Submitted to: Avian Diseases
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/18/2002
Publication Date: 6/5/2003
Citation: Avian Diseases 47:319-329, 2003 Interpretive Summary: Newcastle disease virus (NDV) isolates vary in virulence from those that cause severe disease, which is reportable and a cause of trade sanctions, to those that produce only a mild non-reportable disease similar to the common cold of man. Most bird species can be infected with NDV, but because of species differences in susceptibility, new isolates must be tested by inoculation of susceptible chickens to determine their inherent virulence. To simulate the consequences of virus entry and bird to bird spread of NDV in a chicken flock, six selected NDV isolates were passaged serially in chickens. Chickens infected with the passaged isolates were then evaluated for clinical disease and pathology. Only the viruses from a dove and a pheasant produced severe clinical disease and mortality. Gross and microscopic examination of tissues from these birds revealed severe damage to cells known for their immune function which are critical to generating a protective response to infection. The isolate from an anhinga produced no clinical disease but microscopic changes were evident in nervous tissue, heart, and trachea. Isolates from chickens and from a parrot produced no clinical disease and only very mild microscopic changes. Any tissue damage evident microscopically in infected chickens can impact productivity. In this study NDV isolates from different species produced disease forms in chickens that ranged from asymptomatic, but with detectable microscopic lesions, to severe disease. An effective biosecurity system that prevents entry of viruses like these into the production house is the best protection against the unpredictable impact of infection of commercial chickens with NDV isolates, particularly those from different bird species.
Technical Abstract: The pathogenesis of six Newcastle disease virus (NDV) isolates recovered from chickens and wild (anhinga) and exotic (yellow nape parrot, pheasant, and dove isolate) birds was examined after four passages of the isolates in domestic chickens. Groups of four-week-old specific-pathogen-free White Leghorn chickens were inoculated intraconjunctivally with the passaged isolates. The infected birds were observed for clinical disease and were sampled at selected times from 12 hours to 14 days postinoculation (DPI) or at the occurrence of mortality. Tissues were examined by histopathology, by immunohistochemistry for presence of NDV nucleoprotein (N), by in situ hybridization for viral mRNA, and were double labeled for apoptosis and viral N protein. Among the six passaged isolates, birds infected with pheasant and dove isolates had the most severe clinical disease, which was characterized by marked depression and mortality at 4 and 5 DPI. Severe diffuse necrosis of the lymphoid organs and lymphoid aggregates was the most prominent and consistent histologic finding in birds inoculated with those two isolates. Although birds inoculated with the Anhinga isolate did not show clinical signs, microscopically there were encephalitis, myocarditis, and tracheitis. Birds inoculated with the chicken and parrot isolates did not become clinically ill and had only mild histologic lesions. In chickens infected with the most virulent viruses, viral N protein and mRNA were detected among most of the affected organs especially at 2, 4, and 5 DPI. Double-labeled sections of the lymphoid organs revealed the presence of apoptotic cells containing intracytoplasmic viral N protein.