Molecular and pathological investigation of a natural outbreak of Newcastle disease caused by genotype XVII in White Leghorn chickens

Newcastle disease (ND) is an infectious viral poultry disease with great economic consequences. In developing countries, outbreaks of ND caused by virulent Newcastle disease virus (NDV) have been identified as a limiting factor to the growth of the poultry industry. Limited reports exist on the pathology of natural field infection caused by NDV genotype XVII in chickens. Here, we present clinical, pathological and molecular investigation of confirmed ND in a 24-week-old layer-type, semi-intensive poultry flock with recorded mortality of over 50%. During PM examination, tissues were harvested for virus isolation, histopathology and immunohistochemistry. Virus isolation was performed in 10-day-old embryonated chicken eggs, and a haemagglutinating agent thereof identified by one-step reverse transcription-polymerase chain reaction (RT-PCR). For the genotyping of the isolate, the full fusion gene was sequenced. Clinical signs observed included general body lethargy, inappetence and greenish diarrhoeic faeces from the cloaca before death with daily mortality exceeding 100 chickens. The pathology was characteristic of a viral haemorrhagic infection, with serosal haemorrhages, mucosal surface erosion and ulceration. In most of the carcasses, the main lesions seen included airsacculitis, meningeal congestion, haemorrhagic oophoritis, pancreatic necrosis, enteritis and faecal matting of the vent. Virus isolation and RT-PCR made a confirmatory diagnosis of ND. Based on the cleavage site motif sequence ( 112 RRQKR/F 117 ), the isolate was identified as a virulent strain with phylogenetic analysis showing clustering in genotype XVII viruses. To the best of the authors' knowledge, this is the first report describing the pathological findings of a natural outbreak caused by NDV involving viruses of genotype XVII. RESEARCH HIGHLIGHTS First report of a natural outbreak of Newcastle disease in White Yarkon Leghorns. The outbreak was caused by virulent NDV belonging to genotype XVII. Pathology differed slightly from those in experimental studies using SPF and other unvaccinated chickens.