Marek's disease virus infection in the eye: chronological study of the lesions, virus replication, and vaccine-induced protection.
Marek's disease virus (MDV) infection in the eye was studied chronologically after inoculating 1-day-old chickens with a very virulent MDV strain, Md5. The ocular lesions could be classified as early lesions (6-11 days postinoculation [dpi]) and late lesions (26 and 56 dpi), based upon the location and severity of the lesions. The early lesions involved iris, ciliary body, and choroid layer, and were characterized by endothelial cell hypertrophy, vasculitis, and infiltration of lymphocytes (mainly CD8+), plasma cells, macrophages, and heterophils. Expression of early MDV-antigen pp38 in the cells infiltrating choroid layer was detected as early as 11 dpi. Late lesions consisted of severe lymphohistiocytic uveitis, keratitis, pectenitis, vitreitis, retinitis, and segmental to diffuse retinal necrosis. Cell infiltration included macrophages, granulocytes, plasma cells, and both CD4+ and CD8+ cells of various sizes. Expression of early MDV-antigen pp38 was readily found within the retina, uveal tract, and corneal epithelium. No expression of late-antigen gB or oncoprotein meq was detected in any of the eyes examined. A second experiment was conducted to study the effect of vaccination on the development of ocular lesions. Both HVT and CVI988 were able to protect against the development of early ocular lesions in chickens infected with very virulent plus strain MDV 648A. However, only CVI988 conferred complete protection against the development of late ocular lesions. HVT conferred partial protection, as it reduced the frequency and severity of the late ocular lesions. These results enhance our understanding of the nature and pattern of MDV infection in the eye.