Brain lesions in young broiler chickens naturally infected with a mesogenic strain of Newcastle disease virus.

Thirty-nine 4- to 5-week-old broiler chickens from an outbreak of Newcastle disease (ND) in Japan were examined pathologically. The causative agent was identified as a mesogenic strain of ND virus. Predominant gross lesions included haemorrhage in the lungs, congestion of the trachea, splenomegaly, atrophy of the thymus and bursa of Fabricius, and whitish discolouration of the brain. Microscopically, there was mild haemorrhagic pneumonia, catarrhal tracheitis, lymphoid necrosis in the spleen, thymus, bursa of Fabricius and caecum and diffuse non-suppurative encephalitis. Lesions associated with encephalitis were characterized by multifocal perivascular cuffing, malacia, demyelination and proliferative vasculitis. Malacic lesions occurred in the hyperstriatum, neostriatum, subleptomeningeal and periventricular regions of the cerebrum, whereas demyelination was seen mainly in the brain stem. The morphological changes that occurred in the brain in these cases were distinctive and the lesions in the lymphoid tissues were related to concurrent infection with infectious bursal disease virus.