CHRONOLOGICAL STUDY OF ULTRASTRUCTURAL CHANGES IN THE PERIPHERAL NERVES IN MAREK'S DISEASE

Abstract

A chronological study was made of the ultrastructural changes in peripheral nerves following inoculation of 1-day-old chicks with a neurogenic strain of Marek''s disease virus. No virus particles were found in nerves. Cellular infiltration of nerves was detected as early as 5 days after inoculation and by 3 wk some nerves contained proliferative lesions which possessed many of the ultrastructural features characteristic of normal, reactive lymphoid tissue. About 4 wk after inoculation, coinciding with the onset of neurological signs, areas of widespread demyelination appeared within these lesions; lymphocytes and macrophages penetrated and destroyed the myelin sheath, but spared Schwann cells and most axons. Later edematous, sparsely infiltrated B type lesions were observed, some of which contained demyelinated nerve fibers undergoing repair; these were therefore a stage in the regression of the proliferative lesions. These observations do not favor the hypothesis that cellular infiltration of nerves in Marek''s disease is the direct result of auto-sensitization to normal myelin. They are consistent with the hypothesis that demyelination is a secondary feature and that the primary lesions are preferential sites for immune demyelination.