الفهرس 
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Abstract
A study was conducted to evaluate the effect of some epidemiological and immunological factors may affect genetic changes of live Newcastle Disease virus (NDV) vaccinal strain LaSota. A total number of two hundred and seventy ( 270) one-day-old commercial broiler Cobb chicks were divided into 9 equal groups (30 birds in each) according to different selected factors; route of administration, exposure to mycotoxins naturally contaminated ration, and co-infection with low pathogenic Avian Influenza (AI) virus (H9N2). The Humoral immune response, Cell-mediated immune response in addition to protection rates against Velogenic Viscerotropic Newcastle disease virus VVNDV were recorded. One hundred and eighty shedded vaccinal virus samples were collected. Only eighteen positive HA samples were submitted to sequence analysis to evaluate any genetic changes under the effect of the selected factors. Results proved that Mycotoxins and H9N2 co-infection have a negative effect on immune response to live (ND) vaccines even by spray route of vaccine application. Nucleotide alignment and Phylogenetic analysis revealed that all shedded samples are belong to Genotype II with only one genetic change at site 118 Amino Acid of the F gene after the end of the cleavage site of the F gene at site 117 at only three samples from birds with H9N2 Co-infection which affect virus stability and virulence. This study considered as the first study in Egypt in where this genetic deviation was monitored. All results were in comparing with negative control group.