THE NEUROPATHOLOGY AND EPIDEMIOLOGY OF BOVINE SPONGIFORM ENCEPHALOPATHY

Bovine spongiform encephalopathy (BSE), defined originally from its ch aracteristic neuropathology, retains a place of particular interest in the scrapie-like or prion disease group, presenting uniquely an examp le of such diseases occurring as a nationwide food-borne epidemic in G reat Britain. Comprehensive monitoring of the epidemic, both pathologi cally and epidemiologically, has facilitated our present understanding of the disease. BSE presents the classical neuropathological features of the transmissible spongiform encephalopathies. Although particular ly similar to natural scrapie of sheep, BSE has, unlike scrapie, a ste reotypic lesion profile from which it has been concluded that host and agent factors, including probably the strain of agent, which influenc e the profile, are constant in this disease. Neuronal loss in BSE may make an important but hitherto inapparent contribution to functional d eficits. Preliminary ultrastructural studies have confirmed tight micr oscopic features of brain changes in BSE but have as yet not establish ed significant new findings. Immunohistochemical studies of PrP accumu lation reveal distinctive forms and distributions of immunolabelling, confirming features reported previously in experimental models of scra pie, including perineuronal and perineuritic ''synapse-like'' reactivi ty. The histopathological diagnosis of BSE, validated on a single sect ion of the medulla for the statutory diagnosis of large numbers of cas es, is supplemented where necessary by fibril (SAF) examination which performs similarly to the histological diagnosis in the majority of ca ses. Epidemiological studies of BSE have supported the pathological fi ndings that there is no detectable variation in susceptibility within the cattle population. The detailed monitoring of the epidemic has rev ealed the expected effects on the incidence as a result of statutory m easures intended to prevent food-borne exposure after July 1988. The m ain effect has been a reduction in the national incidence during 1993 which has been continued into 1994. Analytical studies have not reveal ed any means of transmission, other than the food-borne source, capabl e of maintaining the epidemic in Great Britain. An international compa rison of risk factors for the occurrence of BSE indicates that an epid emic of similar magnitude outside the British Isles is unlikely.