|Witter, Richard - USDA-ARS- RETIRED|
Submitted to: Workshop on Molecular Pathogenesis of Marek's Disease and Avian Immunology
Publication Type: Abstract Only
Publication Acceptance Date: October 6, 2002
Publication Date: December 31, 2002
Citation: Witter, R.L., Gimeno, I.M. 2002. Continued evolution of marek's disease?--biocharacteristics and challenge models [abstract]. Workshop on Molecular Pathogenesis of Marek's Disease and Avian Immunology. p. 36. Technical Abstract: Marek's disease (MD) virus has become more virulent since the advent of vaccination. This trend has caused concern in the poultry industry and prompted renewed efforts to develop more effective vaccines. This report examines the status of the virus and the disease in the United States. Condemnations of young broiler chickens for MD lesions at time of slaughter continue to decline. Vaccine breaks in layer and breeder flocks are increasingly rare. However, viruses isolated from 1995-1999 tended to be highly virulent, and usually pathotyped in the vv+ class. Several of these strains induced severe neuropathology in SPF chickens. In contrast to our earlier report, comparison of recent isolates with earlier isolates in chickens vaccinated with CVI988 vaccine revealed no evolution of pathogenicity. One of the recent isolates, strain 686, appeared more virulent than 648A (the prototype vv+ strain) and was utilized to develop a new challenge model to test vaccine efficacy. Several highly virulent strains caused a high rate of lymphomas in adult SPF chickens whereas such chickens were refractory to less virulent strains. Adult chickens previously vaccinated or exposed to low virulence strains were refractory to challenge with even the most virulent MD viral strains. Thus, even though MD is controlled for the moment in the United States, recent MD virus isolates have acquired unique biological characteristics. Examination of evolutionary trends with MD virus may help predict when current control strategies will cease to be effective.