Marek’s Disease is an economically important viral disease of chickens caused by Marek’s disease virus (MDV), an oncogenic herpesvirus characterized by lymphoproliferative and neuropathic syndrome. Despite vaccination, field isolates of MDV-1 have tended to increase in virulence, developing symptoms of MD with obvious visceral lymphoma. In the present study, molecular characterization and phylogenetic analysis of the oncogene pp38 from five field MDV strains of Marek’s Disease in vaccinated flocks of Punjab, India were studied. The MDVs were detected using BamH primers which distinguished field MDV strain from vaccine strain. Oncogenic gene pp38 was amplified using specific primers and further cloned and sequenced. Homology comparison of the nucleotide sequences of 5 MDV field isolates and other reference MDVs was found to be 99.8-100%. High nucleotides and amino acids identities between MDVs detected from the field cases of MD in vaccinated flocks and the reference MDVs were observed. The present study indicated that virulent oncogenic MDV-1 is circulating in this region and the point mutation which occurs in the pp38 gene may be one of the factors responsible for increasing the virulence of MDV and thereby failure in protection from MD by commercial vaccines.
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