Marek's disease herpesvirus: a model of vaccine-dependent adaptation

Marek's disease (MD) virus (MDV) is an alphaherpesvirus that causes a rapid-onset T-cell lymphoma in chickens. In order to preserve the viability of poultry industry, non sterilizing vaccines have been used since fifty years, preventing lymphoma development but leading to an imperfect control of MD. Vaccination has been accompanied with the increase in virulence of MDV forcing the development of new vaccine formulations. Several loci of MDV genome are variable and have evolved in link with virulence of MDV strains. It has been shown that MDV is in fact constituted by a dynamic population of genetic variants with a distribution linked to viral strain phenotype. In this context, we have shown that CVI988/Rispens vaccine, still the most efficient one against hypervirulent MDV strains, is composed of twenty variants, variable from one batch to another, evolving likely as RNA virus quasispecies.