Molecular detection and phylogenetic analysis of Marek's disease virus
oncogenes from vaccinated flocks in southern India reveals circulation
of very virulent MDV genotype
Abstract
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.