Submitted to: International Marek's Disease Symposium Abstracts and Proceedings
Publication Type: Proceedings
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 and 21 isolates were classified in the v and vv pathotypes, respectively. 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+). Lymphoma mortality and frequency of visceral lymphomas and ocular lesions were positively correlated with virulence. Both vv and vv+ 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.