Monocytosis is associated with the onset of leukocyte and viral infiltration of the brain in chickens infected with the very virulent Marek's disease virus strain C12/130

Marek's disease (MDV) virus is mainly known for the induction of visceral lymphomas and lymphoid infiltration of peripheral nerves. Recently, additional tropism for the central nervous system has been recognised as a distinct feature of disease induced by very virulent MDV isolates. During the analysis of changes in the peripheral blood leukocyte subpopulations in chickens infected with either a virulent (HPRS-16) or a very virulent (C12/130) strain of MDV, we observed a marked monocytosis in chickens infected with C12/130. Perivascular cuffing in brain and mononuclear cell infiltration into the meninges of chickens infected with C12/130 were associated with the appearance of the monocytosis from 6-10 days post-infection. Our results show that a peripheral blood monocytosis may be a contributory factor in establishing or accelerating the severity of mononuclear infiltration into the meninges and perivascular spaces in the brain during infection by very virulent C12/130 strain of MDV.