Submitted to: Poultry Disease Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: October 3, 1996
Publication Date: N/A
Technical Abstract: The virulence of 31 isolates of serotype 1 Marek's disease (MD) virus obtained from layer or broiler flocks during 1989-1995 was determined by challenge at 6 days of nonvaccinated and vaccinated chickens and compared to challenge with prototype viruses JM/102W and Md5. Three isolates did not differ from JM/102W and were classified in the vMDV pathotype. Twenty-one isolates produced significantly higher levels of MD in HVT-vaccinated chickens than the JM/102W control and were classified in the vvMDV pathotype. Seven isolates produced significantly higher levels of MD in HVT+SB-1-vaccinated chickens than the Md5 control and were provisionally designated as a new pathotype (vv+MDV). Lymphoma mortality and frequency of visceral lymphomas or ocular lesions in nonvaccinated chickens were positively correlated with virulence. vv+MDV isolates induced transient paralysis and lymphomas in chickens challenged at 3 or 18 weeks. These findings support the continued evolution of MD virus towards greater virulence and identify some unique features of vv+ strains.