Effect of in vitro adaptation of Marek's disease virus on pock induction on the chorioallantoic membrane of embryonated chicken eggs

Cell-associated preparations of several isolates of Marek's disease virus produced more pocks on the chorioallantoic membrane of embryonated chicken eggs than plaques in duck embryo fibroblasts, thus indicating that lesion response in eggs was more sensitive than cytopathic response in duck embryo fibroblasts for assaying low-passage Marek's disease virus. Adaptation of the virus to cell cultures by serial passages, however, substantially reduced its pock response so that the titer ratio (plaque-forming units in duck embryo fibroblasts/pock-forming units in eggs) of cell culture-adapted Marek's disease virus was 1 or higher. The decreased pock response could not be attributed to selection of preexisting virus variant(s) with low affinity for chorioallantoic membrane because cloned Marek's disease virus had a good pock response at low cell culture passage levels, but this response decreased as the virus was attenuated by serial cell culture passage.