I don't know much about chicken diseases. Is it moist? Is there exudate? It almost looks circular in the picture. Is it? There are common indications of inflammation (red, exudate, edema) If it was parasitic, I would think it would not be so perfectly shaped. Just my thought. I would be worried about trauma and subsequent superficial dermatitis, probably bacterial. Like someone said a minute ago, just because you don't see it doesn't mean the roo isn't visiting the hen house. Try a triple antibiotic ointment like neosporin. Good LUck