Central nervous system lesions induced experimentally by a very virulent strain of Marek's disease virus in Marek's disease-resistant chickens.

The histological lesions of central nervous system (CNS) in Marek's disease-resistant chickens inoculated with a very virulent Marek's disease virus strain (vvMDV), Md/5, were examined. This vvMDV induced high incidences of CNS lesions as well as visceral and peripheral nerve lesions. The principal histopatho-logical changes of CNS consisted of non-suppurative meningoencephalomyelitis and lymphomatous lesion, which were categorized into two types, non-necrotizing and necrotizing. The main changes in the former type were perivascular cuffing of lymphoid cells of variable thickness. Although diffuse lymphomatous lesions were sometimes observed, gliosis, neuronal and axonal degeneration, and satellitosis as well as demyelination in the myelinated fibre paths, were infrequent. The most significant change of the latter type was necrotizing lymphomatous and, sometimes, non-suppurative inflammatory lesions (malacia). In the malacic foci, all the cells, including infiltrating lymphoid cells, neurons and glial cells, were necrotic or degenerated. The malacic lesions were frequently accompanied by fibrinoid necrosis of blood vessels. The present results indicate that necrotizing vasculitis associated with vvMDV may lead to ischaemic damage which in turn induces necrotizing lymphomatous and sometimes, meningoencephalomyelitis lesions. In addition, it is considered that vvMDV can induce a high incidence not only of CNS lesions, but also of visceral and nerve lesions in Marek's disease-resistant chickens.