The Ark-attenuated virus, which is adapted to embryonated eggs, only killed Chicks inoculated with virus at 1 day of age showed statistical differences (P B 0.1) in the amount of virus detected in the trachea between the Ark/Ark-DPI/81 and Ark-attenuated viruses at 24, 48, 72, and 96 h post-inoculation with the pathogenic Ark/Ark-DPI/81 having the higher amount of virus at each of the sample times (Fig. 4b) .