الفهرس 
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Abstract
In this study, we investigate the effect of coinfection with avian influenza (subtype H9N2) and velogenic Newcastle disease virus on the pathogenesis and virulence both viruses. In co-infection experiment, one strain of H9N2 by titer 106 EID50 and one strain of VNDV by titer 103 EID50 were used. In experimental design, 70 specific pathogen free chicks (four weeks old) were grouped into 7 groups (G1-G7) ten birds/group. group 1 was uninfected control group, group 2 and 3 were positive control for H9N2 and VNDV respectively, group 4 infected firstly with VNDV and after 3days it was infected by H9N2 and vice versa in group 5 while in group 6 two studied viruses were inoculated simultaneously. The birds in group 7 were infected firstly by H9N2 and after 14 days were infected with vNDV. The obtained results showed that the mortality due to vNDV infection (G 3) and in group 7 started from 6 Days post infection (DPI) and reached 100% at 9DPI, while the mortalities started at 2 DPI in G4 and at 4DPI in G5,6 also the mortalities were reached 100% at 6,7,8DPI in G4,5and 6. The VNDV shedding in control group (G3); G7; G6