Bovine spongiform encephalopathy: epidemiological factors associated with the emergence of an important new animal pathogen in Great Britain

Bovine spongiform encephalopathy was first recognized in Great Britain in 1986. Subsequent studies indicated that this novel disease was a transmissible spongiform encephalpathy associated with a scrapie-like agent. Epidemiological studies indicated that the vehicle of infection for cattle was ruminant derived protein included in commercial feedstuffs and that the probable origin of infection was sheep scrapie. Further exposure from this food-borne source was prevented in July 1988 by legislation. The effects of this intervention became apparent first in 1991 and have been substantiated subsequently. In the period before the legislative action it is apparent that the epidemic was enhanced by the recycling of subclinically infected cattle tissues, exposure commencing from this source in 1984/85 following the initial effective exposure in 1981/82. The effects of this recycling, together with a discussion of the resultant working hypotheses, are described.