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

Abstract

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=22.1) and 3rd (HI=23.2) vaccination doses, respectively, and velogenic NDV destroyed chickens
in these farms, the LB farm showed no deaths with apparently healthy birds (HI=23.3). 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.