Bovine spongiform encephalopathy. Epidemiology, low dose exposure and risks.

BSE has occurred in the U.K. as an extended common source epidemic since 1985/86. The vehicle of infection was concentrated feeds containing meat and bone meal produced by the rendering of ovine, bovine and other animal wastes. The epidemic was probably initiated in 1981/82 when a sudden decline in the use of solvents in rendering allowed a low incidence of scrapie-like infection to occur in cattle. However, the presence in feed of bovine material that, from 1984/85 (or earlier), was increasingly infected with a cattle-adapted strain of agent amplified the epidemic greatly. Nevertheless, the incidence of BSE cases nationally has been low because of the generally low effective exposure of cattle to infection in feed. This, and a combination of risk factors that were probably unique to the U.K. can explain why relatively few cases of BSE have occurred in other countries. The feeding of ruminant-derived protein to all species of ruminants was banned in Great Britain in 1988, and in Northern Ireland in 1989. A more selective approach was subsequently adopted to minimize the risks of BSE infection of other species, including man. This was based on excluding from food a small number of bovine offals whose use and predicted infectivity titers would constitute the greatest potential source of infection. Recent studies of BSE support the basis of the specified bovine offals ban and suggest that more tissues were restricted than may have been necessary.