Molecular characteristics of very virulent European MDV isolates.

Marek's disease virus (MDV) strains with increasing virulence have been reported from many parts of the world. Many of these recent MDV isolates produce an acute early cytolytic disease with high mortality and severe atrophy of the lymphoid organs, thymus and the bursa of Fabricius. Although the degree of the atrophic changes and the virulence of the virus are correlated, the molecular basis of the increased virulence is not known. We examined the characteristics of the disease induced by 3 such MDV isolates, C12/130, MR36 and MR48, isolated from Europe. All the three viruses produce high early mortality and atrophy of the lymphoid organs. As a first step in understanding the determinants of the increased virulence of these isolates, we have compared the sequences of MEQ and the ICP4 genes of these three viruses with that of the published sequences. Some of the amino acid changes seen within the Meq and ICP4 proteins were conserved in all the three isolates and could account for the increased virulence characteristics.