A 30-year old homosexual male presented with a one week history of painful vesicular perinal lesions. There was an associated history of body aches, fever and joint pains. He claimed that his paner had similar lesion on his penis about one month age with associated dysuria which has resolved now. The microscopy of the lesion revealed multinucleated syncytial cells with intranuclear inclusion bodies and perinuclear halo. The most liekly causative agent for these lesions is -