Molecular characteristics of very virulent European MDV isolates

Marek's disease virus (MDV) strains with increasing virulence have been rep orted from many parts of the world. Many of these recent MDV isolates produ ce an acute early cytolytic disease with high mortality and severe atrophy of the lymphoid organs, thymus and the bursa of Fabricius. Although the deg ree of the atrophic changes and the virulence of the virus are correlated, the molecular basis of the increased virulence is not known. We examined th e characteristics of the disease induced by 3 such MDV isolates, C12/130, M R36 and MR48, isolated from Europe. All the three viruses produce high earl y mortality and atrophy of the lymphoid organs. As a first step in understa nding the determinants of the increased virulence of these isolates, we hav e compared the sequences of MEQ and the ICP4 genes of these three viruses w ith that of the published sequences. Some of the amino acid changes seen wi thin the Meq and ICP4 proteins were conserved in all the three isolates and could account for the increased virulence characteristics.