Have you lost the plot?; A three-part series that explains the issues behind the news

Are all our cows mad?

By the late Seventies, Britain's farmers had turned their cattle into carnivores and cannibals by feeding them the boiled-down remains of sheep and other cattle. In the early Eighties, the result of making this "rendering" process cheaper and more economical was that the remains of scrapie-infected sheep passed into cattle feed, sparking an epidemic of mad cow disease, or bovine spongiform encephalopathy (BSE).

Between November 1986, when BSE was first recognised, and 12 January this year, 156,712 cattle have had to be destroyed because of the disease. This is an enormous number. But it's far from being all of Britain's cows. In 1994, the UK's dairy herds numbered 2.7 million cows, while there were 1.8 million beef cattle. The disease is now in decline, with only 300 cattle a week being diagnosed compared with more than 1,000 a week when the epidemic was at its peak.

No one knows if humans can catch BSE. The best scientific consensus is that the risk to humans is tiny, if it exists at all.

There is a similar human degenerative brain condition, Creutzfeldt-Jakob disease, but this occurs in countries free of BSE and sheep scrapie, and also in lifelong vegetarians. Most scientists believe that the agent responsible for transmitting these diseases is neither a bacterium nor a virus but a prion, an aberrant protein, although it has yet to be definitively identified and examined. Neither beef nor milk contain the disease-carrying prions. Since 1989, slaughterhouses have been required to remove and incinerate "prohibited offals" (brain, thymus, spleen and spinal cord, which are thought to have higher concentrations of infectious agent) from all cattle.

About one person a week died of CJD in Britain during 1994, but the disease seems to have declined in 1995, according to early reports.