groups (G1-G5) except for the control were challenged with velogenic NDV genotype VIIj circulating among vaccinated chickens in Egypt. The protection efficiency was evaluated by HI test pre-and post-challenge, and the virus shedding post-challenge was quantitated by real-time RT-PCR. The results of the field study indicated that although LA and QC farms showed clinical signs accompanied with high mortality after the 2nd (HI=2(2.1)) and 3rd (HI=2(3.2)) vaccination doses, respectively, and velogenic NDV destroyed chickens in these farms, the LB farm showed no deaths with apparently healthy birds (HI=2(3.3)). In the laboratory inactivated and alternative vaccinations have the highest protection HI titer with no virus shedding while live vaccination either in DW or eye drops showed low protection and the virus was detected in chickens from the DW live vaccinated group. In conclusion, the currently used in field vaccination programs and their routes of administration should be reconsidered to combat NDV infection.