Bovine spongiform encephalopathy: an appraisal of the current epidemic in the United Kingdom.

Bovine spongiform encephalopathy (BSE) is a food-borne infection of cattle caused by the use of contaminated meat and bone meal in concentrated feeds. The UK epidemic was initiated by a sudden exposure to infection in 1981-1982, which was associated with a dramatic reduction in the use of organic solvents in the manufacture of meat and bone meal. This change almost certainly removed two partial disinfection steps and allowed enough contamination with a scrapie-like agent to infect cattle. Although it is assumed that the epidemic originated with scrapie infection crossing the species barrier, cattle-to-cattle recycling of infection, via feed, amplified the epidemic very considerably. There would have been a strong tendency for recycling to select a single cattle-adapted strain of agent, and this strain of BSE could well be different from scrapie. There is evidence to support both predictions. Because the median incubation period of BSE is 4-6 years, clinical cases did not appear until 1985-1986, by which time the recycling of infection in cattle was probably well established. However, the average food-borne exposure to infection has remained low resulting in a mainly sporadic occurrence of BSE. Signs of an imminent decline in the epidemic were unmistakable early in 1993, which is over 4 years after the feeding of ruminant-derived protein was banned to prevent new infections of cattle.