Clinicopathological and Molecular Investigation of Newcastle Disease Outbreaks in Vaccinated and Non-Vaccinated Broiler Chicken Flocks in Nepal.

Newcastle disease (ND) is a highly contagious viral disease caused by the paramyxovirus, which is a single-stranded ribonucleic acid (RNA) virus. This study was conducted to investigate ND outbreaks in 10 vaccinated or non-vaccinated broiler farms, collectively housing 9840 birds of various ages in the Chitwan and Nawalpur districts of Nepal from July to December 2021. Clinically, the affected birds exhibited symptoms such as limb paralysis, greenish diarrhea (seven out of ten flocks), torticollis (two out of ten flocks), inappetence, and drowsiness (ten out of ten flocks). Birds that succumbed during the clinical course underwent a necropsy for gross pathology and samples were collected for the histopathology and molecular diagnosis. The gross and microscopic examination revealed hemorrhages in the proventriculus, erosions and ulcers in the small intestine, congestion, as well as sero-mucosal hemorrhages in the trachea of affected birds, which are typical of ND. Rapid test kits further confirmed the presence of the ND virus antigen while excluding the avian influenza virus. Furthermore, M gene-based real time polymerase chain reaction (RT-PCR) was performed in the pooled samples from the affected birds and the presence of a velogenic strain of the ND virus was identified. The phylogenetic analysis of the RT-PCR positive strain based on the partial F gene nucleotide sequence revealed these strains as genotype VII.2 (formerly VIIi). The findings highlight the occurrence of clinical ND outbreaks in farms despite adherence to recommended vaccination protocols in broiler flocks, underscoring the need for a regular comprehensive investigation involving in-depth examinations of available vaccines and genetic analyses.