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.