Variable nature of border disease on a single farm: clinical and pathological description of affected sheep.

The clinical, pathological and virological findings in cases of Border disease occurring on a farm were studied. Fourteen abnormal lambs approximately three months old were obtained and were necropsied within 18 months at death or slaughter. Another lamb born during the study to a persistently infected ewe was also examined. The findings of these 15 animals are presented in three groupings. The animals of group A were persistently viraemic and hypomyelinogenesis was present in two lambs at one and 14 weeks old. Systemic, chronic, multifocal inflammation including nephritis, myocarditis and pneumonitis was found in older sheep. Group B lambs were non-viraemic and the predominant necropsy finding was cystic cerebral cavitation of variable severity. Histological examination showed cerebellar dysplasia and in two animals the retinas also showed multifocal photoreceptor cell atrophy. Lambs in group C were also non-viraemic and although these failed to thrive, no consistent pathological changes were found. It is concluded that the dams of animals in group A were infected in early pregnancy while those of animals in groups B and C were infected at mid-gestation or later.