BSE IN GREAT-BRITAIN - CONSISTENCY OF THE NEUROHISTOPATHOLOGICAL FINDINGS IN 2 RANDOM ANNUAL SAMPLES OF CLINICALLY SUSPECT CASES

Two annual, random samples of clinically suspect cases of bovine spong iform encephalopathy (BSE) were taken in 1992-93 (year 1, 1500 cases) and 1993-94 (year 2, 1000 cases). From each sample, 100 positive cases were examined in detail to establish the severity of the vacuolation in 17 specific neuroanatomical locations. The resultant 'lesion profil es' were compared with the profile obtained from a similar sample of B SE-affected cattle from early in the epidemic (1987-89); the compariso n showed that the distribution and severity of vacuolation in BSE has remained unchanged. The cases not confirmed as BSE on histological exa mination (172 in year 1 and 162 in year 2) were examined for evidence of any alternative neurohistological diagnosis. As in previous studies , the majority of these cases showed no significant lesions (61.6 and 61.7 per cent). The remainder consisted of bilateral focal spongiosis of unknown significance (26.7 and 21.0 per cent), inflammatory conditi ons (8.1 and 11.1 per cent) and a small number of cases with tumours, cerebrocortical necrosis or idiopathic brainstem neuronal chromatolysi s. No evidence was found of any cases of BSE with an atypical distribu tion of lesions. These findings support the theory that the BSE epidem ic is sustained by a single, stable strain of the BSE agent, and confi rm that the existing statutory diagnostic criteria continue to be appr opriate.