Fifty-three Marek's disease (MD) virus (MDV) isolates were obtained from turkey-herpesvirus-vaccinated broiler or layer flocks with excessive MD losses, from control field flocks without excessive MD losses, and from laboratory collections. Twelve isolates were typed as very virulent (vvMDVs) on the basis of excessive pathogenicity for vaccinated chickens, 31 were typed as virulent (vMDVs), and 10 were typed as nonpathogenic (npMDVs). The npMDVs could be distinguished from turkey herpesviruses by cultural and serologic criteria. Compared with standard vMDVs, the vvMDVs appeared clearly more pathogenic; they caused greater depression in body and bursal weights and induced more deaths through the early mortality syndrome, more lymphomas, and more visceral and fewer neural lymphomas in susceptible and resistant chickens. However, no antigenic differences between vvMDVs and vMDVs were detected. The vvMDVs were obtained from both broiler and layer flocks in five widely separated states but may only recently have become prevalent, since none were represented among 10 MDV isolates obtained before 1975. The frequency of isolation of vvMDVs from flocks with excessive MD losses (9/27) was threefold higher than that from control flocks (1/10), suggesting an association of this viral pathotype with vaccine breaks. The npMDVs were also widely distributed but were isolated only from control flocks, thus suggesting that npMDVs may augment turkey-herpesvirus-induced vaccinal immunity.
Animals Chickens