Objectives: Virulent Newcastle disease is a serious viral disease of domestic poultry causing severe losses in poultry industry worldwide. Lentogenic, mesogenic and velogenic are the three main pathotypes of the virus. Based on pathological manifestation, velogenic forms of NDV are further classified as neurotropic or viscerotropic. Virulence of the virus is mainly determined by the amino acid sequence of the cleavage site of fusion (F0) protein and the ability of specific cellular proteases to cleave the protein of different pathotypes. During the last few months severe outbreaks of Newcastle disease were observed among poultry population of Khorasan Razavi province, located in Materials & Methods: Based on the history of vaccination against NDV, affected industrial laying flocks experienced various degrees of drops in egg production and mortality. Laying flocks, even with HI titer of Log211, presented with drop in egg production. From three flocks, brain samples were collected and homogenized, followed by inoculation in embryonated chicken eggs, using standard protocols. The allantoic fluids were evaluated by hemagglutination (HA) and hemagglutination inhibition (HI) assays with NDV-specific antibodies. Positive allantoic fluids were employed for RNA extraction, followed by RT-PCR assay using specific primers, able to amplify the cleavage site of fusion gene. The PCR products were sequenced and analyzed. Results & Conclusion: All three isolates from laying flocks with viscerotropic velogenic ND presentation were positive in HA and HI assays. RT-PCR assay proved the NDV identity of all isolates. The nucleotide sequence of amplicons revealed that the isolates were identical to recent isolates from ND outbreaks in broilers. Analysis of the deduced amino acid sequences of the F protein cleavage site confirmed that all recent isolates were virulent strains. They had the amino acid sequence 111-GRRQKRF-117 in their F0 cleavage site. This investigation indicate the NDV virus circulating in recent outbreak of ND in laying flocks of Northeast Iran, are virulent and identical to the virus isolated from broiler flocks in the same region.

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