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.

The ultrastructure of the lesions of the central nervous system (CNS) of chickens was examined at intervals after intra-abdominal inoculation of Marek's disease virus (MDV). No transient paralysis occurred. Peri-vascular accumulations of lymphocytes and macrophages (cuffing) were accompanied by invasion of the CNS by these blood-borne leukocytes in the most severe lesions. Only minor damage to axons, myelin or glial cells was detected and no structural evidence for viral replication was observed. Intracerebral inoculation of cell-free MDV induced similar lesions in the CNS and there was no evidence for local exacerbation of their severity, although a few chickens developed transient paralysis. It is concluded that replication of MDV in the CNS is unlikely to be the direct cause of the CNS lesions.