Newcastle disease is caused by which type of virus?

• A. Virulent strains of avian paramyxovirus serotype 1
• B. Highly pathogenic strains of the avian influenza virus
• C. Pasteurella multocida
• D. Virulent strains of gallid herpesvirus 2 (lymphotropic alphaherpesvirus)

Answer: (A)

Newcastle disease is caused by virulent strains of avian paramyxovirus serotype 1, an RNA virus in the Paramyxoviridae family. This virus, formed by the Newcastle disease virus (NDV), has strains with varying virulence; the virulent ones cause severe disease in birds. A key factor in virulence is the F (fusion) protein cleavage site, where multiple basic amino acids allow widespread infection by enabling activation by host proteases. Other options refer to entirely different pathogens: an influenza A virus causes avian influenza, Pasteurella multocida is a bacterium causing fowl cholera, and gallid herpesvirus 2 is Marek’s disease virus, a herpesvirus. Therefore, the correct cause is virulent strains of avian paramyxovirus serotype 1.