Ultrastructure of the central nervous system in marek's disease and the effect of route of infection on lesion incidence in the central nervous system

Marek's disease virus infection in the brain: virus replication, cellular infiltration, and major histocompatibility complex antigen expression

Marek's disease virus (MDV) infection in the brain was studied chronologically after inoculating 3-week-old chickens of two genetic lines with two strains of serotype I MDV representing two pathotypes (v and vv+). Viral replication in the brain was strongly associated with the development of lesions. Three viral antigens (pp38, gB, and meq) were detected in the brain of infected chickens. Marked differences between v and vv+ pathotypes of MDV were identified for level of virus replication, time course of brain lesions, and expression of major histocompatibility complex (MHC) antigens. Two pathologic phenomena (inflammatory and proliferative) were detected in the brain of chickens inoculated with vv+MDV, but only inflammatory lesions were observed in those inoculated with vMDV. Inflammatory lesions, mainly composed of macrophages, CD4+ T cells, and CD8+ T cells, started at 6-10 days postinoculation (dpi) and were transient. Proliferative lesions, characterized by severe infiltrates of CD4+CD8- T cells (blasts), started at 19-26 dpi and persisted. Expression of MHC antigens in endothelial cells and infiltrating cells within the brain was influenced by MDV infection. Upregulation of MHC class II antigen occurred in all treatment groups, although it was more severe in those inoculated with vv+MDV. MHC class I antigen was downregulated only in those groups inoculated with vv+MDV. These results enhance our understanding of the nature and pattern of MDV infection in the brain and help to explain the neurovirulence associated with highly virulent MDV.

Electron microscopic and immunohistochemical localization of Marek's disease (MD) herpesvirus particles in MD skin lymphomas

Skin lymphomas induced in 11 specific-pathogen-free chickens by inoculation at 1 day of age with Marek's disease virus (MDV) were biopsied weekly and examined by electron microscopy and immunohistochemistry. In the sequentially biopsied lymphomas, immature MDV particles (abortive replication) were found only in the nuclei of necrotic lymphoblasts within necrotizing neoplasms. The necrotizing lymphomas were observed in two of the 11 experimental birds and were associated with prominent vascular endothelial cell injury, including fibrinoid necrosis of blood vessels. Nonnecrotizing lymphomas biopsied sequentially from the 11 experimental birds did not contain virus particles of any kind in the lymphoblasts and had no distinct vascular lesions. Immunohistochemically, MDV early antigen (pp38), but not late antigens (glycoproteins B and C), was detected only in the necrotizing lymphomas. These findings indicate that abortive MDV replication mainly occurred in necrotic lymphoblasts, which might have been induced by ischemia.

Marek's disease virus-induced transient paralysis in chickens: electron microscopic lesions

A study was made to determine the causative lesion of Marek's disease virus (MDV)-induced transient paralysis (TP) in chickens by comparing the ultrastructure of brain tissue from MDV-infected genetically susceptible and resistant birds. There were numerous intramyelinic vacuoles in the brains of TP-affected birds. Many of these vacuoles contained particulate material compatible with precipitated protein from edema. Astrocyte processes often were distended with similar particulate material. Most intramyelinic vacuoles were either adjacent to the axolemma or within inner myelin lamellae. Myelin sheaths of affected axons, while being displaced, were relatively normal with no vesiculation. Most affected axons were also otherwise normal. Cell processes adjacent to occasional affected axons were distended by degenerating mitochondria, vacuoles, and amorphous material. Some of these processes appeared to be inner loops of oligodendrocyte cytoplasm. The cell bodies of most oligodendrocytes were normal, but a few contained vacuoles similar to those seen in processes adjacent to axons. There were scattered necrotic cells. While most of these could not be specifically identified, some appeared to be oligodendrocytes. Mononuclear inflammatory cells were present both perivascularly and within the parenchyma. Although these cells occasionally contacted myelinated axons and there was myelin phagocytosis, there was no indication that they initiated demyelination. Brain tissue from virus-inoculated resistant birds had perivascular aggregates of mononuclear cells, but there were no intramyelinic vacuoles and few necrotic cells. These findings suggest that intramyelinic vacuolation contributes to the pathogenesis of transient paralysis. Potential pathophysiological mechanisms contributing to the vacuoles, including brain edema and oligodendrocyte injury, are discussed.