Comparison of histopathological changes induced by the VII.2 and VII.1.1 genotypes of Newcastle disease virus in SPF chicks and commer-cial layers

Despite the widespread use of vaccination against Newcastle disease virus (NDV), the virus is still circulating in poultry farms. The present study was conducted to measure different histopathological effects and protective antibody levels in experimentally infected chickens with VII.2 and VII.1.1 NDV virulent strains. In this study, commercial layers and SPF(Specific Pathogen-Free) chickens were challenged with NDV genotypes VII.1.1 and VII.2 via oculo-nasal route.  Tracheal and cloacal swabs were collected from all chickens on the fourth day after challenge, then chicks were euthanized in order to collect tissue samples. Tissue specimens were chosen from trachea, proventriculus, heart, kidney, lung, spleen, brain, intestine and liver for evaluation and histopathological grading. All SPF chicks were died within the first 48h of experiment. The HI results of a day before challenge revealed the antibody titer of 6.46 in commercial layers and 0 in SPF chicks. The histopathological results did not show any severe damage in tissues of proventriculus and heart. Both SPF groups showed hemorrhages in tracheal tissues. The most lesions were observed in lung and kidney tissues with more severity in SPF chickens and also more severe in VII.2 infected groups rather than the two VII.1.1 infected groups. In conclusion, according to higher antibody titers and lower histopathological lesions in commercial layers, it can be concluded that antibodies in commercial flocks can prohibit viral replication to a great extent.