Marek's disease (MD) is a re‐emerging viral disease of chickens and a serious economic threat to the poultry industry worldwide. Continuous surveillance with molecular investigation is essential to monitor the emergence of virulent Marek's disease virus (MDV) strains and to devise any appropriate vaccination strategy and implement bio‐security programmes. In the present study, we investigated the cases of MD outbreaks in vaccinated poultry flocks. The MD outbreak was confirmed through necropsy (mainly visceral tumours), histopathology and viral gene specific PCR. The pathotypes of the field MDV strains were assessed by molecular analysis of three virulence‐associated genes, meq, pp38 and vIL‐8. The Meq sequence of the field strains analyzed in this study lacked the 59 aa unique to mild strains, indicating that they are potentially virulent strains. Mutation at position 71 and the presence of five proline rich repeats in the transactivation domain, both associated with virulence were observed in these strains; however, the signature sequences specific to very virulent plus strains were absent. Phylogenetic analysis of meq oncogene sequences revealed clustering of the field strains with North Indian strains and with a very virulent plus ATE 2539 strain from Hungary. Analyses of pp38 protein at positions 107 and 109 and vIL‐8 protein at positions 4 and 31 showed signatures of virulence. Sequence and phylogenetic analysis of oncogene and virulence‐associated genes of field MDVs from vaccinated flock indicated that these strains possessed molecular features of virulent strains.

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Marek’s disease (MD) is a re-emerging viral disease of chicken and a serious economic threat to poultry industry worldwide. Continuous surveillance with molecular investigation is mandatory to monitor the emergence of virulent MDV strains and to devise any appropriate vaccination strategy and implement bio-security programs. In the present study, we investigated the cases of MD outbreaks in vaccinated poultry flocks. The MD outbreak was confirmed through necropsy (majorily visceral tumors), histopathology and viral gene specific PCR. The pathotypes of the field MDV strains were assessed by molecular analysis of three oncogenes -Meq, pp38 and vIL-8. The Meq sequence of the field strains analyzed in this study lacked the 59 aa unique to mild strains indicating that they are virulent strains. Mutation at position 71 and presence of five proline rich repeats in the transactivation domain, both associated with virulence were observed in these strains, however, the signature sequences specific to very virulent plus strains were absent. Phylogenetic analysis of Meq gene sequences revealed clustering of the field strains with North Indian strains and with a very virulent plus ATE 2539 strain from Hungary. Analyses of pp38 protein at positions 107 and 109 and vIL-8 protein at positions 4 and 31 showed signatures of virulence. Sequence and phylogenetic analysis of oncogenes from field MDVs from vaccinated flock indicated these strains possessing molecular features of very virulent strains. Our data shows here that Meq, vIL-8 and pp38 genes can be used as markers for molecular analysis to decipher the pathotype of MDV strains. Our present study suggests evolution of virulent MDV induced by vaccination.