The stress index: a method for indicating the pathogenicity of vaccinal Newcastle disease virus when administered by aerosol.

Exposure of young chicks to a standard aerosol of strains of Newcastle disease live virus vaccine under fully controlled conditions has permitted observation of their respiratory health to be used to give an index of respiratory stress. The pathogenicity of 14 strains of vaccine has been studied and the test has been found to be reproducible. When used in combination with tests for immunising capacity, it is suggested that the test offers a method for selecting the most effective vaccinal strains for use by aerosol application.