الفهرس 
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Abstract
In the present study, a molecular epidemiological investigation was conducted to better understand the epidemiology of Newcastle disease virus (NDV) in the chicken farms in Egypt during the last 4 years from 2015 to 2018. A total of 114 positive Newcastle diseases (ND) from 442 chicken farms were suffering from problems such as mortality, respiratory signs, greenish diarrhea and nervous signs, Out of positive NDV flocks (114) 17 were infected with NDV only. A total of 7 NDV isolates recovered from different outbreaks were analyzed for in vivo pathogenicity, biological property and amplification of F cleavage site gene. Mean death time was (MDT: 40.5-59.5), Intracerebral Pathogenicity Index was (ICPI: 1.53-1.86) while the intravenous pathogenicity index was (IVPI: 2.01-2.35) and deduced amino acid sequences of the F0 proteolytic cleavage site (112RRQKR117) revealed that all 7 field isolates were velogenic isolates. Phylogenetic analysis showed that these isolates could be classified into velogenic group (genotype VIId) and one of them had been used to evaluate protective efficacy of 6 different vaccination programs of ND under lab condition in broiler chickens. Two hundred one-day-old commercial broiler chicks were divided into 8 groups (25 birds each). Groups 1, 2, 3 and 6 were vaccinated at 1 and 14 day- old with live attenuated vaccine (lasota) and group 2 and 3 were vaccinated at 1 and 6 day old with inactivated vaccine respectively. Also group 6 was vaccinated at one day old with vectored-ND vaccine. But group 4 and 5 vaccinated at 1 and 14 day old with live attenuated genotype VII vaccine. group 5 was vaccinated at 6 day-old with inactivated genotype VII vaccine. group 7 and 8 were kept as unvaccinated controls. At 24 day of age all vaccinated groups as well as chickens of group7 were challenged with 106 EID50 /bird of vVNDV genotype VII. Our obtained results showed that groups 3 and 5 showed the highest rate of protection (100%) with the lowest shedding of virus. while the groups 3 and 5 were able to stop shedding at 10th dpc. Conclusions: The sequences of the F genes of the 2015-2018 Egypt isolates from the same province are closely related to that of the 2011 Egypt isolate (genotype VIId) suggesting that these strains are probably circulating in the vaccinated bird population in Egypt until development of an outbreak. Regardless to the heterologous or homologous inactivated vaccines, they are very important to decrease the shedding of the virus.