Newcastle disease is primarily caused by which virus?

Newcastle disease is caused primarily by virulent strains of avian paramyxovirus serotype 1. This virus is a member of the Paramyxoviridae family and is specifically noted for its impact on various bird species, particularly poultry. It can lead to severe respiratory, neurological, and gastrointestinal symptoms in infected birds, making it a significant concern in avian health and poultry production.

The other options refer to different viruses associated with other diseases or specific conditions in birds. For instance, gallid herpesvirus 1 is responsible for infectious bursal disease, while highly pathogenic strains of avian influenza virus are known for causing avian influenza outbreaks, which also affect poultry but differ in their pathogenesis and clinical signs. Lastly, virulent strains of gallid herpesvirus 2 are associated with Marek's disease. Understanding these distinctions is crucial for effective disease management in poultry health.