Single-nucleotide polymorphisms in two Marek's disease virus genes (Meq and gD): application to a retrospective molecular epidemiology study (1982-1999) in France

Laboratoire de Virologie et Barrière d'Espèces, UR 086, INRA, Centre de recherches de Tours, 37380 Nouzilly, France Correspondence S. Laurent slaurent{at}tours.inra.fr Marek's disease virus (MDV) is a herpesvirus that causes a lymphoproliferative disease in chickens. Vaccines against MDV are available, but the virus is gradually becoming more virulent. A molecular epidemiology study of MDV was carried out by assessing nucleotide variation in two different genes, Meq and gD , in 68 French field isolates circulating from 1982 to 1999, compared with reference strains. Viral DNA was amplified by nested PCR and sequenced directly. Comparison of the nucleotide sequences revealed a high nucleotide sequence identity (98 %). Single-nucleotide polymorphisms were identified, leading to the identification of three gene alleles for gD and six for Meq . Nine combinations of alleles were identified. A majority of French isolates (60·5 %) clustered in the C1 type, which has been present for over 17 years. Waves of non-C1-type isolates appeared when vaccine efficacy decreased. Furthermore, specific discriminating sequences were obtained for the CVI-988 vaccine strain.