Marek's disease vaccines: a solution for today but a worry for tomorrow?

Marek's disease (MD) is a lymphoproliferative disease of chickens that, in the absence of control measures, is capable of causing devastating losses in commercial poultry flocks. MD has been successfully controlled by vaccination since 1968. However, vaccine efficacy has decreased concomitantly with the increase in virulence of Marek's disease virus (MDV). The constant evolution of MDV has forced the development of new vaccines or vaccine strategies that control the more virulent emergent strains. However, this race between the introduction of new vaccines and the evolution of MDV represents a major threat for the poultry industry. In addition to vaccination, other factors might have contributed to the evolution of MDV (intensive methods of chicken production, early exposure of the chickens to MDV and administration of vaccines at very low doses). From all the possible factors influencing MDV evolution, the effect of vaccination has received the greatest attention. MD vaccines protect with great efficacy against the development of the disease but they do not prevent infection or transmission. Sterilizing immunity could be a solution to stop the evolution of the virus but it has been proven to be extremely difficult, if at all possible, to obtain with MDV or with other herpesviruses. Other solutions to improve vaccine-induced protection are discussed in this paper.