An overview of bovine spongiform encephalopathy.

None of the diseases caused by the "unconventional slow viruses" is highly infectious in its natural host. Transmission of infection to other species only occurs if the effective dose is high enough to overcome the species barrier. The current epidemic of bovine spongiform encephalopathy (BSE) in the U.K. is believed to have been initiated by scrapie infection of cattle via contaminated meat and bone meal in concentrated feedstuffs. But the effective exposure was extremely low. Subsequently the epidemic was driven by the re-cycling of infected cattle material which by-passed the species barrier and effectively "passaged" the infection within the cattle population. It is important that re-cycling would not apply to any public health risks arising from BSE. The safety of food can be ensured by excluding all potentially high titre tissues from the food chain. With medicines, risks can most simply be avoided by not using bovine materials from U.K. sources.