BACKGROUND: Newcastle disease virus (NDV) is one of the most principal contagious diseases with a significant role in multifactorial respiratory diseases in the poultry industry. OBJECTIVES: This study aims to evaluate and detect NDVs in multicausal respiratory diseases. METHODS: A total of 180 tracheal swabs were collected from 20 commercial broiler flocks during 2014- 2015. Three tracheal swabs from each flock were pooled. Sixty samples were amplified in PCR by specified primers for fusion gene that flanked cleavage site. Twelve samples were purified and sequenced by a two-way direction method. Based on the amino acids and nucleotide similarities in the part of the F gene, the phyloge- netic analysis of the fusion genes was generated. RESULTS: Based on PCR detection, 26 of 60 (43%) samples and 12 of 20 (60 %) flocks were positive for NDV. Sequence analysis of 12 positive samples revealed that 5 viruses (41.66 %) belonged to the velogenic strains (genotype II, subgenotype VIId) and 7 viruses (58.33 %) were included in the lentogenic pathotypes. Based on the nucleotide similarities, it is concluded that some of the lentogenic pathotypes have 100% close- ness to B1 and Lasota strains. Likewise, the 5 virulent ND virus strains of this study show 100% homogeneity to some previously identified velogenic isolates. CONCLUSIONS: A high percentage of ND viruses were detected in broiler flocks with respiratory com- plexes. Among these detected viruses, it is concluded that the lentogenic isolates and vaccinal strains can be effective in exacerbating respiratory problems.

BACKGROUND: Newcastle disease virus (NDV) majorly infects the poultry, and despite high rates of vaccination, it is still circulating in different geographical regions. Due to the high mortality rate, the economic loss of Newcastle disease (ND) is enormous. OBJECTIVES: The molecular characterization of NDV isolates from chicken farms in Northern Iran, during 2017-2018, was the main goal of this study. METHODS: We isolated and characterized five NDVs from commercial broiler and backyard chicken farms during severe disease outbreak. The partial coding sequence of fusion (F) genes of isolates was determined and compared with those of other published NDVs. RESULTS: Phylogenetic analysis revealed that all of the isolates were grouped into sub-genotype VII.1.1 (formerly known as VIIl). All isolates carried multi‐basic amino acid residues at the cleavage site of fusion protein, typical of virulent strains. Studied NDV isolates had high homology with the prevalent genotype NDV strains that currently circulate in China and Republic of Korea (96.94%). CONCLUSIONS: Our results suggested that NDV sub-genotype VII.1.1 (VIIl), circulating among chicken farms, may be a dominant sub-genotype. Considering the genetic variation between the used vaccine strains (B1, LaSota, and Clone 30, all belonging to genotype II) and circulating NDVs, it is recommended that a contemporary homologous virus should be developed as the vaccine strain to avert the outbreaks of genotype VII viruses.