The neuropathology and epidemiology of bovine spongiform encephalopathy

Abstract

Bovine spongiform encephalopathy (BSE), defined originally from its characteristic neuropathology, retains a place of particular interest in the scrapie-like or prion disease group, presenting uniquely an example of such diseases occurring as a nationwide food-borne epidemic in Great Britain. Comprehensive monitoring of the epidemic, both pathologically and epidemiologically, has facilitated our present understanding of the disease. BSE presents the classical neuropathological features of the transmissible spongiform encephalopathies. Although particularly similar to natural scrapie of sheep, BSE has, unlike scrapie, a stereotypic lesion profile from which it has been concluded that host and agent factors, including probably the strain of agent, which influence the profile, are constant in this disease. Neuronal loss in BSE may make an important but hitherto inapparent contribution to functional deficits. Preliminary ultrastructural studies have confirmed light microscopic features of brain changes in BSE but have as yet not established significant new findings. Immunohistochemical studies of PrP accumulation reveal distinctive forms and distributions of immunolabelling, confirming features reported previously in experimental models of scrapie, including perineuronal and perineuritic "synapse-like" reactivity. The histopathological diagnosis of BSE, validated on a single section of the medulla for the statutory diagnosis of large numbers of cases, is supplemented where necessary by fibril (SAF) examination which performs similarly to the histological diagnosis in the majority of cases. Epidemiological studies of BSE have supported the pathological findings that there is no detectable variation in susceptibility within the cattle population. The detailed monitoring of the epidemic has revealed the expected effects on the incidence as a result of statutory measures intended to prevent food-borne exposure after July 1988. The main effect has been a reduction in the national incidence during 1993 which has been continued into 1994. Analytical studies have not revealed any means of transmission, other than the food-borne source, capable of maintaining the epidemic in Great Britain. An international comparison of risk factors for the occurrence of BSE indicates that an epidemic of similar magnitude outside the British Isles is unlikely.