Abstract
Marek's disease (MD) is a common lymphoproliferative disease of the domestic chicken caused by a cell associated herpesvirus. Vaccines used for the prophylaxis of MD have been derived from non-pathogenic field Marek's disease virus (MDV), pathogenic MDV and the herpesvirus of turkeys (HVT). Vaccines derived from MDV have been developed by attenuation of virus strains which produce the acute form of MD and virus strains which produce the classical form. They have also been developed by suitable modification, where necessary, of non-pathogenic MDV. All MDV derived vaccines have to be used in the cell associated form. The turkey herpesvirus vaccine can be used in either the cell associated or cell free lyophilized form. All these types of vaccine appear to be safe and to provide significant levels of protection under field conditions.