Bovine Spongiform Encephalopathy: Hypothetical Risk of Emergence as a Zoonotic Foodborne Epidemic

Bovine spongiform encephalopathy (BSE) is a fatal neurological disease of cattle, recognized in Great Britain in 1986. Cases in other countries have been attributed to imports from Great Britain. The disease has not occurred in the U.S. BSE is one of a group of diseases (other examples are scrapie of sheep and Creutzfeld-Jacob disease of humans) referred to as prion diseases or transmissible spongiform encephalopathies. Under some circumstances prion diseases can be transmitted by injection or by feeding infected (abnormal prion protein-containing) tissue to susceptible hosts. BSE was disseminated by feeding meat and bone meal containing BSE agent which was not completely inactivated by rendering. BSE is hypothesized to have emerged from scrapie via recycling of rendered by-products in cattle. There is also evidence of spontaneous feed-borne transmission of BSE to wild ruminants in zoological parks and to domestic cats. It has been hypothesized that foodborne transmission of BSE to humans has occurred or could occur. This hypothesis can neither be definitively refuted nor supported. However, it seems unlikely. In spite of hundreds of years of human exposure to scrapie, there is no evidence of transmission of scrapie to humans. Even if BSE is ultimately found to be somehow transmissible to humans, the risk of foodborne transmission appears to be low for several reasons: (i) The oral route is several orders of magnitude less sensitive than the parenteral route for transmission of prion diseases; (ii) the BSE agent is only detectable in brain, spinal cord, and intestine of infected cattle, tissues infrequently used for human food; and (iii) Great Britain (where the disease occurs) destroys and bans the use of all tissues from BSE-infected cattle as well as the brains, spinal cords, and intestinal tracts from clinically normal cattle.