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Duck virus hepatitis: the effect of attenuation on virus stability in ducklings.

The effect of further attenuation by serial passage in specific pathogen free embryonated hens' eggs, on the stability of the H strain of duck hepatitis virus (DHV) when serially passaged in ducklings was investigated. The 65th, 75th, 86th and 94th passage viruses (H65, H75, H86 and H94) which were considered to be potential vaccines all reverted to virulence on serial passage in ducklings. The virulent Q strain of DHV was similarly attenuated and tested for vaccine potential and stability. Virus passaged serially 10 times was still pathogenic. The 20th and 30th passage virus was apathogenic but did not protect as well as the 40th, 50th, 60th, 70th and 80th passage virus. The 90th passage virus was less protective. All these potential vaccines reverted to virulence on serial passage in ducklings.