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

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.