Submitted to: International Marek's Disease Symposium Abstracts and Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: September 7, 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, early (8-20 day) mortality, and frequency of visceral lymphomas or ocular lesions in nonvaccinated chickens were positively correlated with virulence. Some vv+MDV isolates induced high mortality at 8-12 days post inoculation in chickens challenged at 3 weeks or older. These findings support the continued evolution of MD virus towards greater virulence.